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THE  LIBRARY 

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THE  UNIVERSITY 

OF  CALIFORNIA 

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WORKS  OF  PROF.  I.  0.  BAKER 

PUBLISHED    BY 

JOHN   WILEY   &   SONS. 


A  Treatise  on  Hasonry  Construction. 

Containing  Materials  and  Methods  of  Testing 
Strengtli.etc;  Combinations  of  Materials — Composi- 
tion, etc.;  Foundations  — Testing  the  Bearing  Power 
of  Soils,  etc  ;  Masonry  Structure— Stabi.itv  Against 
Sliding,  Overturning,  Crushing,  etc.,  etc.,  etc. 
Ninth  Edition,  E.xtensively  Revised.  8vo,  about 
600  pages,  ito  figures  and  6  folding  plates,  cloth, 
^5.00. 

Engineers'  Surveying  Instruments. 

Their  Construction,  Adjustment,  and  Use.  Second 
Edition.  Revi'ied  and  Hreat  y  Enlarged.  i2mo,  ix  + 
391  pages,  Sd  figures,  clotli,  53.00. 


Ready  December  i,  iqoz : 
A  Treatise  on  Roads  and  Pavements. 

8vo,  600  pages,  150  figures,  cloth,  ^5  00. 


A  TREATISE 


OK 


MASONRY  CONSTRUCTION. 


IRA   C,  BAKEE,  C.  E,, 

PROFESSOR  OF   CIVIL   EN-GIXEERING,  UNIVERSITY  OF  ILLINOIS, 


^IXTH  EDITION,   REVISED  AND  PARTIALLY  REWRITTEN, 

FIFTH    THOUSAifD 


NEW    YORK : 

JOHN    WILEY    &    SONS. 

London;    CHAPMAN    &    HALL,    Limited. 

1902. 


Copyright,  !389,  189' 

BY 

IRA  O.   BAKER. 


ROBERT    DRUMMOND,    PRINTER,    NEW    YORK. 


Urban  PianiUni 


l^no 


PREFACE.       i>nt 


The  present  volume  is  an  outgrowth  of  the  needs  of  the  author's 
own  class-room.  The  matter  is  essentially  that  presented  to  his 
classes  for  a  number  of  years  past,  a  considerable  part  having  been 
used  in  the  form  of  a  blue-print  manuscript  text-book.  It  is  now 
published  for  the  greater  convenience  of  his  own  students,  and  with 
the  hope  that  it  may  be  useful  to  others.  The  author  knows  of  no 
work  which  treats  of  any  considerable  part  of  the  field  covered  by 
this  volume.    Nearly  all  of  the  matter  is  believed  to  be  entirely  new. 

The  object  has  been  to  develop  principles  and  methods  and  to 
give  such  examples  as  illustrate  them,  rather  than  to  accumulate 
details  or  to  describe  individual  structures.  The  underlying  prin- 
ciples of  ordinary  practice  are  explained  ;  and,  where  needed,  ways 
are  pointed  out  whereby  it  may  be  improved.  The  common  theo- 
ries are  compared  with  the  results  of  actual  practice  ;  and  only 
those  are  recommended  which  have  been  verified  by  experiments 
or  experience,  since  true  theory  and  good  practice  are  always 
in  accord.  The  author  has  had  the  benefit  of  suggestions  and 
advice  from  practical  masons  and  engineers,  and  believes  that  the 
information  here  presented  is  reliable,  and  that  the  examples  cited 
represent  good  practice.  The  general  prices  are  the  average  of  a 
large  number  actually  paid  ;  and  the  special  prices  are  representa- 
tive. The  structures  illustrated  are  actual  ones.  The  accredited 
illustrations  are  from  well-autlienticated  copies  of  working  drawings, 
and  are  presented  without  any  modification  whatever  ;  while  those 
not  accredited  are  representative  of  practice  so  common  that  a  single 
name  could  not  properly  be  attached. 

In  the  preparation  of  the  book  the  endeavor  has  been  to  observe 
a  logical  order  and  a  due  proportion  between  different  parts.  Great 
care  has  been  taken  in  classifying  and  arranging  the  matter.  It 
will  be  helpful  to  the  reader  to  notice  that  tlie  volume  is  divided 
successively  into  parts,  chapters,  articles,  sections  having  small-cap- 
ital black-face  side-heads,  sections  having  lower-case  black-face  side- 
heads,  sections  having  lower-case  italic  side-heads,  and  sections  hav- 
ing simply  the  serial  number.     In  some  cases  the  major  subdivis* 


IV  PREFACE. 

ions  of  the  sections  are  indicated  by  small  numerals.  The  constant 
aim  has  been  to  present  tiie  subject  clearly  and  concisely. 

Every  precaution  has  been  taken  to  present  the  work  in  a  form 
for  convenient  practical  use  and  ready  reference.  Numerous  cross 
references  are  given  by  section  number  ;  and  whenever  a  figure  or  a 
table  is  mentioned,  the  citation  is  accompanied  by  the  number  of 
the  page  on  which  it  may  be  found.  The  table  of  contents  shows 
the  general  scope  of  the  book  ;  the  running  title  assists  in  finding 
the  different  parts ;  and  a  very  full  index  makes  everything  in  the 
book  easy  of  access.  There  are  also  a  number  of  helps  for  the 
student,  which  the  experienced  teacher  will  not  fail  to  recognize 
and  appreciate. 

Although  the  book  has  been  specially  arranged  for  engineering 
and  architectural  students,  it  is  hoped  that  the  information  con- 
cerning the  strengths  of  the  materials,  the  data  for  facilitating  the 
making  of  estimates,  the  plans,  the  tables  of  dimensions,  and  the 
costs  of  actual  structures,  will  prove  useful  to  the  man  of  experience. 
Considering  the  large  amount  of  practical  details  presented  and 
the  great  difference  in  the  methods  employed  by  various  construc- 
tors, it  is  probable  that  practical  men  will  find  much  to  criticise 
The  views  here  expressed  are,  however,  the  results  of  observation 
throughout  the  entire  country,  and  of  consultation  and  corresj^ond- 
ence  with  many  prominent  and  practical  men,  and  represent  average 
good  practice.  The  experienced  engineer  may  possibly  also  feel 
that  some  subjects  should  have  been  treated  more  fully  ;  but  it  is 
neither  wise  nor  possible  to  give  in  a  single  volume  minute  details. 
These  belong  to  technical  journals,  proceedings  of  societies,  and 
special  reports  of  particular  work. 

No  pains  have  been  spared  in  verifying  data  and  checking  re- 
sults. The  tables  of  cubic  contents  have  been  computed  by  differ- 
ent processes  by  at  least  two  persons,  and  to  at  least  one  more  place 
than  is  recorded.  Should  any  error,  either  of  printer  or  author, 
be  discovered — as  is  very  possible  in  a  work  of  so  much  detail, 
despite  the  great  care  used, — the  writer  will  be  greatly  obliged  by 
prompt  notification  of  the  same. 

The  author  gi-atefully  acknowledges  his  indebtedness  to  many 
engineers  for  advice  and  data,  and  to  his  former  pupil  and  present 
co-laborer.  Prof.  A.  N.  Talbot,  for  many  valuable  suggestions. 

Champaign,  III.,  July  9,  1889. 


PREFACE  FOR  NINTH  EDITION. 


The  order  of  the  subdivisions  of  Art.  2,  Chap.  I,  has  been 
changed,  and  pages  7-11  have  been  rewritten.  Chapter  III — Cemenfc 
and  Lime — and  Chapter  lY — Mortar  and  Concrete — have  been 
entirely  rewritten.  Chapter  IIIa — Sand,  Gravel,  and-  Broken 
Stone — has  been  added.  The  Definitions  of  Kinds  of  Masonry — 
pages  136  and  137 — have  been  rewritten  and  new  illustrations  have 
been  prepared.  The  specifications  for  the  different  classes  of  stone 
masonr}^ — pages  142,  144,  and  147 — have  been  rewritten.  Many 
minor  changes  have  been  made  in  various  parts  of  the  book. 

Champaign,  III.,  June  27,  1899. 


TABLE  OF  CONTENTS. 


PART  I.     THE    MATERIALS. 

PAGE 

CHAPTER  I.     NATURAL  STONE. 

Introduction 1 

Aet.  1.     Requisites  for  Good  Building  Stone.  ........      3 

Art.  2.     Testing  Building  Stone 5 

Weight ,     .       6 

Hardness  and  Toughness 7 

Strength.     Crushing  Strength.     Transverse  Strength.     Elasticity.  .       8 
Durability.     Destructive  Agents  :  mechanical,  chemical.     Resisting 
Agents  :  chemical  composition,  physical  structure,  seasoning.     Meth- 
ods of  Testing  Durability  :  absorptive  power,  methods  and   results ; 
effect  of  frost,  methods  and  results ;  effect  of  atmosphere,  methods 

and  results.     Methods  of  Preserving 14 

Art.  3.     Classification  and  Description  of  Building  Stones.  .     .     23 
Classification  :  geological,  chemical,  physical.    Description  of  Trap, 
Granite,  Marble,  Limestone,  and  Sandstone.     Location  of  Quarries. 
Weight  of  Stone. 

CHAPTER  II.     BRICK. 

Process  of  manufacture.  Classification.  Requisites  for  good  Brick. 
Methods  of  Testing  :  absorbing  power,  transverse  strength,  crushing 
strength ;  results.     Size.     Cost 33 

CHAPTER  III.     LIME  AND  CEMENT. 

Classification ,     .     .    48 

Art.  1.     Common  Lime 4^ 

Methods  of  manufacturing,  testing,  and  preserving.     Cost. 

Art.  2.     Hydraulic  Lime ,     .     51 

Art.  3.     Hydraulic  Cement ,     ,     51 

Description  :  Portland,  Natural,  Pozzuolana.     Weight.     Cost. 

Art.  4.     Methods  of  Testing  Hydraulic  Cement 56 

Color,  Thoroughness  of  Burning,  Activity,  Soundness,  Fineness, 
Strength. 

vii 


vni  TABLE    OF   CONTENTS. 


PAG»; 

Art.  5.     Specifications  for  Cement (57 

Quality :  Germau,  English,  FrcDch,  American,   Philadelphia.    De- 
livery aud  Storage. 

CHAPTER  IIIa.     SAND,   GRAVEL.   AND   BROKEN  STONE. 

Art.  1.     Sand 79a 

Requisites  for  Good  Sand  :  Durability,  Sharpness,  Cleanness,  Fine- 
ness, Voids.     Stone  Screenings.     Cost.     Weight. 

Art.  2.     Gravel  and  Broken  Stone 79A 

Gravel.     Broken  Stone.     Voids.     Weight.     Cost. 


PAET  II.    PREPARING  AND  USING  THE  MATERIALS. 

CHAPTER  IV.    MORTAR,  CONCRETE,  AND  ARTIFICIAL  STONE. 

Art.  1    Mortar 81 

Lime  Mortar.  Cement  Mortar  :  proportions  and  preparation. 
Data  for  Estimates.  Strength  :  tensile,  compressive,  adhesive.  Cost. 
Effect  of  Re-tempering.  Lime  vrith  Cement.  Mortar  Impervious  to 
Water.     Effect  of  Freezing. 

Art.  3.     Concrete 106 

Mortar.  Aggregate.  Proportions  :  theory,  determination,  data 
for  estimates      Mixing.     Laying.     Strength.     Cost. 

Art.  3.     Aiitificial  Stone 1136 

Portland.     McMurtrie.     Frear.     Ransome.     Sorel. 

CHAPTER  V.     QUARRYING. 

Methods  of  Quarrying  :  by  hand  tools  ;  by  explosives, — the  drills, 
the  explosives  ;  by  channeling  and  wedging 116 

CHAPTER  VI.     STONE   CUTTING. 

Art.  1.     Tools 125 

Eighteen  hand  tools  illustrated  and  described.  Machine  tools  de- 
scribed. 

Art.  2.     Methods  op  Forming  the  Surfaces 129 

Four  methods  illu.strated  and  described. 

Art.  3.     Methods  of  Finishing  the  Surfaces 131 

Eight  methods  illustrated  and  described. 

CHAPTER  VII.  STONE  MASONRY. 
Definitions  :  parts  of  the  wall,  kinds  of  masonry.  Ashlar  Masonry : 
dressing,  bond,  backing,  pointing,  mortar  required,  when  employed, 
specifications.  Squared-stone  Masonry  :  description,  mortar  required, 
specifications.  Rubble  Masonry :  description,  mortar  required,  when 
employed,  specifications.     Slope-wall  Masonry.     Stone  Paving.     Rip- 


TABLE    OF   CONTEXTS. 


PAGE 

rap.  Strength  of  Stone  Masonry  :  examples,  safe  pressure.  Meas- 
urement of  masonry.  Cost :  quarrying,  dressing,  price  of  stone  ; 
examples — U.  S.  public  buildings,  railroads,  tunnels,  bridge  piers, 
arch  culverts  ;  summary 135 

CHAPTER  VIII.  BRICK  MASONRY. 
Mortar.  Bond.  Compressive  Strength :  results  of  experiments, 
safe  pressure.  Transverse  Strength  :  strain  on  lintel.  Measurement 
of  Brick-work.  Data  for  Estimates :  brick,  labor,  mortar  required. 
Cost.  Specifications  :  for  buildings,  sewers,  arches.  Brick  vs.  Stone 
Masonry.     Brick  Masonry  Impervious  to  Water.     Efflorescence.  .     .  161 


PART   III.     FOUNDATIONS. 
CHAPTER  IX.     INTRODUCTORY. 

Definitions,  and  Plan  of  Proposed  Discussion 183 

CHAPTER  X.     ORDINARY  FOUNDATIONS. 

Outline  of  Contents 186 

Art.  1.     The  Soil 186 

Examination  of  the  Site.  Bearing  power  of  Soils  :  rock,  clay,  sand, 
semi-liquid  soils  ;  summary.  Methods  of  Improving  Bearing  Power : 
increasing  depth,  drainage,  springs,  consolidating  the  soil,  sand  piles, 
layers  of  sand. 

Art.  2.     Designing  the  Footings 199 

Load  to  be  Supported.     Area  Required.     Center  of  Pressure  and 
Center  of  Base.     Independent  Piers.     Effect  of  AVind.     Offsets  for      ' 
Masonry  Footings.     Timber  Footings.     Steel-rail  Footings.     Inverted 
Arches. 

Art.  3.     Preparing  the  Bed 213 

On  Rock.  On  Firm  Earth,  In  "Wet  Ground  :  coffer-dam,  con- 
crete, grillage. 

CHAPTER  XI.     PILE  FOUNDATIONS. 

Definitions ...  216 

Art.  1.     Descriptions,  and  Methods  of  Driving  Piles 216 

Description  :  iron  piles  ;  screw  piles  ;  disk  piles  ;  sheet  piles  ;  bear 
ing  piles, — specifications,  caps  and  shoes,  splicing.  Pile  Driving  Ma- 
chines :  drop-hammer, — friction  clutch,  nipper  ;  steam-hammer,  drop- 
hammer  vs.  steam-hammer  ;  gunpowder  pile-drivers  ;  driving  with 
dynamite  ;  driving  with  water  jet ;  jet  vs.  hammer.  Cost  of  Piles. 
Cost  of  Pile  Driving :  railroad  construction,  bridge  construction, 
[       bridge  repairs,  foundations,  harbor  and  river  work. 


TABLE    OF   CONTENTS. 


PAGW 

Art.  2.     Bearing  Power  of  Piles 233 

Methods  of  Determining  Supporting  Power.  Rational  Formula. 
Comparison  of  Empirical  Formulas:  Beaufoy's,  Nystrom's,  Mason's, 
Sander's,  Mc Alpine's,  Trautwine's,  the  Author's.  Supporting  Power 
Determined  by  Experiment :  examples,  factor  of  safety ;  supporting 
power  of  screw  and  disk  piles. 

Art.  3.     Arrangement  of  the  Foundation 250 

Position  of  Piles.  Sawing-ofE.  Finishing  Foundation  :  piles  and 
grillage,  piles  and  concrete,  lateral  yielding.  Cushing's  Pile  Founda- 
tion. 

CHAPTER  XII.     FOUNDATIONS  UNDER  WATER. 
Difficulties  to  be  0\t;rcome.     Outline  of  Contents.  .     .  257 

Art.  1.     The  Coffer-Dam  Process 258 

Construction  of  the  Dam.  Leakage,  pumps.  Preparing  the 
Foundation 

Art.  2.     The  Crib  and  Open  Caisson  Process 266 

Definitions.  Principle.  Construction  of  the  Caisson.  Construc- 
tion of  the  Crib.     Excavating  tne  Site. 

Art.  '6.     Dredging  through  Wells 271 

Principle.  Excavator.  Noted  Examples :  Poughkeepsie,  Atcha- 
falaya,  aud  Hawkesbury  bridges;  brick  cylinders.  Frictional  Resist- 
ance.    Cost. 

Art.  4.     Pneumatic  Process 278 

Vacuum  Process.  Plenum  Process.  History.  Pneumatic  Piles, 
bearing  power.  Pneumatic  Caissons  :  the  caisson,  the  crib,  the  coffer- 
dam, machinery,  air-lock.  Excavators:  sand  lift,  mud-pump,  water 
column,  blasting.  Rate  of  Sinking.  Guiding  the  Caisson.  Noted 
Examples  :  Havre  de  Grace,  Blair,  St.  Louis,  Brooklyn,  Forth  Bridges. 
Physiological  Effects  of  Compressed  Air.  Examples  of  Cost :  at 
Havre  de  Grace,  Blair,  and  Brooklyn,  and  in  Europe. 

Art.  5.     The  Freezing  Process 307 

Principle.    History.    Details  of  Process.    Examples.     Advantages. 
Cost. 
Art.  6.     C(jmparison  of  Methods 309 


PAET   IV.     MASONRY   STRUCTURES. 

CHAPTER  XIII.     MASONRY  DAMS. 

Classification  op  Dams 311 

Art.  1.     Stability  of  Gravity  Dams 312 

Principles.  ■  Stability  against  Sliding  :  destroying  forces,  resisting 
forces,  co-efficient  of  friction,  condition  of  equilibrium,  factor  of 
safety.   Stability  against  Overturning  :  by  moments, — overturning  mo- 


TABLE    OF    CONTEXTS. 


PAGE 

ment,  resisting  moment,  condition  for  equilibrium,  factor  of  safety ; 
by  resolution  of  forces..  Stability  against  Crushing  :  method  of  find- 
ing maximum  pressure,  tension  on  masonry,  limiting  pressure. 

Art.  2.     Outlines  op  the  Design 336 

"Width  on  Top.  The  Profile :  theory,  examples.  The  Plan  : 
straight  crest  vs.  straight  toe  ;  gravity  vs.  arch  dams  ;  curved  gravity 
dams.     Qualitj"  of  Masonry.     Bibliography. 

Art.  3.     Rock-Fill  Dams 334 

Wood.    Earth.     Rock-fill  and  masonry  dams  compared. 

CHAPTER  XIV.     RETAINING  WALLS. 

Definitions.     Methods  of  Failure.     Difficulties.     .......  338 

Art.  1.     Theoretical  Formulas 340 

The  Thre-j  Assumptions.  Theories  :  Coulomb's,  Weyrauch's, 
Rankine's. 

Abt.  3.     Empirical  Rules 349 

English  Rules.  American  Rules.  Details  of  Construction  :  quality 
of  masonry,  drainage,  land  ties,  relieving  arches. 

CHAPTER   XV.     BRIDGE  ABUTMENTS. 

Discussion  of  General  Forms.  Quality  of  Masonr}-.  Foundation. 
Wing  Abutment,  — design,  and  table  of  contents  of  various  sizes. 
U- Abutment, — design  and  table  of  contents  of  various  sizes.  T- Abut- 
ment,— design  and  table  of  contents  of  various  sizes 353 

CHAPTER   XVI.     BRIDGE   PIERS. 

Selection  of  Site  and  Arrangement  of  Spans.        .     .     .  366 

Art.   1.     Theory  of  Stability ,     .  367 

Methods  of  Failure.  Stability  against  Sliding :  effect  of  wind,  cur- 
rent, ice  ;  resisting  forces.  Stability  against  Overturning:  by  mo- 
ments; by  resolution  of  forces.  Stability  against  Crushing.  Example 
of  method  of  computing  stability. 

Art.  2.     Det.uls  of  Construction 377 

Dimensions  :  on  top,  at  bottom.  Batter.  Cross  Section.  Specifica- 
tions. Examples;  Cairo,  Grand  Forks,  Blair,  Henderson,  St.  Croix 
River  ;  iron  tubular  ;  -wooden  barrel.  Tables  of  Contents  of  different 
styles  aud  sizes  of  bridge  piers.     Specifications. 

CHAPTER  XVII.     CULVERTS. 

Art.  1.     Water  Way  Required 391 

The  Factors.    The  Formulas  :  Meyer's,  Talbot's.    Practical  method 
of  finding  area  of  water  way. 
Art.  2.     Box  and  Pipe  Culverts 396 

Stone  ^ox  CulveH  :    foundation,  end   walls,    cover,    specifications. 


Xii  TABLE    OF    CONTENTS. 


PAQg 

Examples :  Standard,  "West  Shore.  Canadian.     Table  of  Contents  and 
cost  of  the  various  styles  and  sizes 396 

Vitrijkd  Pipe  Culverts  :  Construction.  Example.  Table  of  Con- 
tents  407 

Iron  Pipe  Culverts  :  Construction.  Size  and  Weight  of  Pipe.  Ex- 
amples :  A.,  T.  &  S.  F.,  and  C,  B.  &  Q.  standards.  Table  of  Quan- 
tity of  Materials  Required 412 

Timber  Culvert :     C,  M.  &  St.  P.  standard  box  culverts.     C,  B.  & 

Q.  standard  barrel  culvert 417 

Art.  3.    Akch  Ctjlvekt 419 

General  Form  :  splay  of  wing  walls,  joining  wings  and  body,  seg- 
mental vs.  semi-circular.  Examples  :  diagrams  illustrating  details,  and 
also  tables  giving  dimensions,  and  contents,  and  cost,  of  all  sizes  of 
each  of  the  standard  forms  of  the  Illinois  Central.  C,  K.  &  N.,  A.,  T. 
&  S.-F.  (both  semi-circular  and  segmental),  and  a  standard  form. 
Specifications. 

CHAPTER  XVIII.     MASONRY  ARCHES. 

Definitions :  parts  and  kinds  of  arches ;  line  of  resistance 440 

Art.  1.     Theory  of  the  Masonry  Arch 444 

External  Forces.  Methods  of  Failure.  Criteria  of  Safety  :  sliding, 
rotation,  crushing, — unit  pressure,  open  joints.  Location  of  Line  of 
Resistance  :  hypothesis  of  least  pressure  ;  hypothesis  of  least  crown 
thrust,  joint  of  rupture  ;  Winkler's  hypothesis ;  Navier's  principle. 
Rational  Theory  of  the  Arch  :  symmetrical  load, — two  methods ; 
unsymmetrical  load  ;  criterion  for  line  of  resistance.  Scheflier's 
Theory :  two  examples  ;  erroneous  application  ;  reliability  of.  Ran- 
kiue's  Theory  :  curvature  of  linear  arch,  method  of  testing  stability, 
reliability.  Other  Theories.  Theory  of  the  Elastic  Arch.  Stability 
of  Abutments  and  Piers. 

Art.  2.    Rules  Derived  from  Practice 194 

Empirical  Formulas :  thickness  of  the  arch  at  the  crown, — Ameri- 
can, French,  English  practice  ;  thicknessat  the  springing, — American, 
French,  English  practice  ;  dimensions  of  abutments.  Dimensions  of 
Actual  Arches  and  Abutments.  Illustrations  of  Arches.  Minor  De- 
tails: backing,  spandrel  filling,  drainage.  Brick  Arches  ;  bond  ;  ex- 
amples,— tunnel,  Philadelphia  sewers,  Washington  sewers.  Specifica- 
tions :  stone  arches,  brick  arches. 

Art.  3.     Arch  Centers 515 

Load  to  be  supported,  method  of  computing.  Outline  forms  of 
Centers  :  solid  rib,  built  rib,  braced  wooden  rib,  trussed  frame.  Ex- 
amples: centers  for  Vosburg  tunnel,  stone  bridges,  and  Cabin  John 
Arch.     Striking  Centers :  method,  time. 


TABLE   OF    CONTENTS.  xiii 


APPENDIX   I.     SPECIFICATIONS   FOR  MASONRY. 


PAGE 


General  Railroad  Masonry 529 

Masonry  of  Railroad  Buildings 534 

Architectural  Masonry 539 


APPENDIX   11.     SUPPLEMENTARY   NOTES. 

Labor  Required  in  Quarrying 544 

Cost  of  Cutting  Granite ,     .  545 

Cost  of  Laying  Cut  Stone .  545 

Cost  of  Breaking  Stone  for  Concrete ^     .  54^ 

Cost  of  Imbedding  Large  Stones  in  Concrete 547 

Crushing  Strength  of  Sewer  Pipe 547 

Holding  Power  of  Drift-bolta 547 


MASONRY  CONSTRUCTIOK 


INTRODUCTION. 

Under  this  general  head  will  be  discussed  the  subjects  relating 
to  the  use  of  stone  and  brick  as  employed  by  the  engineer  or  archi- 
tect in  the  construction  of  buildings,  retaining  walls,  bridge  piers, 
culverts,  arches,  etc.,  including  the  foundations  for  the  same. 
For  convenience,  the  subject  will  be  divided  as  follows  : 

Part     I.  Description  and  Characteristics  of  the  Materials- 

Part    II.  Methods  of  Preparing  and  Using  the  Materials. 

Part  III.  Foundations. 

Part  I\^.  Masonry  Structures. 


"  The  first  cost  of  masonry  should  be  its  only  cost.  Though  supersti  ucturefr 
decay  and  drift  away,  though  embankments  should  crumble  aud  wash  out, 
masonry  should  stand  as  one  great  mass  of  solid  rock,  firm  and  enduring.'^ 

— Anonyiau»L8. 


F>ARX    I. 

THE  MATERIALS, 


CHAPTER  I. 
NATURAL   STONE. 

Art.  1.  Requisites  for  Good  Building  Stone. 

1.  The  qualities  which  are  most  important  in  stone  used  for 
construction  are  cheapness^,  durability,  strength,  and  beauty. 

2.  Cheapness.  The  primary  factor  which  determines  the  value 
of  a  stone  for  structural  purposes  is  its  cheapness.  The  items  which 
contribute  to  the  cheapness  of  a  stone  are  abundance,  proximity  of 
quarries  to  place  of  use,  facility  of  transportation,  and  the  ease  with 
which  it  is  quarried  and  worked. 

The  wide  distribution  and  the  great  variety  of  good  building 
stone  in  this  country  are  such  that  suitable  stone  should  everywhere 
be  cheap.  That  such  is  not  the  case  is  probably  due  either  to  a 
lack  of  the  developmcT^t  of  home  resources  or  to  a  lack  of  confidence 
in  home  products.  Tlie  several  State  and  Government  geological 
surveys  have  done  much  to  increase  our  knowledge  of  the  building 
stones  of  this  country. 

The  lack  of  confidence  in  home  resources  has  very  frequently 
caused  stones  of  demonstrated  good  quality  to  be  carried  far  and 
wide,  and  frequently  to  be  laid  down  upon  the  outcropping  ledges 
of  material  in  every  way  their  equal.  The  first  stone  house  erected 
in  San  Francisco,  for  example,  was  built  of  stone  brought  from 
China  ;  and  at  the  present  day  the  granites  mostly  employed  there 
are  brought  from  New  England  or  from  Scotland.  Yet  there  are 
no  stones  in  our  country  more  to  be  recommended  than  the  Califor- 
nia granites.  Some  of  tlie  prominent  public  and  private  buildings 
in  Cuicinnati  are  constructed  of  stone  that  was  carried  by  water  and 

3 


NATURAL  STONE.  [CHAP.   I. 


railway  a  distance  of  about  1500  miles.  Within  150  miles  of  Cin- 
cinnati, in  the  sub-carboniferous  limestone  district  of  Kentucky, 
there  are  very  extensive  deposits  of  dolomitic  limestone  that  afford 
a  beautiful  building  stone,  which  can  be  quarried  at  no  more  ex- 
pense than  that  of  the  granite  of  Maine.  Moreover,  this  dolomite 
is  easily  carved,  and  requires  not  more  than  one  third  the  labor  to 
give  it  a  surface  that  is  needed  by  granite.  Experience  has  shown 
that  the  endurance  of  this  stone  under  the  influence  of  weather  is 
very  great ;  yet  because  it  has  lacked  authoritative  indorsement 
there  has  been  little  market  for  it,  and  lack  of  confidence  in  it  has 
led  to  the  transportation  half-way  across  the  continent  of  a  stone 
little,  if  any,  superior  to  it. 

Development  of  local  resources  follows  in  the  wake  of  good  in- 
formation concerning  them,  for  the  lack  of  confidence  in  home  prod- 
ucts can  not  be  attributed  to  prejudice. 

The  facility  with  which  a  stone  may  be  quarried  and  worked  is 
an  element  affecting  cheapness.  To  be  cheaply  worked,  a  stone 
must  not  only  be  as  soft  as  durability  will  allow,  but  it  should  have 
no  flaws,  knots,  or  hard  crystals. 

3.  Dtjeability.  Next  in  importance  after  cheapness  is  dura- 
bility. Rock  is  supposed  to  be  the  type  of  all  that  is  unchangeable 
and  lasting ;  but  the  truth  is  that,  unless  a  stone  is  suited  to  the 
conditions  in  which  it  is  placed,  there  are  few  substances  more  liable 
to  decay  and  utter  failure.  The  durability  of  stone  is  a  subject 
upon  which  there  is  very  little  reliable  knowledge.  The  question 
of  endurance  under  the  action  of  weather  and  other  forces  can  not 
be  readily  determined.  The  external  aspect  of  the  stone  may  fail 
to  give  any  clue  to  it ;  nor  can  all  the  tests  we  yet  know  determine 
to  a  certainty,  in  the  laboratory,  just  how  a  given  rock  will  with- 
etand  the  effect  of  our  variable  climate  and  the  gases  of  our  cities. 
If  our  land  were  what  is  known  as  a  rainless  country,  and  if  the 
temperature  were  uniform  throughout  the  year,  the  selection  of  a 
durable  building  stone  would  be  much  simplified.  The  cities  o'' 
northern  Europe  are  full  of  failures  in  the  stones  of  importanl 
structures.  The  most  costly  building  erected  in  modern  times,  per* 
haps  the  most  costly  edifice  reared  since  the  Great  Pyramid, — the 
Parliament  House  in  London, — was  built  of  a  stone  taken  on  thfc 
recommendation  of  a  committee  representing  the  best  scientific  and 
technical  skill  of  Great  Britain.     The  stone  selected  was  submitted 


ART.    2.  J  TESTS   OF  BvJILDilfG   STONES.  5 

to  various  tests,  but  the  corroding  influence  of  a  London  atmc^phere 
was  overlooked.  The  great  structure  was  built,  and  now  it  seems 
questionable  whether  it  can  be  made  to  endure  as  long  as  a  timber 
building  would  stand,  so  great  is  the  efEect  of  the  gases  of  the 
atmosphere  upon  the  stone.  This  is  only  one  of  the  numerous  in- 
stances that  might  be  cited  in  which  a  neglect  to  consider  the 
climatic  conditions  of  a  particular  locality  in  selecting  a  building 
material  has  proved  disastrous. 

"  The  great  difference  which  may  exist  in  the  durability  of  stones 
of  the  same  kind,  presenting  little  difference  in  appearance,  is 
strikingly  exemplified  at  Oxford,  England,  where  Christ  Church 
Cathedral,  built  in  the  twelfth  or  thirteenth  century  of  oolite  from 
a  quarry  about  fifteen  miles  away,  is  in  good  preservation,  while 
many  colleges  only  two  or  three  centuries  old,  built  also  of  oolite 
from  a  quarry  in  the  neighborhood  of  Oxford,  are  rapidly  crumbling 
to  pieces. "  * 

4.  Strength.  The  strength  of  stone  is  in  some  instances  a 
cardinal  quality,  as  when  it  is  to  form  piers  or  columns  to  support 
great  weights,  or  capstones  that  span  considerable  intervals.  It  is 
also  an  indispensable  attribute  of  stone  that  is  ';o  be  exposed  to 
mechanical  violence  or  unusual  wear,  as  in  steps,  Imtels,  door- jambs, 
e^e. 

5.  Beauty.  This  element  is  of  more  importance  to  the  archi- 
tect than  to  the  engineer  ;  and  yet  the  latter  can  not  afford  to 
neglect  entirely  the  element  of  beauty  in  the  design  of  his  most 
utilitarian  structures.  The  stone  should  have  a  durable  and  pleas- 
ing color. 

Art.  2.  Tests  of  the  Quality  of  Building  Stones. 

6.  As  a  general  rule,  the  densest,  hardest,  and  most  uniform 
stone  will  most  nearly  meet  the  preceding  requisites  for  a  good 
building  stone.  The  fitness  of  stone  for  structural  purposes  can  ba 
determined  approximately  by  examining  a  fresh  fracture.  It  should 
be  bright,  clean,  and  sharp,  without  loose  grains,  and  free  from  any 
dull,  earthy  appearance.  The  stone  should  contain  no  "drys,"«.e., 
seams  containing  material  not  thoroughly  cemented  together,  nor 
"crow-foots,'*  i.e.,  veins  containing  dark-colored,  uncemented 
material. 

*  Rankine's  Civil  Engineering,  p.  362. 


NATURAL    STONE.  [CHAP.  I. 


The  more  formal  tests  employed  to  determine  the  qualities  of  a 
building  stone  are:  (1)  weight  or  density,  (2)  hardness  and  tough- 
ness, (3)  strength,  (4)  durability. 

1.  Weight  of  Stone. 

7.  Weight  or  density  is  an  important  property,  since  upon  it 
depends  to  a  large  extent  the  strength  and  durability  of  the  stone. 

If  it  is  desired  to  find  the  exact  weight  per  cubic  foot  of  a  given 
stone,  it  is  generally  easier  to  find  its  specific  gravity  first,  and  then 
multiply  by  62.4, — weight,  in  pounds,  of  a  cubic  foot  of  water. 
This  method  obviates,  on  the  one  hand,  the  expense  of  dressing  a 
sample  to  regular  dimensions,  or,  on  the  other,  hand,  the  in- 
accuracy of  measuring  a  rough,  irregular  piece.  Notice,  however, 
that  this  method  determine^  the  weight  of  a  cubic  foot  of  the  solid 
stone,  which  will  be  more  than  the  weight  of  a  cubic  foot  of  the 
material  as  used  for  structural  purposes.  In  finding  the  specific 
gravity  there  is  some  difficulty  in  getting  the  correct  disj^lacement 
of  porous  stones, — and  all  stones  are  more  or  less  porous.  There 
are  various  methods  of  overcoming  this  difficulty,  which  give 
slightly  different  results.  The  following  method,  recommended  by 
General  Gillmore,  is  most  frequently  used: 

All  loose  grains  and  sharp  corners  having  been  removed  from 
the  sample  and  its  weight  taken,  it  is  immersed  in  water  and 
weighed  there  after  all  bubbling  has  ceased.  It  is  then  taken  out 
of  the  water,  and,  after  being  compressed  lightly  in  bibulous  paper 
to  absorb  the  water  on  its  surface,  is  weighed  again.  The  specific 
gravity  is  found  by  dividing  the  weight  of  the  dry  stone  by  the 
difference  between  the  weight  of  the  saturated  stone  in  air  and  in 
water.     Or  expressing  this  in  a  formula, 

W„ 
Specific  gravity  = 


w;-  w/ 


in  which  IF„  represents  the  weight  of  dry  stone  in  air,  W^  the 
weight  of  saturated  stone  in  air,  IF,-  the  weight  of  stone  immersed 
in  water. 

The  following  table  contains  the  weight  of  the  stones  most  fre- 
quently met  with. 


ART.  2.] 


TESTS   OF    BUILDING    STONES. 


TABLE   1. 
Weight  of  Building  Stones. 


Kind  of  Stonb. 


Pounds  per  Cubic  Foot. 


Min. 


Max. 


Mean. 


Granites . . 
Limestones 
Marbles. . . 
Sandstones 
Slates 


161 
146 
157 
127 
160 


178 
174 
180 
151 
175 


167 
158 
170 
139 

174 


2.  Hardness  and  Toughness. 

8.  The  apparent  hardness  of  a  stone  depends  upon  (1)  the 
hardness  of  its  component  minerals  and  (2)  their  state  of  aggrega- 
tion. The  hardness  of  the  component  minerals  is  determined  by 
the  resistance  they  offer  to  being  scratched;  and  varies  from  that 
of  talc  which  can  easily  be  scratched  with  the  thnmb-nail,  to  that 
of  quartz  which  scratches  glass.  But  however  hard  the  mineral 
constituents  of  a  stone  are,  the  apparent  hardness  of  the  stone  itself 
depends  upon  the  state  of  aggregation  of  the  particles.  Many 
rocks  composed  of  hard  materials  work  readily,  because  their  grains 
are  loosely  coherent ;  while  others  composed  of  softer  materials  are 
quite  tough  and  difficult  to  work,  owing  to  the  tenacity  with  which 
the  particles  adhere  to  each  other.  Obviously  a  stone  in  which  the 
grains  adhere  closely  and  strongly  one  to  another  will  be  stronger 
and  more  durable  than  one  which  is  loose  textured  and  friable. 

The  toughness  of  a  stone  depends  upon  the  force  with  which  the 
particles  of  the  component  minerals  are  held  together. 

Both  hardness  and  toughness  should  exist  in  a  stone  used  for 
stoops,  pavements,  road-metal,  the  facing  of  piers,  etc.  No  experi- 
ments have  been  made  in  this  country  to  test  the  resisting  power  of 
stone  when  exposed  to  the  different  kinds  of  service.  A  table  of  the 
resistance  of  stones  to  abrasion  is  often  quoted,*  but  as  it  contains 
only  foreign  stones,  which  are  described  by  local  names,  it  is  not  of 
much  value. 


*  For  example,  Mahan's  Civil  Engineering,  p.  13. 


NATURAL    STONE.  [CHAP.  I. 


3.  Strength. 

Under  this  head  will  be  included  (1)  crnshing  or  compressive 
strength,  (2)  transverse  strength,  (3)  elasticity.  Usually,  when 
simply  the  strength  is  referred  to,  the  crushing  strength  is  intended. 

9.  Crushing  Strength.  The  crushing  strength  of  a  stone  is 
tested  by  applying  measured  force  to  cubes  until  they  are  crushed. 
The  results  for  the  crushing  strength  vary  greatly  with  the  details 
of  the  experiments.  Several  points,  which  should  not  be  neglected 
either  in  planning  a  series  of  experiments  or  in  using  the  results 
obtained  by  experiment,  will  be  taken  up  separately,  although  they 
are  not  entirely  independent. 

10.  Form  of  Test  Specimen.  Experiments  show  that  all  brittle 
materials  when  subjected  to  a  compressive  load  fail  by  shearing  on 
certain  definite  angles.  For  brick  or  stone,  the  plane  of  rupture 
makes  an  angle  of  about  60°  with  the  direction  of  the  compressing 
force.  For  this  reason,  the  theoretically  best  form  of  test  specimen 
would  be  a  prism  having  a  height  of  about  one  and  a  half  times  the 
least  lateral  dimension.  The  result  is  not  materially  different  if 
the  height  is  three  or  four  times  the  least  lateral  dimension.  But 
if  the  test  specimen  is  broader  than  high,  the  material  is  not  free 
to  fail  along  the  above  plane  of  rupture,  and  consequently  the 
strength  per  unit  of  bed-area  is  greater  than, when  the  height  is 
greater  than  the  breadth. 

However,  notwithstanding  the  fact  that  theoretically  the  test 
specimen  should  be  higher  than  broad,  it  is  quite  the  universal 
custom  to  determine  the  crushing  strength  of  stone  by  testing 
cubes. 

11.  Size  of  the  Cube.  Although  the  cube  is  the  form  of  test 
specimen  generally  adopted,  there  is  not  equal  unanimity  as  to  the 
size  of  the  cube;  but  it  is  conclusively  proven  that  the  strength  per 
square  inch  of  bed-area  is  independent  of  the  size  of  the  cube,  and 
therefore  the  size  of  the  test  specimen  is  immaterial. 

General  Gillmore,  in  1875,  made  two  sets  of  experiments  which 
seems  to  prove  that  the  relation  between  the  crushing  strength  and 
the  size  of  the  cube  can  be  expressed  by  the  formula 

?/  =  a  Vx, 

in  which  y  is  the  total  crnshing  pressure  in  pounds  per  square  inch 


AET.  2.]  STRENGTH    OF    BUILDING    STOKES.  9 

of  bed-area,  a  is  the  crushing  pressure  of  a  1-inch  cube  of  the  same 
material,  and  x  is  the  length  in  inches  of  an  edge  of  the  cube  under 
trial.  For  two  samples  of  Berea  (Ohio)  sandstone,  a  was  7000  and 
9500  lbs.,  respectively.* 

Eesults  by  other  observers  with  better  machines,  particularly  by 
General  Gillmore  f  with  the  large  and  accurate  testing-machine  at 
Watertown  (Mass.)  Arsenal,  I  uniformly  show  this  supposed  law  to 
be  without  any  foundation.  Unfortunately  the  above  relation 
between  strength  and  bed-area  is  frequently  quoted,  and  has  found 
a  wide  acceptance  among  engineers  and  architects. 

Two  inches  is  the  most  common  size  of  the  cube  for  compression 
tests. 

12.  Cushions.  Homogeneous  stones  in  small  cubes  appear  in 
all  cases  to  break  as  shown  in  Fig.  1.  The 
forms  of  the  fragments  a  and  b  are,  approxi- 
mately, either  conical  or  pyramidal.  The 
more  or  less  disk-shaped  pieces  c  and  d  are 
detached  from  the  sides  of  the  cube  with  a 
kind  of  explosion.  In  the  angles  e  and  /,  the 
stone  is  generally  found  crushed  and  ground 
into  powder.  This  general  form  of  breakage 
occurs  also  in  non-homogeneous  stones  when  ^iq.  i. 

crushed  on  their  beds,  but  in  this  case  the  modification  which  the 
grain  of  the  stone  produces  must  be  taken  into  account. 

The  nature  of  the  material  in  contact  with  the  stone  while 
nnder  pressure  is  a  matter  of  great  moment.  If  the  materials  which 
press  upon  the  top  and  bottom  of  the  specimen  are  soft  and  yielding 
and  press  out  sidewise,  they  introduce  horizontal  forces  which 
materially  diminish  the  apparent  crushing  strength  of  the  stone. 
If  the  pressing  surfaces  are  hard  and  unyielding,  the  resistance  of 
these  surfaces  adds  considerable  to  the  apparent  strength. 

*  Report  on  Strength  of  Building  Stone,  Appendix,  Report  of  Chief  of  Engineers 
of  U.  S.  A.  for  1875. 

t  Notes  on  the  Compressive  Resistance  of  Freestone,  Brick  Piers,  Hydraulic 
Cements,  Mortars,  and  Concrete,  Q.  A.  GiUmore.  John  Wiley  &  Sons,  New  York, 
1888. 

X  Report  on  the  "  Tests  of  Metals,"  etc.,  for  the  year  ending  June  30,  1884,  pp. 
126,  166,  167,  197,  212,  213,  215 ;  the  same  being  Sen.  Ex.  Doc.  No.  35,  49th  Cong., 
1st  Session.  For  a  discussion  of  these  data  by  the  author,  see  Engineering  News, 
vol.  lix.  pp.  511-512. 


10  NATURAL    STONE.  [CHAP.  I. 

Formerly  steel,  wood,  lead,  and  leather  were  much  used  as 
pressing  surfaces.  Under  certain  limitations,  the  relative  crushing 
strengths  of  stones  with  these  different  pressing  surfaces  are  100, 
89,  65,  and  62  respectively.* 

Tests  of  the  strength  of  blocks  of  stone  are  useful  only  in  com- 
paring different  stones,  and  give  no  idea  of  the  strength  of  struct- 
ures built  of  such  stone  (see  §  246)  or  of  the  crushing  strength  of 
stone  in  large  masses  in  its  native  bed  (see  §  273). 

Then,  since  it  is  not  possible  to  have  the  stone  under  the  same 
conditions  while  being  tested,  that  it  is  in  the  actual  structure,  it  is 
best  to  test  the  stone  under  conditions  that  can  be  accurately 
described  and  readily  duplicated.  Therefore  it  is  rapidly  coming  to 
be  the  custom  to  test^  the  stone  between  metal  pressing  surfaces. 
Under  these  conditions  the  strength  of  the  specimen  will  vary  greatly 
with  the  degree  of  smoothness  of  its  bed-surfaces.  Hence,  to  obtain 
definite  and  precise  results,  these  surfaces  should  be  rubbed  or 
ground  perfectly  smooth ;  but  as  this  is  tedious  and  expensive,  it  is 
quite  common  to  reduce  the  bed-surfaces  to  planes  by  plastering 
them  with  a  thin  coat  of  plaster  of  Paris.  With  the  stronger 
stones,  specimens  with  jjlastered  bed  will  show  less  strength  than 
those  having  rubbed  beds,  and  this  difference  will  vary  also  with 
the  length  of  time  the  plaster  is  allowed  to  harden.  With  a 
stone  having  a  strength  of  5,000  to  6,000  pounds  per  square  inch, 
allowing  the  plaster  to  attain  its  maximum  strength,  this  differ- 
ence varied  from  5  to  20  per  cent.,  the  mean  for  ten  trials  being 
almost  10  per  cent,  of  the  strength  of  the  specimen  with  rubbed 
beds. 

13.  Dressing  the  Cube.  It  is  well  known  that  even  large 
stones  can  be  broken  by  striking  a  number  of  comparatively  light 
blows  along  any  particular  line ;  in  which  case  the  force  of  the  blows 
gradually  weakens  the  cohesion  of  the  particles.  This  principle  finds 
application  in  the  preparation  of  test  specimens  of  stone.  If  the 
specimen  is  dressed  by  hand,  the  concussion  of  the  tool  greatly 
affects  its  internal  conditions,  particularly  with  test  specimens  of 
small  dimensions.  With  2-inch  cubes,  the  tool-dressed  specimen 
usually  shows  only  about  60  per  cent,  of  the  strength  of  the  sawed 


*  Report  on  Building  Stones,  in  Report  of  Chief  of  Engineers,  U.  S.  A.,  1875,  App. 
II. ;  also  bound  separately,  page  29. 


ART.  2.] 


STRENGTH    OF    BUILDING    STONES. 


11 


sample.  The  sawed  sample  most  nearly  represents  the  conditions 
of  actual  practice. 

Unfortunately,  experimenters  seldom  state  whether  the 
specimens  were  tool-dressed  or  sawed.  The  disintegrating 
effect  of  the  tool  in  dressing  is  greater  with  small  than  with 
large  specimens.  This  may  account  for  the  difference  in 
strength  of  different  sizes  of  test  specimen  as  seems  to  be 
shown   by   some    experiments. 

All  stones  are  strongest  when  laid  on  their  natural  bed, 
i.e.,  when  the  pressure  is  perpendicular  to  the  stratification ; 
and  with  sedimentary  rocks  there  is  a  very  great  difference 
in  the  two  positions.  Hence,  in  preparing  the  test  specimen 
the  natural  bed  should  be  marked,  and  the  cube  should  be 
tested  upon  its  native   bed. 

14.  Data  on  Crushing  Strength.  The  strength  of  the  principal 
classes  of  building  stone  in  use  in  the  United  States  is  about  as 
follows  : 

TABLE  2. 
CBUsmNG  Strength  of  Cubes  of  Stone. 


Ultimate  Crushing  Strength. 

Kinds  of  Stone. 

Pounds  per  Square  Inch. 

Tons  per  Square  Foot. 

Min. 

Max. 

Min. 

Max. 

Trap  Rocks  of  N.  J 

Granite 

20,000 

12,000 

8,000 

7,000 

5,000 

24,000 
21,000 
20,000 
20,000 
15,000 

1,440 
860 
580 
500 
360 

1.730 
1,510 
1,440 
1,440 
1,080 

Marble 

Limestorie 

Sandstbne 

15.  Crushing  Strength  of  Slabs.  Oniy  a  few  experiments  have 
been  made  to  determine  the  crushing  strength  of  slabs  of  stone. 
The  strength  per  square  inch  of  bed-surface  was  considerably 
greater  than  that  for  cubes ;  but  a  study  of  the  results  of  all  of  the 
reliable  experiments  *  fails  to  discover  any  simple  relation  between 

*See  Report  on  "Tests  of  Metals,  etc.,"  for  1884.— Sen.  Ex.  Doo.  No.  35,  49th 
Cong.,  1st  Sesoion,— pp.  126  and  212. 


12  NATURAL    STONE.  [CHAP.    I. 


the  crushing  strength  of  cubes  and  slabs.  It  is  probable  that  the 
effect  of  the  pressing  surface  is  so  great  as  to  completely  mask  the 
variation  due  to  height  of  specimen.  More  experiments  on  this 
subject  are  very  much  needed. 

16.  Transverse  Strength.  When  stones  are  used  for  lintels, 
etc.,  their  transverse  strength  becomes  important.  The  ability  of  a 
stone  to  resist  as  a  beam  depends  upon  its  tensile  strength,  since 
that  is  always  much  less  than  its  compressive  strength.  A  knowl- 
edge of  the  relative  tensile  and  compressive  strength  of  stones  is 
valuable  in  interpreting  the  effect  of  different  pressing  surfaces  in 
compressive  tests,  and  also  in  determining  the  thickness  required 
for  lintels,  sidewalks,  cover-stones  for  box  culverts,  thickness  of 
footing  courses,  etc. 

Owing  to  the  small  cross  section  of  the  specimen  employed  in 
determining  the  transverse  strength  of  stones, — usually  a  bar  1  inch 
square, — the  manner  of  dressing  the  sample  affects  the  apparent 
transverse  strength  to  a  greater  degree  than  the  compressive  strength 
(see  §  13) ;  and  it  is  even  more  unfortunate,  since  the  strength  of 
the  stone  as  used  in  actual  practice  is  nearly  proportional  to  the 
strength  of  sawed  samples. 

The  following  formulas  are  useful  in  computing  the  breaking 
load  of  a  slab  of  stone.  Let  W  represent  the  concentrated  center 
load  phis  half  of  the  weight  of  the  beam  itself,  in  pounds ;  and  let 
b,  d,  and  I  represent  the  breadth,  depth,  and  length,  in  inches, 
respectively.  Let  R  =  the  modulus  of  rupture,  in  lbs.  per  sq.  in. ; 
let  C  =  the  weight,  in  pounds,  required  to  break  a  bar  1  inch 
square  and  1  foot  long  between  bearings  ;  and  let  L  =  the  length 
of  the  beam  in  feet.     Then 

The  equivalent  uniformly  distributed  weight  is  equal  to  twice  the 
concentrated  center  load.  ' 

Table  3  on  the  following  page  gives  the  values  of  R,  the  mod- 
ulus of  rupture,  and  of  C,  the  co-efficient  of  transverse  strength, 
required  in  the  above  formulas. 

Example, — To  illustrate  the  method  of  using  the  above  formulas, 
assume  that  it  is  desired  to  know  the  breaking  load  for  a  limestone 
slab  3  inches  thick,  4  feet  wide,  and  6  feet  long.     Then  Z*  =  48 ; 


ART.    2.] 


STRENGTH    OF    BUILDING   STONES. 


13 


TABLE   3. 
Tbansverse  Strength  of  Stone,  Brick,  and  Mortar. 


MaTEKIAIj. 


,,  „  -D liCo  EFFICIENT  OF  TRANS- 

Modulus  of  Rupture.  |,       ^^^^^^  Strength. 


Max.    I    Min.    '  Aver,     i    Max. 


Min. 


Blue-stone  flagging 

Granite 

Limestone 

"      oolitic,  from  Ind.,  sawed 

Marble 

Sandstone 

Slate 

Brick  (§59) 

Concrete — see  §  157/ 

Mortar,  neat  Portland,  1  year  old. . 
Mortar,  1  part  Portland  cement,  1 

part  sand,  1  j^ear  old 

Mortar,  1  part  Portland  cement,  2 

parts  sand,  1  year  old 

Mortar,  neat  Rosendale,  1  year  old. 
Mortar,  1  part  Rosendale  cement, 

1  part  sand,  1  year  old 

Mortar,  1  part  Rosendale  cement, 

2  parts  sand,  1  year  old 


4,511 
2,700 
3,500 
2,590 
2,880 
2,340 
9,000 
1,796 


715 
690 
479 


360 

2,700 

251 

20 

900  1,800  1 

150 

50 

140  1  1,500 

140 

8 

2,190  ,  2,338 

144 

122 

144  1  2,160 

160 

8 

576  1,260 

130 

32 

1,800 

5,400 

500 

100 

269 

800 
1,158* 
945* 
682* 

100 

15 

415 

600 

39 

23 

348 

526 

38 

19 

338 

405 

26 

18 

Aver. 


150 
100 

83 
130 
120 

70 
300 

45 

64* 

52* 

38* 
33 

29 

22 


d  =  ^;  I  =  72;    R  =  1500  lbs.,— the  "average"  value   from   the 
table; — and  C  =  83.     Substituting  these  values,  we  have 


2  h  ff  2  V  48  V  Q 


GOOD  pounds; 


or,  using  the  other  form, 


W  = 


Id 


48  v  0 
(7  =       :^    83  =  5976  pounds. 


L    ^  6 

which  agrees  with  the  preceding  except  for  omitted  decimals. 
Hence  the  breaking  load  for  average  quality  of  limestone  is  6000 
pounds  concenti'ated  along  a  Ime  half-way  between  the  ends ;  the 
uniformly  distributed  load  is  twice  this,  or  12,000  pounds.  The 
*  Only  one  experiment. 


14 


Natural  stone. 


[chap.  I. 


question  of  wliat  margin  should  be  allowed  for  safety  is  one  that  can 
not  be  determined  in  the  abstract ;  it  depends  upon  the  accuracy 
with  which  the  maximum  load  is  estimated,  upon  the  manner  the 
load  is  applied — whether  with  shock  or  not, — upon  the  care  with 
which  the  stone  was  selected,  etc.  This  subject  will  be  discussed 
further  in  connection  with  the  use  of  the  data  of  the  above  table  in 
subsequent  parts  of  this  volume. 

17.  Elasticity.  But  very  few  experiments  have  been  made  to 
determine  the  co-efficient  of  elasticity,  the  elastic  limit,  and  the 
•'set''  of  stone.  Data  on  these  points  would  be  valuable  in  deter- 
mining the  effect  of  combining  masonry  and  metal,  of  joining 
different  kinds  of  masoni-y,  or  of  joining  new  masonry  to  old  ;  in 
calculating  the  effect  of  loading  a  masonry  arch  ;  in  proportioning 
abutments  and  piers  of  railroad  bridges  subject  to  shock,  etc. 
The  following  is  all  the  data  that  can  be  found: 

TABLE  4. 
Co-efficient  of  Elasticity  op  Stone,  Brick,  and  Mortak. 


Material. 


Haverstraw  Freestone  * 

Portland  Stone  (oolite  limestone)f 

Marblef 

Portland  Granite:}: 

Siatef 

Grafton  Limestone^ 

Richmond  Granite:}:  

Brick,  medium — mean  of  16  experiments* 

Louisville  Cement  Mortar,  4  months  old  :  X 

Neat  cement 

1  part  cement,  1  part  sand 

1  part  cement,  2  parts  sand 

Ulster   Co.    (N.    Y.)    Cement    Mortar,    23    months 

old:* 

2  parts  cement,  3  parts  sand 

1  part  cement,  3  parts  sand 

l*K>rtland  Cement  Mortar,  22  months  old  * 


Pounds  per  Square  Inch. 


950,000 
1,530,000 
2,500,000 
5,500,000 
7,000,000 
8,000,000 
13,000,000 
3,500,000 

800,000 

600,000 

1,300,000 


640,000 

535.000 

1,525,000 


*  U.  S.  testing-machine,  Watertown,  Mass.        +  Tredgold,  as  quoted  by  Stoney. 
X  History  of  St.  Louis  Bridge,  pp.  334-28. 


ART.    2.]  STRENGTH    OF    BUILDING    STONES.  15 

18.  Bibliographical.  A  large  number  of  tests  have  been 
ajiplied  lo  the  building  stones  of  the  United  States.  For  the 
results  and  details  of  some  of  the  more  important  of  these  tests 
see:  Keporfc  on  Strength  of  Building  Stone,  Gen.  Q.  A.  Gillmore, 
Ajjpeu.  II,  Eeport  of  Chief  of  Engineers,  U.  S.  A.,  for  1875; 
Teuth  Census  of  the  U.  S.,  Vol.  X,  Eeport  on  the  Quarry 
Industry,  pp.  330-35;  the  several  annual  reports  of  tests  made 
with  the  U.  S.  Government  testing  machine  at  the  Watertown 
(Mass.)  Arsenal,  published  by  the  U.  S.  War  Department  under 
the  title  Eeport  on  Tests  of  Metals  and  Other  Materials; 
Transactions  of  the  American  Society  of  Civil  Engineers,  Vol.  II, 
pp.  145-51  and  pp.  187-92;  Journal  of  the  Association  of  Ea- 
gineering  Societies,  Vol.  Y,  pp.  176-79,  Yol.  IX,  pp.  33-43; 
Engineering  Keivs,  Vol.  XXXI,  p.  135  (Feb.  15,  1884);  and  the 
reports  of  the  various  State  Geological  Surveys,  and  the  com- 
missioners of  the  various  State  capitols  and  of  other  public 
buildings. 

By  way  of  comparison  the  following  reports  of  tests  of  building 
stones  of  Great  Britain  may  be  interesting:  Proceedings  of  the 
Institute  of  Civil  Engineers,  Vol.  CVII  (1891-92),  pp.  341-69; 
abstract  of  the  above.  Engineering  JVeios,  Vol.  XXVIII,  pp.  279-82 
(Sept.  22,  1892). 

In  consulting  the  above  references  or  in  using  the  results,  the 
details  of  the  manner  of  making  of  the  experiments  should  be 
kept  clearly  in  mind,  particularly  the  method  of  prej^aring  and 
bedding  the  specimen. 

4.    DUEABILITY. 

19.  '*  Although  the  art  of  building  has  been  practiced  from  the 
earliest  times,  and  constant  demands  have  been  made  in  every  age 
for  the  means  of  determining  the  best  materials,  yet  the  process  of 
ascertaining  the  durability  of  stone  appears  to  have  received  but 
little  definite  scientific  attention,  and  the  processes  usually  employed 
for  solving  this  question  are  still  in  a  very  unsatisfactory  state. 
Hardly  any  department  of  technical  science  is  so  much  neglected  as 
that  which  embraces  the  study  of  the  nature  of  stone,  and  all  the 
varied  resources  of  lithology  in  chemical,  microscopical,  and  physical 
methods  of  investigation,  wonderfully  developed   within   the  last 


16  NATURAL   STONE.  [CHAP.    l. 


quarter  century,  have  never  yet  been  properly  applied  to  the  selec- 
tion and  protection  of  stone  used  for  building  purposes/'* 

Examples  of  the  rapid  decay  of  building  stones  have  already  been 
referred  to,  and  numerous  others  could  be  cited,  in  which  a  stone 
which  it  was  supposed  would  last  forever  has  already  begun  to 
decay.  In  every  way,  the  question  of  durability  is  of  more  interest 
to  the  architect  than  to  the  engineer  ;  although  it  is  of  enough 
importance  to  the  latter  to  v/arrant  a  brief  discussion  here. 

20.  Destructive  Agents.  The  destructive  agents  may  be  clas- 
sified as  mechanical,  chemical,  and  organic.  The  last  are  unim- 
portant, and  will  not  be  considered  here. 

21.  Mechanical  Agents.  For  our  climate  the  mechanical  agents 
are  the  most  efficient.  These  are  frost,  wind,  rain,  fire,  pressure, 
and  friction. 

The  action  of  frost  is  usually  one  of  the  main  causes  of  rapid 
decay.  Two  elements  are  involved, — the  friability  of  the  material 
and  its  power  of  absorbing  moisture.  In  addition  to  the  alter- 
nate freezing  and  thawing,  the  constant  variations  of  temperature 
from  day  to  day,  and  even  from  hour  to  hour,  give  rise  to  molecular 
motions  which  affect  the  durability  of  stone  as  a  building  material. 
This  effect  is  greatest  in  isolated  columns, — as  monuments,  bridge 
piers,  etc. 

The  effect  of  rain  depends  upon  the  solvent  action  of  the  gases 
which  it  contains,  and  upon  its  meclianical  effect  in  the  wear  of 
pattering  drops  and  streams  trickling  down  the  face  of  the  wall. 

A  gentle  breeze  dries  out  the  moisture  of  a  building  stone  and 
tends  to  j)reserve  it;  but  a  violent  wind  wears  it  away  by  dashing 
sand  grains,  street  dust,  ice  particles,  etc.,  against  its  face.  The 
extreme  of  such  action  is  illustrated  by  the  vast  erosion  of  the  sand- 
.stone  in  the  jilateaus  of  Colorado,  Arizona,  etc.,  into  tabular  mesasi, 
isolated  joillars,  and  grotesquely-shaped  hills,  by  the  erosive  force  of 
sand  grains  borne  by  the  winds.  The  effect  is  similar  to  that  of  the 
.  sand  blast  as  used  in  various  processes  of  manufacture..  A  violent 
wind  also  forces  the  rain-water,  with  all  the  corrosive  acids  it  con- 
. tains,  into  the  pores  of  stones,  and  carries  off  the  loosened  grains, 
*thus  keeping  a  fresh  surface  of  the  stone  exposed.  Again,  the 
gwaying  of  tall  edifices  by  the  wind  must  cause  a  continual  motion, 

*  Tenth  Census  of  the  U.  S.,  Vol.  X.,  Report  on  the  Quarry  Industry,  p.  864. 


ART.    2.]  DURABILITY    OF    BUILDING    STONES.  17 

not  only  in  the  joints  between  the  blocks,  but  among  the  grains  of 
the  stones  themselves.  Many  of  these  have  a  certain  degree  of 
flexibility,  it  is  true;  and  yet  the  play  of  the  grains  must  gradually 
increase,  and  a  tendency  to  disintegration  result. 

Experience  in  great  fires  in  the  cities  shows  that  there  is  no 
stone  which  can  withstand  the  fierce  heat  of  a  mass  of  burning 
buildings.  Sandstones  seem  to  be  the  least  affected  by  great  heat, 
and  granite  most. 

Friction  affects  sidewalks,  pavements,  etc.,  and  has  already 
been  referred  to  (§  8).  It  may  also  affect  bridge  piers,  sea-walls, 
docks,  etc. 

The  effect  of  pressure  in  destroying  stone  is  one  of  the  least 
impoitance,  provided  the  load  to  be  borne  does  not  too  nearly  equal 
the  crushing  strength.  The  pressure  to  which  stone  is  subjected 
does  not  generally  exceed  one  tenth  of  the  ultimate  strength  as 
determined  by  methods  already  described. 

22.  Chemical  Agents.  The  principal  ones  are  acids.  Every 
constituent  of  stone,  except  quartz,  is  subject  to  attack  by  acids; 
and  the  carbonates,  which  enter  as  chief  constituents  or  as  cement- 
ing materials,  yield  very  readily  to  such  action.  Oxygen  and  am- 
monia by  their  chemical  action  tend  to  destroy  stones.  In  cities  or 
manufacturing  districts  sulphur  acids  and  carbonic  acid  have  a 
very  marked  effect.  These  all  result  from  the  combustion  of  gas, 
coal,  etc.,  and  some  are  also  the  residuary  gases  of  many  kinds  of 
manufactories.  The  nitric  acid  in  the  rain  and  the  atmosphere 
exerts  a  perceptible  influence  in  destroying  building  stone. 

23.  Resisting  Agents.  The  durability  of  a  building  stone  de- 
pends upon  three  conditions;  viz.,  the  chemical  and  mineralogical 
nature  of  its  constituents,  its  physical  structure,  and  the  character 
and  position  of  its  exposed  surfaces. 

24.  Chemical  Composition.  The  chemical  composition  of  the 
principal  constituent  mineral  and  of  the  cementing  material  has  an 
important  effect  upon  the  durability  of  a  stone. 

A  siliceous  stone,  other  things  being  equal,  is  more  durable  than 
a  limestone;  but  the  durability  of  the  former  plainly  depends  upon 
the  state  of  aggregation  of  the  individual  grains  and  their  cement- 
ing bond,  as  well  as  on  the  chemical  relation  of  the  silica  to  the 
other  chemical  ingredients.  A  dolomitic  limestone  is  more  durable 
than  a  pure  limestone. 


18  NATL'EAL   STOXE,      ^  [CHAP.    I.. 

A  stone  that  absorbs  moisture  abundantly  and  rapidly  is  apt  to- 
be  injured  by  alternate  freezing  and  thawing;  hence  clayey  constit- 
uents are  injurious.  An  argillaceous  stone  is  generally  compact, 
and  often  has  no  pores  visible  to  the  eye;  yet  such  will  disintegrate 
rapidly  either  by  freezing  and  thawing,  or  by  corrosive  vapors. 

The  presence  of  calcium  carbonate,  as  in  some  forms  of  marble 
and  in  earthy  limestones,  renders  a  building  material  liable  to  rapid 
attack  by  acid  vapors.  In  some  sandstones  the  cementing  material 
is  the  hydrated  form  of  ferric  oxide,  which  is  soluble  and  easily 
removed.  Sandstones  in  Avhich  the  cementing  material  is  siliceous 
are  likely  to  be  the  most  durable,  although  they  are  not  so  easily 
w^orked  as  the  former.  A  stone  that  has  a  high  per  cent,  of  alumina 
(if  it  be  also  non-crystalline),  or  of  organic  matter,  or  of  protoxide 
of  iron,  will  usually  disintegrate  rapidly.  Such  stones  are  gen- 
erally of  a  bluish  color. 

25.  Seasoning.  The  thorough  drying  of  a  stone  before,  and  the 
preservation  of  this  dryness  after,  its  insertion  in  masonry  are  com- 
monly recognized  as  important  factors  of  its  durability;  but  the 
exact  nature  of  the  process  of  seasoning,  and  of  the  composition 
of  the  quarry-sap  removed  by  thorough  drying,  have  never  been 
determined.  The  quarry  water  may  contain  little  else  than  ordinary 
Well-water,  or  may  be  a  solution  more  or  less  nearly  saturated,  at  the 
ordinary  temperature,  with  carbonate  of  calcium,  silica,  double  salts 
of  calcium  and  magnesium,  etc.  In  the  latter  case,  hardening  re- 
sults from  the  drying,  and  an  exact  knowledge  of  its  nature  might 
throw  important  light  on  the  best  means  for  the  artificial  jJreserva- 
tion  of  stone.  Again,  water  may  exist  in  large  quantity,  in  chemical 
combination,  in  the  silicates  (e.g.,  chlorite,  kaolin,  etc.),  or  in  the 
hydrated  iron  oxides  which  constitute  the  cement  of  a  building 
stone. 

26.  Physical  Structure.  The  physical  properties  v/hich  con- 
tribute to  durability  are  hardness,  toughness,  homogeneity,  con- 
tiguity of  the  grains,  and  the  structure — whether  crystalline  or 
amorphous. 

Although  hardness  (resistance  to  crushing)  is  often  regarded  as 
the  most  important  element,  yet  resistance  to  weathering  does  not 
necessarily  depend  upon  hardness  alone,  but  upon  hardness  and  the 
non-absorbent  properties  of  the  stone.  A  hard  material  of  close 
and  firm  texture  is,  however,  in  those  qualities  at  least,  especially 


ART.    2.]  DURABILITY    OF   BUILDING    STONES.  V.* 

fitted  to  resist  friction,  as  in  stoops,  pavements,  and  road  metal,  and 
the  wear  of  rain-drops,  dripping  rain-water,  the  blows  of  the  waves, 
etc. 

Porosity  is  an  objectionable  element.  An  excessive  porosity  in- 
creases the  layer  of  decomposition  which  is  caused  by  the  acids  of 
the  atmosphere  and  of  the  rain,  and  also  deepens  the  penetration  of 
frost  and  promotes  its  work  of  disintegration. 

If  the  constituents  of  a  rock  differ  greatly  in  hardness,  texture^ 
solubility,  porosity,  etc.,  the  weathering  is  unequal,  the  surface  is 
roughened,  and  the  sensibility  of  the  stone  to  the  action  of  frost  is 
increased. 

The  principle  which  obtains  in  applying  an  artificial  cement, 
such  as  glue,  in  the  thinnest  film  in  order  to  secure  the  greatest 
binding  force  finds  its  analogy  in  the  building  stones.  The  thinner 
the  films  of  the  natural  cement  and  the  closer  the  grains  of  the  pre- 
dominant minerals,  the  stronger  and  more  durable  the  stone.  One 
source  of  weakness  in  the  famous  brown-stone  of  New  York  City 
lies  in  the  separation  of  the  rounded  grains  of  quartz  and  feldspar 
by  a  superabundance  of  ocherous  cement.  Of  course  the  further 
sejxiration  produced  by'  fissure,  looseness  of  lamination,  empty 
<3avities  and  geodes,  and  excess  of  mica  tends  to  deteriorate  still 
further  a  weak  building  stone. 

Experience  has  generally  shown  that  a  crystalline  structure  re- 
sists atmospheric  attack  better  than  an  amorphous  one.  This  prin- 
ciple has  been  abundantly  illustrated  in  the  buildings  of  New  York 
City.  The  same  fact  is  generally  true  with  the  sedimentary  rocks 
also,  a  crystalline  limestone  or  good  marble  resisting  erosion  better 
than  earthy  limestone.  A  stone  that  is  compactly  and  finely  granular 
will  exfoliate  more  easily  by  freezing  and  thawing  than  one  that  is 
•coarse-grained.  A  stone  that  is  laminar  in  structure  absorbs  mois- 
ture unequally  and  will  be  seriously  affected  by  unequal  expansion 
and  conti-action, — especially  by  freezing  and  thawing.  Such  a  stone 
will  gradually  separate  into  sheets.  A  stone  that  has  a  gninular 
texture,  as  contrasted  with  one  that  is  crystalline  or  fibrous,  will 
crumble  sooner  by  frost  and  by  chemical  agents,  because  of  the 
easy  dislodgment  of  the  individual  grains. 

The  condition  of  the  surface,  whether  rough  or  polished,  in- 
fluences the  durability, — the  smoother  surface  being  the  better. 


20  NATURAL   STONE.  [CHAP. 


The  stone  is  more  durable  if  the  exposed  surface  is  vertical  than  if 
inclined.     The  lamination  of  the  stone  should  be  horizontal. 

27.  Methods  of  Testing  Durability.  It  has  long  been  recog- 
nized that  there  are  two  ways  in  which  we  can  form  a  judgment  of 
the  durability  of  a  building  stone,  and  these  may  be  distinguished 
as  natural  and  artificial. 

28.  Natural  Methods,  These  must  always  take  the  precedence 
whenever  they  can  be  used,  because  they  involve  (1)  the  exact 
agencies  concerned  in  the  atmospheric  attack  upon  stone,  and  (2) 
long  periods  of  time  far  beyond  the  reach  of  artificial  experiment. 

One  method  is  to  visit  the  quarry  and  observe  whether  the  ledges 
that  have  been  exposed  to  the  weather  are  deeply  corroded,  or 
whether  these  old  surfaces  are  still  fresh.  In  apj^lying  this  test, 
consideration  must  be  given  to  the  modifying  effect  of  geological 
phenomena.  It  has  been  pointed  out  that  ^'tlie  length  of  time  the 
ledges  have  been  exposed,  and  the  changes  of  actions  to  which 
they  may  have  been  subjected  during  long  geological  periods,  are 
unknown;  and  since  different  quarries  may  not  have  been  exposed 
to  the  same  action,  they  do  not  always  afford  definite  data  for  re- 
liable comparative  estimates  of  durability,  except  where  different 
specimens  occur  in  the  same  quarry." 

North  of  the  glacial  limit,  all  the  products  of  decomposition 
have  been  planed  away  and  deposited  as  drift-formation  over  the 
length  and  breadth  of  the  land.  The  rocks  are  therefore,  in  gen- 
eral, quite  fresh  in  appearance,  and  possess  only  a  slight  dej)th  of 
cap  or  worthless  rock.  The  same  classes  of  rock,  however,  in  the 
South  are  covered  with  rotten  products  fi-om  long  ages  of  atmos- 
pheric action. 

A  study  of  the  surfaces  of  old  buildings,  bridge  piers,  monu- 
ments, tombstones,  etc.,  which  have  been  exposed  to  atmospheric 
influences  for  years,  is  one  of  the  best  sources  of  reliable  information 
concerning  the  durability  of  stone.  A  durable  stone  will  retain  tlie 
tool-marks  made  in  working  it,  and  preserve  its  edges  and  corners 
sharp  and  true. 

29.  Artificial  Methods  of  Testing  Durability.  The  older  but 
less  satisfactory  methods  are:  determining  (1)  the  resistance  to 
crushing,  (2)  the  absorjitive  power,  (3)  the  resistance  to  the  expan- 
sion of  frost,  by  saturating  the  stone  with  some  solution  which  will 
crystallize  in  the  pores  of  the  stone  and  produce  an  effect  similar  to 
frost,  (4)  the  solubility  ia  acids,  and  (5)  microscopical  examination. 


A  i;t. 


2.] 


DURABILITY    OF   BUILDING    STONES. 


21 


30.  Absorptive  Power.  The  ratio  of  absorption  depends  largely 
on  the  density, — a  dense  stone  absorbing  less  water  than  a  lighter, 
more  porous  one.  Com^Dactness  is  therefore  a  matter  of  impor- 
tance, especially  in  cold  climates;  for  if  the  water  in  a  stone  is  once 
allowed  to  freeze,  it  destroys  the  surface,  and  the  stone  very  speedily 
crumbles  away.  Other  things  being  equal,  the  less  the  absorption 
the  better  the  stone. 

To  determine  the  absorptive  power,  dry  the  specimen  and  weigh, 
it  carefully;  then  soak  it  in  water  for  24  hours,  and  weigh  again. 
The  increase  in  weight  will  be  the  amount  of  absor2:)tion.  Table  4 
shows  the  weight  of  Avater  absorbed  by  the  stone  as  compared  with 
the  weight  of  the  dry  stone — that  is,  if  300  units  of  dry  stone  weigh 
301  units  after  immersion,  the  absorption  is  1  in  300,  and  is  recorded 
fis  1-300. 

Dr.  Hiram  A.  Cutting,  State  Geologist  of  Vermont,  determined 
the  absorptive  power  *  by  placing  the  specimens  in  water  under  the 
receiver  of  an  air-pump,  and  found  the  ratio  of  absorption  a  little 
larger  than  is  given  in  the  following  table.  It  is  believed  that  the 
results  given  below  more  nearly  represent  the  conditions  of  actual 
practice.  The  values  in  the  ''Max."  column  are  the  means  of  two 
or  three  of  the  largest  results,  and  those  in  the  "Min.''  column  of 
two  or  three  of  the  smallest.  The  value  in  the  last  column  is  the 
mean  for  20  or  more  specimens. 

TABLE    5. 
Absorptive  Power  of  Stoxe,  Brick,  and  Mortar. 


Ratio  of  Absorption. 


Kind  of  Material. 

Max. 

Miu. 

Average. 

Granites 

1-150 

1-150 

1-20 

1-15 

1-4 

1-2 

0 

0 
1-500 
1-240 
1-50 
1-10 

1-750 
-   300 

Marbles 

Limestones 

1-38 

Sandstones 

Bricks 

1-10 

Mortars 

31.  Effect  of  Frost.     To  determine  the  probable  effect  of  frost 
upon  a  stone,  carefully  wash,  dry,  and  weigh  samples,  and  then  wet 
*  Yan  Nostrand's  Engin'g  Mag.,  vol.  xxiv.  pp.  491-95. 


22  NATURAL    STONE.  [CHAP.    I. 

them  and  expose  to  alternate  freezing  and  thawing,  after  which 
wash,  dry,  and  weigh  again.  The  loss  in  weight  measures  the  rela- 
tive durability. 

A  quicker  way  of  accomplishing  essentially  the  same  result  is  to 
heat  the  specimens  to  500'^  or  600°  F.,  and  plunge  them,  Avhile  hot, 
into  cold  water.  The  following  comparative  results  were  obtained 
by  the  latter  method  :  * 

Relative  Ratio  of  Loss. 

White  brick 1 

Red  brick 2 

Brown-stone  (sandstone  from  Conn.). . , 5 

Nova  Scotia  sandstone 14 

32.  Brard's  Test.  Brard's  method  of  determining  the  effect  of 
frost  is  much  used,  although  it  does  not  exactly  conform  to  the  con- 
ditions met  with  in  nature.  It  consists  in  weighing  carefully  some 
small  pieces  of  the  stone,  which  are  then  boiled  in  a  concentrated 
sohition  of  sulphate  of  soda  and  afterwards  hung  up  for  a  few  days 
in  the  open  air.  The  salt  crystallizes  in  the  pores  of  the  stone, 
expands,  and  produces  an  effect  somewhat  similar  to  frost,  as  it 
causes  small  pieces  to  separate  in  the  form  of  dust.  The  specimens 
are  again  weighed,  and  those  which  suffer  the  smallest  loss  of  weight 
are  the  best.  The  test  is  often  repeated  several  times.  It  will  be 
seen  that  this  method  depends  upon  the  assumption  that  the  action 
of  the  salt  in  crystallizing  is  similar  to  that  of  water  in  freezing. 
This  is  not  entirely  correct,  since  it  substitutes  chemical  and 
mechanical  action  for  merely  mechanical,  to  disintegrate  the  stone, 
thus  giving  the  specimen  a  worse  character  than  it  really  deserves. 
The  following  results  were  obtained  by  this  method:  f 

Relative  Ratio  of  Loss. 

Hard  brick 1 

Light  dove-colored  sandstone  from  Seneca,  Ohio 2 

Coarse-grained  sandstone  from  Nova  Scotia 2 

Coarse-grained  sandstone  from  Little  Falls,  N.  J 5 

Coarse  dolomite  marble  from  Pleasautville,  N.  Y 7 

Coarse-grained  sandstone  from  Conn 13 

Soft  brick 16 

Fine-grained  sandstone  from  Conn 19 

*  Tenth  Census  of  the  U.  S.,  vol.  x.,  Report  on  the  Quarry  Industry,  p.  384.  For 
&  table  showing  essentially  the  same  results,  see  Van  Nostrand's  Engin'g  Mag.,  vol. 
Xiv.  p.  537. 

t  Tenth  Census,  vol.  x.,  Report  of  the  Quarry  Industry,  p.  385. 


AI;T.    2. J  DURABILITY   OF   BUILDIKG   STONES.  23 

33.  Effect  of  the  Atmosphere.  To  determine  the  effect  of  the 
atmosphere  of  a  large  city,  where  coal  is  used  for  fuel,  soak  clean 
small  pieces  of  the  stone  for  several  days  in  water  which  contains  one 
per  cent,  of  sulphuric^and  hydrochloric  acids,  agitating  frequently. 
If  the  stone  contains  any  earthy  matter  likely  to  be  dissolved  by  the 
gases  of  the  atmosphere,  the  water  will  be  more  or  less  cloudy  or 
muddy.     The  following  results  were  obtained  by  this  method:  * 

Relative  Ratio  of  Loss. 

White  brick 1 

Red  brick 5 

Nova  Scotia  stone , 9 

Brown-stone 30 

34.  Microscopical  Examination.  It  is  now  held  that  the  best 
method  of  determining  the  probable  durability  of  a  building  stone 
is  to  study  its  surface,  or  thin,  transjDarent  slices,  under  a  micro- 
scope. This  method  of  study  in  recent  years  has  been  most  fruit- 
ful in  developing  interesting  and  valuable  knowledge  of  a  scientific 
and  truly  practical  character.  An  examination  of  a  section  by  means 
of  the  microscope  will  show,  not  merely  the  various  substances  which 
compose  it,  but  also  the  method  according  to  which  they  are 
arranged  and  by  which  they  are  attached  to  one  another.  For 
example,  "^pyrites  is  considered  to  be  the  enemy  of  the  quarryman 
and  constructor,  since  it  decomposes  with  ease,  and  stains  and  dis- 
colors the  rock.  Pyrites  in  sharp,  well-defined  crystals  sometimes 
decomposes  with  great  difficulty.  If  a  crystal  or  grain  of  pyrites  is 
embedded  in  soft,  porous,  light-colored  sandstones,  like  those  which 
come  from  Ohio,  its  presence  will  with  certainty  soon  demonstrate 
itself  by  the  black  spot  which  will  form  about  it  in  the  porous 
stone,  and  which  will  permanently  disfigure  and  mar  its  beauty. 
If  the  same  gi-ain  of  pyrites  is  situated  in  a  very  hard,  compact,  non- 
absorbent  stone,  the  constituent  minerals  of  which  are  not  rifted  or 
cracked,  this  grain  of  pyrites  may  decompose  and  the  products  bo 
washed  away,  leaving  the  stone  untarnished." 

35.  Methods  of  Preserving.  Vitruvius,  the  Eoman  architect, 
two  thousand  years  ago  recommended  that  stone  should  be  quarried 
in  summer  when  driest,  and  that  it  should  be  seasoned  by  being 
allowed  to  lie  two  years  before  being  used,  so  as  to  allow  the  natural 

*  Tenth  Census,  vol.  x.,  Report  on  the  Quarry  Industry,  p.  385. 


24  NATURAL    STONE.  [CHAP.    I. 

sap  to  evaporate.  It  is  a  notable  fact,  that  in  the  erection  of  St. 
Paul's  Cathedral  in  London,  England,  Sir  Christopher  Wren  re« 
quired  that  the  stone,  after  being  quarried,  should  be  exposed  for 
three  years  on  the  sea-beach,  before  its  introduction  into  the 
building. 

The  surfaces  of  buildings  are  often  covered  vpith  a  coating  of 
paint,  coal-tar,  oil,  paraflBne,  soap  and  alum,  rosin,  etc.,  to  preserve 
them. 

Another  method  of  treatment  consists  in  bathing  the  stone  ia 
successive  solutions,  the  chemical  actions  bringing  about  the  forma- 
tion of  insoluble  silicates  in  the  pores  of  the  stone.  For  example,  if 
a  stone  front  is  first  washed  with  an  alkaline  fluid  to  remove  dirt, 
and  this  followed  by  a  succession  of  baths  of  silicate  of  soda  or 
potash,  and  the  surface  is  then  bathed  in  a  solution  of  chloride  of 
lime,  an  insoluble  lime  silicate  is  formed.  The  soluble  salt  is  then 
washed  away,  and  the  insoluble  silicate  forms  a  durable  cement  and 
checks  disintegration.  If  lime-water  is  substituted  for  chloride  oi 
lime,  there  is  no  soluble  chloride  to  wash  away. 

There  are  a  gi-eat  many  applications  that  have  been  used  for  the 
prevention  of  the  decay  of  building  stones,  as  paint,  oil,  coal-tar, 
bees- wax,  rosin,  paraffine,  etc. ,  and  numerous  chemical  preparations 
similar  to  that  mentioned  in  the  paragraph  just  above ;  but  all  are 
expensive,  and  none  have  proved  fairly  satisfactory.* 

It  has  already  been  stated  that,  in  order  to  resist  the  effects  of 
both  pressure  and  weathering,  a  stone  should  be  placed  on  its  nat- 
ural bed.  This  simple  precaution  adds  considerably  to  the  dura- 
bility of  any  stone. 

Aet.  3.  Classification  and  Description  of  Building  Stones. 

36.  Classification.  Building  stones  are  variously  classified 
according  to  geological  position,  physical  structure,  ai~-d  chemical 
composition. 

37.  Geological  Classification.  The  geological  position  of  rocks 
iias  but  little  connection  with  their  properties  as  building  materials. 
As  a  general  rule,  the  more  ancient  rocks  are  the  stronger  and  the 

*  For  an  elaborate  and  valuable  article  by  Prof.  Eggleston  on  the  causes  of  decay 
and  the  methods  of  preserving  building  stones,  see  Trans.  Am.  Soc.  of  C.  E.,  voL 
XV.  pp-  647-704 ;  and  for  a  discussion  on  the  same,  see  same  volume,  pp.  705-16. 


ART     3.]  DESCRIPTION    OF    BUILDING   STONES.  25 

more  durable  ;  but  to  this  there  are  many  exceptions.  According 
to  the  usual  geological  classification,  rocks  are  divided  into  igneous, 
metamorphic,  and  sedimentary.  Greenstone,  basalt,  and  lava  ara 
examples  of  igneous  rocks  ;  granite,  marble,  and  slate,  of  meta- 
morj^hic  ;  and  sandstone,  limestone,  and  clay,  of  sedimentary.  Al- 
though clay  can  hardly  be  classed  with  building  stones,  it  is  not 
entirely  out  of  place  in  this  connection,  since  it  is  employed  in 
making  bricks  and  cement,  which  are  important  elements  ol 
masonry. 

38.  Physical  Classification.  With  respect  to  the  structural 
character  of  large  masses,  rocks  are  divided  into  stratified  and  un» 
stratified. 

In  their  more  minute  structure  the  unstratified  rocks  present, 
for  the  most  part,  an  aggregate  of  crystalline  grains,  firmly  adhering 
together.     Granite,  trap,  basalt,  and  lava  are  examples  of  this  class. 

In  the  more  minute  structure  of  stratified  rocks,  the  follo«-ing 
varieties  are  distinguished  :  1.  Compact  crystalline  structure :  ac- 
companied by  great  strength  and  durability,  as  in  quartz-rock  and 
marble.  2.  Slaty  structure,  easily  split  into  thin  layers ;  accom- 
panied by  both  extremes  of  strength  and  durability,  clay-slate  and 
hornblende-slate  being  the  strongest  and  most  durable.  3.  The 
granular  crystalline  sti-ucture,  in  which  crystalline  grains  either 
adhere  firmly  together,  as  in  gneiss,  or  are  cemented  into  one  masj 
by  some  other  material,  as  in  sandstone  ;  accompanied  by  all  degree? 
of  compactness,  porosity,  strength,  and  durability,  the' lowest  ex- 
treme being  sand.  4.  The  compact  granular  structure,  in  which 
the  grains  are  too  small  to  be  visible  to  the  unaided  eye,  as  in  blue 
limestone  ;  accompanied  by  considerable  strength  and  durability. 
5-  Porous,  granular  structure,  in  which  the  grains  are  not  crystal- 
line, and  are  often,  if  not  always,  minute  shells  cemented  together; 
accompanied  by  a  low  degree  of  strength  and  durability.  6.  The 
conglomerate  structure,  where  fragments  of  one  material  are  embed- 
ded in  a  mass  of  another,  as  graywacke;  accompanied  by  all  degrees 
of  strength  and  durability. 

A  study  of  the  fractured  surface  of  a  stone  is  one  means  of 
determining  its  structural  character.  The  even  fracture,  when  the 
surfaces  of  division  are  planes  in  definite  positions,  is  characteristic 
of  a  crystalline  structure.  The  uneven  fracture,  when  the  broken 
Burface  presents  sharp  projections,  is  characteristic  of  a  gi-anular 


26  NATURAL   STONE.  [CHAP.    I. 

structure.  The  slaty  fracture  gives  an  even  surface  for  planes  of 
division  parallel  to  the  lamination,  and  uneven  for  other  directions 
of  division.  The  conchoidal  fracture  presents  smooth  concave  and 
')onvey  surfaces,  and  is  characteristic  of  a  hard  and  compact  struct- 
ure. The  earthy  fracture  leaves  a  rough,  dull  surface,  and  indi- 
cates softness  and  brittleness. 

39.  Chemical  Classification.  Stones  are  divided  into  three 
classes  with  respect  to  their  chemical  composition,  each  distin- 
guished by  the  earth  which  forms  its  chief  constituent ,  viz.,  sili- 
ceoui;'  stones,  argillaceous  stones,  and  calcareous  stones. 

Siliceous  Stones  are  those  in  which  silica  is  the  characteristic  earthy 
constituent.  With  a  few  exceptions  their  structure  is  crystalline- 
gi-anular,  and  the  crystalline  grains  contained  in  them  are  hard  and 
durable  ;  hence  weakness  and  decay  in  them  generally  arise  from 
the  decomposition  or  disintegration  of  some  softer  and  more  perish- 
able material,  by  which  the  grains  are  cemented  together,  or,  when 
they  are  porous,  by  the  freezing  of  water  in  their  pores.  The  prin- 
cipal siliceous  stones  are  granite,  syenite,  gneiss,  mica-slate,  green- 
stone, basalt,  trap,  porphyry,  quartz-rock,  hornblende-slate,  and 
sand  stone. 

Argillaceous  or  Clayey  Stones  are  those  in  which  alumina,  although 
it  may  not  always  be  the  most  abundant  constituent,  exists  in  suf- 
ficient quantity  to  give  the  stone  its  characteristic  properties.  The 
principal  kinds  are  slate  and  graywacke-slate. 

Calcareous  Stones  are  those  in  which  carbonate  of  lime  pre- 
dominates. They  effervesce  with  the  dilute  mineral  acids,  which 
combine  with  the  lime  and  set  free  carbonic  acid  gas.  Sulphuric 
acid  forms  an  insoluble  compound  with  the  lime.  Nitric  and  mu- 
riatic acids  form  compounds  with  it,  which  are  soluble  in  water. 
By  the  action  of  intense  heat  the  carbonic  acid  is  expelled  in  gas- 
ecus  form,  and  the  lime  is  left  in  its  caustic  or  alkaline  state,  when 
it  is  called  quicklime.  Some  calcareous  stones  consist  of  pure  car- 
bonate of  lime;  in  others  it  is  mixed  with  sand,  clay,  and  oxide 
of  iron,  or  combined  with  carbonate  of  magnesia.  The  durability 
of  calcareous  stones  depends  upon  their  compactness,  those  which 
are  porous  being  disintegrated  by  the  freezing  of  water,  and  by  the 
chemical  action  of  an  acid  atmosphere.  They  are,  for  the  most 
part,  easily  wrought.     The  principal  calcareous  stones  are  marble, 


ART.    3.]  DESCRIPTION    OF   BUILDING   STONEC.  27 

compact  limestone,  granular  limestone  (the  calcareous  stone  of  the 
geological  classification),  and  magnesian  limestone  or  dolomite. 

40.  Description  of  Building  Stones.  A  few  of  the  more 
prominent  classes  of  building  stones  will  now  be  briefly  described. 

41.  Trap.  Although  trap  is  the  strongest  oi  building  materials, 
and  CACcodingly  durable,  it  is  little  used,  owing  to  the  great  diffi- 
culty with  which  it  is  quarried  and  wrought.  It  is  an  exceedingly 
tough  rock,  and,  being  generally  without  cleavage  or  bedding,  is 
especially  intractable  under  the  hammer  or  cliisel.  It  is,  however, 
sometimes  used  with  excellent  effect  in.cyclopean  architecture,  the 
blocks  of  various  shapes  and  sizes  being  fitted  together  with  no 
effort  to  form  regular  courses.  The  '•'  Palisades"  (the  bluff'  skirting 
the  western  shore  of  the  Hudson  Eiver,  opposite  and  above  Xew 
York)  are  composed  of  trap-rock, — much  used  for  road-metal,  street 
pavements,  and  railroad  ballast. 

42.  Granite.  Granite  is  the  strongest  and  most  durable  of  all 
the  stones  in  common  use.  It  generally  breaks  with  regularity, 
and  may  be  quarried  in  simple  shapes  with  facility ;  but  it  is  ex- 
tremely hard  and  tough,  and  therefore  can  only  be  wrought  into 
elaborate  forms  with  a  great  expenditure  of  labor.  For  this  reason 
the  use  of  granite  is  somewhat  limited.  Its  strength  and  durability 
commend  it,  however,  for  foundations,  docks,  piers,  etc.,  and  for 
massive  buildings ;  and  for  these  purposes  it  is  in  use  the  world 
over. 

The  larger  portion  of  our  granites  are  some  shade  of  gi'ay  in 
color,  though  -pink  and  red  varieties  are  not  uncommon,  and  black 
varieties  occasionally  occur.  They  vary  in  texture  from  very  fine 
and  homogeneous  to  coarsely  porphyritic  rocks,  in  which  the  indi- 
vidual grains  are  an  inch  or  more  in  length.  Excellent  granites  are 
found  in  New  England,  throughout  the  Alleghany  belt,  in  the 
Eocky  Mountains,  and  in  the  Sierra  Nevada.  Very  large  granite 
quarries  exist  at  Vinalhaven,  Maine ;  Gloucester  and  Quincy,  Mas- 
sachusetts; and  at  Concord,  New  Hampshire.  These  quarries  fur- 
nish nearly  all  the  granite  used  in  this  country.  An  excellent 
granite,  which  is  largely  used  at  Chicago  and  in  the  Northwest,  is 
found  at  St.  Cloud,  Minnesota. 

At  the  Vinalhaven  quarry  a  single  block  300  feet  long,  20  feet 
wide,  and  6  to  10  feet  thick  was  blasted  out,  being  afterwards  broken 
up.      Until  recently  the  largest  single  block  ever  quarried  and 


28  NATURAL   STONE.  [CIIAP.    I 

dressed  in  this  country  was  that  used  for  the  General  Wool  Mcnu- 
ment,  now  in  Troy,  New  York,  which  measured,  when  completed, 
60  feet  in  height  by  5^  feet  square  at  the  base,  being  only  9  feet 
shorter  than  the  Egyptian  Obelisk  now  in  Central  Park,  New  York. 
In  1887  the  Bodwell  Granite  Company  took  out  from  its  quarries  in 
Maine  a  granite  shaft  115  feet  long,  10  feet  square  at  the  base,  and 
weighing  850  tons.  It  is  claimed  that  this  is  the  largest  single 
quarried  stone  on  record. 

43.  Marbles.  In  common  language,  any  limestone  which  will  take 
a  good  polish  is  called  a  marble  ;  but  the  name  is  properly  applied 
only  to  limestones  which  have  been  exposed  to  metamoi'phic  action, 
and  have  thereby  been  rendered  more  crystalline  in  texture,  and 
have  had  their  color  more  or  less  modified  or  totally  removed. 
Marbles  exhibit  great  diversity  of  color  and  texture.  They  are 
pure  white,  mottled  white,  gray,  blue,  black,  red,  yellow,  or  mot- 
tled with  various  mixtures  of  these  colors.  Marble  is  confessedly 
the  most  beautiful  of  all  building  materials,  but  is  chiefly  employed 
for  interior  decorations. 

44.  Limestones.  Limestones  are  comj^osed  chiefly  or  Ui-gely  of 
carbonate  of  lime.  There  are  many  varieties  of  limestone,  which 
differ  in  color,  composition,  and  value  for  engineering  and  building 
purposes,  owing  to  the  differences  in  the  character  of  the  deposits 
and  chemical  combinations  entering  into  them.  ''If  the  rock  is 
compact,  fine-grained,  and  has  been  deposited  by  chemical  agencies, 
we  have  a  variety  of  limestone  known  as  travertine.  If  it  contains 
much  sand,  and  has  a  more  or  less  conchoidal  fracture,  we  have  a 
siliceous  limestone.  If  the  silica  is  very  fine-grained,  it  is  horn- 
stone.  If  the  silica  is  distributed  in  nodules  or  flakes,  either  in 
seams  or  throughout  the  mass,  it  is  cherty  limestone;  if  it  contains 
silica  and  clay  in  about  equal  proportions,  hydraulic  limestone  ;  if 
clay  alone  is  the  principal  impurity,  argillaceous  limestone  ;  if  iron 
is  the  principal  impurity,  ferruginous  limestone  ;  if  iron  and  clay 
exceed  the  lime,  ironstone.  If  the  ironstone  is  decomposed,  and 
the  iron  hydrated,  it  is  rottenstone;  if  carbonate  of  magnesia  forms 
one  third  or  less,  magnesian  limestone  ;  if  carbonate  of  magnesia 
forms  more  than  one  third,  dolomitic  limestone." 

The  lighter-colored  and  fine-grained  limestones,  when  sawed  and 
nsed  as  ashlars,  are  deservedly  esteemed  as  among  our  best  building 
materials.     They  are,  however,  less  easily  and  accurately  worked 


ART.    3.]  DESCRIPTION   OF    BUILDING   STONES.  21' 

under  the  chisel  than  sandstones,  and  for  this  reason  and  their 
greater  rarity  are  far  less  generally  used.  The  gray  limestones,  like 
that  of  Lockport,  New  York,  when  hammer-dressed,  have  the  ap- 
pearance of  light  granite,  and,  since  they  are  easily  wrought,  they 
are  advantageously  used  for  trimmings  in  buildings  of  brick. 

Some  of  the  softer  limestones  possess  qualities  which  specially 
commend  them  for  building  materials.  For  example,  the  cream- 
colored  limestone  of  the  Paris  basin  {calcau'e  grassier)  is  so  soft  that 
it  may  be  dressed  with  great  facility,  and  yet  hardens  on  exposure, 
and  is  a  duralDle  stone.  "Walls  laid  up  of  this  material  are  frequently 
planed  cown  to  a  common  surface,  and  elaborately  ornamented  at 
small  expense.  The  Topeka  stone,  found  and  now  largely  used  in 
Kansas,  has  the  same  qualities.  It  may  be  sawed  out  in  blocks 
almost  as  easily  as  wood,  and  yet  is  handsome  and  durable  when 
placed  in  j)osition.  The  Bermuda  stone  and  coquina  are  treated  in 
the  same  way. 

Large  quantities  of  limestones  and  dolomites  are  quarried  in 
nearly  all  of  the  "Western  States.  These  are  mostly  of  a  dull  grayish 
color,  and  their  uses  are  chiefly  local.  The  light-colored  oolitic 
limestone  of  Bedford,  Indiana,  is,  however,  an  exception  to  this 
rule.  Not  only  are  the  lasting  qualities  fair  and  the  color  pleasing, 
but  its  fine  even  grain  and  softness  render  it  admirably  adapted  for 
carved  work.  It  lias  been  very  widely  used  within  the  last  few 
years.  This  stone  is  often  found  in  layers  20  and  30  feet  thick,  and 
is  much  used  for  bridge  piers  and  other  massive  work.  There  are 
noted  I'rnestone  quarries  at  Dayton  and  Sandusky,  Ohio;  at  Bedford, 
Ellettsville,  and  Salem,  Indiana;  at  Joliet,  Lemont,  Grafton,  and 
Chester,  Illinois;  and  at  Cottonwood,  Kansas. 

45.  Sandstones.  "Sandstones  vary  much  in  color  and  fitness  for 
architectural  purposes,  but  they  include  some  of  the  most  beautiful, 
durable.  .  nd  highly  valued  materials  used  in  construction.  What- 
ever their  differences,  they  have  this  in  common,  that  they  are 
chiefly  composed  of  sand — that  is,  grains  of  quartz — to  a  greater  or 
less  degree  cemented  and  consolidated.  They  also  frequently  con- 
tain other  ingredients,  as  lime,  iron,  ialumina,  manganese,  etc.,  by 
which  the  color  and  texture  are  modified.  "Where  a  sandstone  is 
composed  exclusively  of  grains  of  quartz,  without  foreign  matter,  it 
may  be  snow-white  in  color.  Examples  of  this  variety  are  known 
in  many  localities.     They  are  rarely  used  for  building,  though  capa- 


30  NATURAL    STONE.  [CHAP.    I. 

"ble  of  being  employed  for  that  purpose  with  excellent  effect.  They 
have  been  more  generally  valued  as  furnishing  material  for  the  man- 
ufacture of  glass.  The  color  of  sandstones  is  frequently  bright  and 
handsome,  and  constitutes  one  of  the  many  qualities  -which  have 
rendered  them  so  popular.  It  is  usually  caused  by  iron;  when  gray, 
blue,  or  green,  by  the  protoxide,  as  carbonate  or  silicate  ;  when 
brown,  by  the  hydrated  oxide  ;  when  red,  by  the  anhydrous  oxide. 
The  purple  sandstones  usually  derive  this  shade  of  color  from  a 
small  quantity  of  manganese. 

"  The  texture  of  sandstones  varies  with  the  coarseness  of  the 
sand  of  which  they  are  composed,  a7id  the  degree  to  which  it  is  con- 
solidated. Usually  the  material  which  unites  the  grains  of  sand 
is  silica;  and  this  is  the  best  of  all  cements.  This  silica  has  been 
deposited  from  solution,  and  sometimes  fills  all  the  interstices  be- 
tween the  grains.  If  the  process  of  consolidation  has  been  carried 
far  enough,  or  the  quartz  grains  have  been  cemented  by  fusion,  the 
sandstone  is  converted  into  quartzite, — one  of  the  strongest  and  most 
durable  of  rocks,  but,  in  the  ratio  of  its  compactness,  difficult  to 
work.  Lime  and  iron  often  act  as  cements  in  sandstones,  but  both 
are  more  soluble  and  less  strong  than  silica.  Hence  the  finest  and 
most  indestructible  sandstones  are  such  as  consist  exclusively  of 
grains  of  quartz  united  by  siliceous  cement.  In  some  sandstones 
part  of  the  grains  are  fragments  of  feldspar,  and  these,  being  liable 
to  decomposition,  are  elements  of  weakness  in  the  stone.  The  very 
fine-grained  sandstones  often  contain  a  large  amount  of  clay,  and 
thus,  though  very  handsome,  are  generally  less  strong  than  those 
which  are  more  purely  siliceous. 

"  The  durability  of  sandstones  varies  with  both  their  physical 
and  chemical  composition.  "When  nearly  pure  silica  and  well  ce- 
mented, sandstones  are  as  resistant  to  weather  as  granite,  and  very 
much  less  affected  by  the  action  of  fire.  Taken  as  a  whole,  they 
may  be  regarded  as  among  the  most  durable  of  building  materials. 
When  first  taken  from  the  quarry,  and  saturated  witli  quarry  water 
(a  weak  solution  of  silica),  they  are  frequently  very  soft,  but  on  ex- 
posure become  much  harder  by  the  precipitation  of  the  soluble  silica 
contained  in  them. 

46.  *'  Since  they  form  an  important  part  of  all  the  groups  of 
sedimentary  rocks,  sandstones  are  abundant  in  nearly  all  countries; 
and  as  they  are  quarried  with  great  ease,  and  are  wrought  with  th» 


ART.    3.]  DESCRIPTION    OF   BUILDING   STONES.  31 

hammer  and  chisel  with  much  greater  facility  than  limestones, 
granites,  and  most  other  kinds  of  rocks,  these  qualities,  joined  to 
their  various  and  pleasant  colors  and  their  durability,  have  made 
them  the  most  popular  and  useful  of  building  stones.  In  the 
United  States  we  have  a  very  large  number  of  sandstones  which  are 
extensively  used  for  building  purposes. 

"  x\mong  these  may  be  mentioned  the  Dorchester  stone  of  New 
Brunswick,  and  Broivn-sione  of  Connecticut  and  New  Jersey. 
These  have  been  much  used  in  the  buildings  of  the  Atlantic  cities. 
The  latter  has  been  very  popular,  but  experience  has  shown  it  to  be 
seriously  lacking  in  durability. 

"^  Among  the  sandstones  most  frequently  employed  in  the  build- 
ing of  the  interior  are  : — 

1.  "  The  Ohio  stone,  derived  from  the  Berea  grit,  a  member  of 
the  Lower  Carboniferous  series  in  Northern  Ohio.  The  principal 
quarries  are  located  at  Amherst  and  Berea.  The  stone  from  Am- 
herst is  generally  light  drab  in  color,  very  homogeneous  in  texture, 
and  composed  of  nearly  pure  silica.  It  is  very  resistant  to  fire  and 
weathering,  and  is,  on  the  whole,  one  of  the  best  and  handsomest 
building  stones  known.  The  Berea  stone  is  lighter  in  color  than 
the  Amherst,  but  sometimes  contains  sulphide  of  iron,  and  is  then 
liable  to  stain  and  decomjjose. 

2.  ''  Tlie  Waverly  sandstone,  also  derived  from  the  Lower  Car- 
boniferous series,  comes  from  Southern  Ohio.  This  is  a  fi  le- 
grained  homogeneous  stone  of  a  light-drab  or  dove  color,  works  with 
facility,  and  is  very  handsome  and  durable.  It  forms  the  material 
of  which  many  of  the  finest  buildings  of  Cincinnati  are  constructed, 
and  is,  justly,  highly  esteemed  there  and  elsewhere. 

3.  "  The  Lake  Siqnrior  sandstone  is  a  dark,  purplish-brown 
stone  of  the  Potsdam  age,  quarried  at  Bass  Island,  Marquette,  etc. 
This  is  rather  a  coarse  stone,  of  medium  strength,  but  homogeneous 
and  durable,  and  one  much  used  in  the  Lake  cities. 

4.  "  TJie  St.  Genevieve  stone  is  a  fine-grained  sandstone  of  a  del- 
icate drab  or  straw  color,  very  homogeneous  in  tone  and  texture. 
It  is  quarried  at  St.  Genevieve,  Missouri,  and  is  one  of  the  hand- 
somest of  all  our  sandstones. 

5.  "  The  Medina  sandstone,  which  forms  the  base  of  the  Upper 
Silurian  series  in  Western  New  York,  furnishes  a  remarkably  strong 


32  KATURAL    STONE.  [CHAP.    L 

and  durable  stone,  much  used  for  pavement  and  curbing  in  the 
Lake  cities. 

6.  "  The  coal-measures  of  Pennsylvania,  Ohio,  and  other  Western 
States  supply  excellent  sandstones  for  building  purposes  at  a  large 
number  of  localities.  These  vary  in  color  from  white  to  dark  red 
or  purple,  though  generally  gray  or  drab.  While  strong  and 
durable,  they  are  mostly  coarser  and  less  handsome  than  the  sand- 
■stones  which  have  been  enumerated  above.  This  is  the  source  from 
■which  are  derived  the  sandstones  used  in  purely  engineering  struct- 
ures."  * 

47.  Other  Names.  There  is  a  great  variety  of  names  of  more 
or  less  local  application,  derived  from  the  appearance  of  the  stone, 
the  use  to  which  it  is  put,  etc.,  which  it  would  be  impossible  to 
classify.  The  same  stone  often  passes  under  entirely  different 
names  in  different  localities;  and  stones  entirely  different  in  their 
essential  characteristics  often  pass  under  the  same  name. 

48.  Location  of  Quarries.  For  information  concerning  the 
location  of  quarries,  character  of  product,  etc.,  see:  Tenth  Census 
of  the  U.  S.,  Vol.  X,  Keport  on  Quarry  Industry,  pp.  107-363; 
Keport  of  Smithsonian  Institution,  1885-86,  Part  II,  pp.  357-488; 
Merrill's  Stones  for  Building  and  Decoration,  pp.  45-312 — substan- 
tially the  same  as  the  preceding — and  the  reports  of  the  various 
State  geological  surveys. 

49.  Cost.     See  §§  226-38. 

*  Prof.  J.  S.  Newberry. 


CHAPTER  II. 
BRICK. 

51.  Brick  is  made  by  submitting  clay,  which  has  been  prepared 
properly  and  moulded  into  shape,  to  a  temperature  which  converts 
it  into  a  semi-vitrified  mass. 

Common  brick  is  a  most  valuable  substitute  for  stone.  Its 
comparative  cheapness,  the  ease  with  which  it  is  transported  and 
handled,  and  the  facility  with  which  it  is  worked  into  structures  of 
any  desired  form,  are  its  valuable  characteristics.  It  is,  when  prop- 
erly made,  nearly  as  strong  as  the  best  building  stone.  It  is  but 
slightly  affected  by  change  of  temperature  or  of  humidity;  and  is 
also  lighter  than  stone. 

Notwithstanding  the  good  qualities  which  recommend  brick  as 
a  substitute  for  stone,  it  is  very  little  used  in  engineering  structures. 
It  is  employed  in  the  construction  of  sewers  and  bridge  piers,  and 
for  the  lining  of  tunnels.  Brick  could  many  times  be  profitably 
substituted  for  iron,  stone,  or  timber  in  engineering  structures. 
This  is  especially  true  since  recent  improvements  in  the  process  of 
manufacture  have  decreased  the  cost  while  they  have  increased  the 
quality  and  the  uniformity  of  the  product.  The  advantages  of 
employing  brick-work  instead  of  stone  masonry  will  be  discussed  in 
connection  with  brick  masonry  in  Chapter  VIII.  Probably  one 
thing  which  has  prevented  the  more  general  use  of  brick  in  engi- 
neering is  the  variable  quality  of  the  product  and  the  trouble  of 
proper  inspection. 

52.  Peocess  of  Manufacture.  The  Clay.  The  quality  of  the 
brick  depends  primarily  upon  the  kind  of  clay.  Common  clays,  of 
which  the  common  brick  is  made,  consist  principally  of  silicate  of 
alumina;  but  they  also  usually  contain  lime,  magnesia,  and  oxide 
of  iron.  The  latter  ingredient  is  useful,  improving  the  product  by 
giving  it  hardness  and  strength;  hence  the  red  brick  of  the  Eastern 
States  is  often  of  better  quality  than  the  white  and  yellow  brick 
made  in  the  West.     Silicate  of  lime  renders  the  clay  too  fusible, 

33 


34  BRICK.  [chap.  II.. 

and  causes  the  bricks  to  soften  and  to  become  distorted  in  the  pro- 
cess of  burning.  Carbonate  of  lime  is  certain  to  decompose  in 
burning,  and  the  caustic  lime  left  behind  absorbs  moisture,  prevents 
the  adherence  of  the  mortar,  and  promotes  disintegration. 

Uncombined  silica,  if  not  in  excess,  is  beneficial,  as  it  preserves 
the  form  of  the  brick  at  high  temperatures.  In  excess  it  destroys 
the  cohesion,  and  renders  the  bricks  brittle  and  weak.  Twenty-five 
per  cent,  of  silica  is  a  good  proportion. 

53.  Moulding.  In  the  old  process  the  clay  is  tempered  with 
water  and  mixed  to  a  plastic  state  in  a  pit  with  a  tempering  wheel, 
or  in  a  primitive  pug-mill;  and  then  the  soft,  plastic  clay  is  pressed 
into  the  moulds  by  hand.  This  method  is  so  slow  and  laborious 
that  it  has  beeu  almost  entirely  displaced  by  more  economical  and 
expeditious  ones  in  which  the  work  is  done  wholly  by  machinery. 
There  is  a  great  variety  of  machines  for  preparing  and  moulding 
the  clay,  which,  however,  may  be  grouped  into  three  classes,  accord- 
ing to  the  condition  of  the  clay  when  moulded:  (1)  soft-mud 
machines,  for  which  the  clay  is  reduced  to  a  soft  mud  by  adding 
about  one  quarter  of  its  volume  of  water;  (2)  stiff-mud  machines, 
for  which  the  clay  ^s  reduced  to  a  stiff  mud;  and  (3)  dry-clay 
machines,  with  which  the  dry,  or  nearly  dry,  clay  is  forced  into  the 
moulds  by  a  heavy  pressure  without  having  been  reduced  to  a  plastic 
mass.  These  machines  may  also  be  divided  into  two  classes,  accord- 
ing to  the  method  of  filling  the  moulds:  (1)  Those  in  which  a  con- 
tinuous stream  of  clay  is  forced  from  the  pug-mill  through  a  die 
and  is  afterwards  cut  up  into  bricks;  and  (2)  those  in  which  the 
clay  is  forced  into  moulds  moving  under  the  nozzle  of  the  pug-mill. 

54.  Burning.  The  time  of  burning  varies  with  the  character  of 
the  clay,  the  form  and  size  of  kiln,  and  the  kind  of  fuel.  With  the 
older  processes  of  burning,  the  brick,  when  dry  enough,  is  built  up 
in  sections — by  brick-makers  called  "arches," — which  are  usually 
about  5  bricks  (3|^  feet)  wide,  30  to  40  bricks  (20  to  30  feet)  deep, 
and  from  35  to  50  courses  high.  Each  section  or  ''arch"  has  an 
opening — called  an  "eye" — at  the  bottom  in  the  center  of  its  width, 
which  runs  entirely  through  the  kiln,  and  in  which  the  fuel  used  in 
burning  is  placed.  After  the  bricks  are  thus  stacked  up,  the  entire 
pile  is  enclosed  with  a  wall  of  green  brick,  and  the  joints  between 
the  casing  bricks  are  carefully  stopped  with  mud.  Burning,  includ- 
ing drying,  occupies  from  6  to  15  days.     The  brick  is  first  subjected 


CLASSIFICATIOX   OF   COMMON"   BEICK.  35 

to  a  moderate  heat,  and  when  all  moisture  has  been  expelled,  the 
heat  is  increased  slowly  until  the  "arch-brick/'  i.  e.,  those  next  to 
the  "eye,"  attain  a  white  heat.  This  temperature  is  kept  up  until 
the  burning  is  complete.  Finally,  all  oj)enings  are  closed,  and  the 
mass  slowly  cools. 

With  the  more  modern  processes  of  burning,  the  principal  yards 
have  permanent  kilns.  These  are  usually  either  a  rectangular  space 
surrounded,  except  for  very  wide  doors  at  the  ends,  by  permanent 
brick  walls  having  fire-boxes  on  the  outside;  or  the  kiln  may  be 
entirely  enclosed — above  as  well  as  on  the  sides — with  brick  masonry. 
The  latter  are  usually  circular,  and  are  sometimes  made  in  com- 
partments, each  of  which  has  a  separate  entrance  and  independent 
connection  with  the  chimney.  The  latter  may  be  built  within  the 
kiln  or  entirely  outside,  but  a  downward  draught  is  invariably 
-secured.  The  fuel,  usually  fine  coal,  is  placed  near  the  top  of  the 
kiln,  and  the  down  draught  causes  a  free  circulation  of  the  flame 
and  heated  gases  about  the  material  being  burned.  While  some 
compartments  are  being  fired  others  are  being  filled,  and  still 
others  are  being  emptied. 

55.  Fire  Brick.  Fire  bricks  are  used  whenever  very  high 
temjDeratures  are  to  be  resisted.  They  are  made  either  of  a  very 
nearly  pure  clay,  or  of  a  mixture  of  pure  clay  and  clean  sand,  or,  in 
rare  cases,  of  nearly  jjure  silica  cemented  with  a  small  propoition 
of  clay.  The  presence  of  oxide  of  iron  is  very  injurious,  and,  as  a 
rule,  the  presence  of  6  per  cent,  justifies  the  rejection  of  the  brick. 
In  specifications  it  should  generally  be  stijiulated  that  fire  brick 
should  contain  less  than  6  j^er  cent,  of  oxide  of  iron,  and  less  tiian 
an  aggregate  of  3  per  cent,  of  combined  lime,  soda,  potash,  and 
magnesia.  The  sulphide  of  iron — pyrites — is  even  worse  in  its 
effect  on  fire  brick  than  the  substances  first  named. 

When  intended  to  resist  only  extremely  high  heat,  silica  should 
be  in  excess;  and  if  to  be  exposed  to  the  action  of  metallic  oxides, 
which  would  tend  to  unite  with  silica,  alumina  should  be  in  excess. 

Good  fire  brick  should  be  uniform  in  size,  regular  in  shape, 
homogeneous  in  texture  and  composition,  easily  cut,  strong,  and 
infusible. 

56.  Classification  of  Common  Brick.  Bricks  are  classified 
accordiug  to  (1)  the  way  in  which  they  are  moulded;  (2)  their 
position  in  the  kiln  while  being  burned;  and  (3)  their  form  or  use. 


36  BRICK.  [chap.  II. 

1'  The  method  of  moulding  gives  rise  to  the  following  terms : 

Soft-mud  Brich.  One  moulded  from  clay  which  has  been  reduced 
to  a  soft  mud  by  adding  water.  It  may  be  either  hand-moulded  or 
machine-moulded. 

Stiff -7)111(1  Brick.  One  moulded  from  clay  in  the  condition  of 
stiff  mud.     It  is  always  machine-moulded. 

Pressed  Brick.     One  moulded  from  dry  or  semi-dry  clay. 

Ee-jjressed  Brick.  A  soft-mud  brick  which,  after  being  par- 
tially dried,  has  been  subjected  to  an  enormous  pressure.  It  is 
also  called,  but  less  appropriately,  pressed  brick.  The  object  of 
the  re-pressing  is  to  render  the  form  more  regular  and  to  increase 
the  strength  and  density. 

Slop  Brick.  In  moulding  brick  by  hand,  the  moulds  are  some- 
times dipped  into  water  just  before  being  filled  with  clay,  to  pre- 
vent the  mud  from  sticking  to  them,  Brick  moulded  by  this 
process  is  known  as  slop  brick.  It  is  deficient  in  color,  and  has  a 
comparatively  smooth  surface,  with  rounded  edges  and  corners. 
This  kind  of  brick  is  now  seldom  made. 

Sanded  Brick.  Ordinarily,  in  making  soft-mud  brick,  sand  is 
sprinkled  into  the  moulds  to  prevent  the  clay  from  sticking ;  the 
brick  is  then  called  sanded  brick.  The  sand  on  the  surface  is  of  no 
serious  advantage  or  disadvantage.  In  hand-moulding,  when  sand 
is  used  for  this  purpose,  it  is  certain  to  become  mixed  with  the  clay 
and  occur  in  streaks  in  the  finished  brick,  which  is  very  undesira- 
ble ;  and  owing  to  details  of  the  process,  which  it  is  here  unneces- 
sary to  explain,  every  third  brick  is  especially  bad. 

MacJi ine-made  Brick.  Brick  is  frequently  described  as  "ma- 
chine-made;" but  this  is  very  indefinite,  since  all  grades  and  kinds 
are  made  by  machinery. 

2.  When  brick  was  generally  burned  in  the  old-style  up-draught 
kiln,  the  classification  according  to  position  was  important ;  but 
with  the  new  styles  of  kilns  and  improved  methods  of  burning,  the 
quality  is  so  nearly  uniform  throughout  the  kiln,  that  the  classifica- 
tion is  less  important.  Three  grades  of  brick  are  taken  from  the 
old-style  kiln: 

Arch  or  Clinker  Bricks.  Those  which  form  the  tops  and  sides  of 
the  arches  in  which  the  fire  is  built.  Being  over-burned  and  par- 
tially vitrified,  they  are  hard,  brittle,  and  weak. 


REQUISITES  FOR   GOOD    BRICK.  3? 

Body,  Cherry,  or  Hard  Bricks.  Those  taken  from  the  interior 
of  the  pile.     The  best  bricks  in  the  kiln. 

Salmon,  Pale,  or  Soft  Bricks.  Those  which  form  the  exterior  of 
the  mass.  Being  underburned,  they  are  too  soft  for  ordinary  work, 
unless  it  be  for  filling.  The  terms  salmon  and  pale  refer  to  the 
color  of  the  brick,  and  hence  are  not  applicable  to  a  brick  made  of 
a  clay  that  does  not  burn  red.  Although  nearly  all  brick  clays  burn 
red,  yet  the  localities  where  the  contrary  is  true  are  sufficiently 
numerous  to  make  it  desirable  to  use  a  different  term  in  designating 
the  quality.  There  is,  necessarily,  no  relation  between  color,  and 
strength  and  density.  Brick-makers  naturally  have  a  prejudice 
against  the  term  soft  brick,  which  doubtless  explains  the  nearly 
universal  prevalence  of  the  less  appropriate  term — salmon. 

3.  The  form  or  use  of  bricks  gives  rise  to  the  following  classifi- 
cation:— 

Compass  Brick,  l^hose  having  one  edge  shorter  than  the  other. 
Used  in  lining  shafts,  etc. 

Feather-edge  Brick.  Those  of  which  one  edge  is  thinner  than 
the  other.  Used  in  arches ;  and  more  properly,  but  less  frequently, 
called  voussoir  brick. 

Face  Brick.  Those  which,  owing  to  uniformity  of  size  and 
color,  are  suitable  for  the  face  of  the  wall  of  buildings.  Sometimes 
face  bricks  are  simply  the  best  ordinary  brick  ;  but  generally  the 
term  is  applied  only  to  re-pressed  or  pressed  brick  made  specially  for 
this  purpose.     They  are  a  little  larger  than  ordinary  bricks  (§  62). 

Sewer  Brick.  Ordinary  hard  brick,  smooth,  and  regular  in 
form. 

Paving  Brick.  Very  hard,  ordinary  brick.  A  vitrified  cla} 
block,  very  much  larger  than  ordinary  brick,  is  sometimes  use<3  for 
paving,  and  is  called  a  paving  brick,  but  more  often,  and  more 
properly,  a  brick  imving-block. 

57.  Requisites  for  Good  Beick.  1.  A  good  brick  should  have 
plane  faces,  parallel  sides,  and  diarp  edges  and  angles.  2.  It  should 
be  of  fine,  compact,  uniform  texture  ;  should  be  quite  hard;  and 
should  give  a  clear  ringing  sound  when  struck  a  sharp  blow.  3.  It 
should  not  absorb  more  than  one  tenth  of  its  weight  of  water.  4. 
Its  specific  gravity  should  be  2  or  more.  5.  The  crushing  strength 
of  half  brick,  when  ground  flat  and  pressed  between  thick  metaJ 


B8  BRICK.  [chap.   II. 

plates,  should  be  at  least  7,000  pounds  per  square  inch.     6.  fts  mod- 
ulus of  rupture  should  be  at  least  1,000  pounds  per  square  mch. 

1.  In  regularity  of  form  re-pressed  brick  ranks  first,  dry-clay 
brick  next,  then  stiff-mud  brick,  and  soft-mud  brick  last.  Regu- 
larity of  form  depends  largely  upon  the  method  of  burning. 

2.  The  compactness  and  uniformity  of  texture,  which  greatly 
influence  the  durability  of  brick,  depend  mainly  upon  the  method 
of  moulding.  As  a  general  rule,  hand-moulded  bricks  are  best  in 
this  respect,  since  the  clay  in  them  is  more  uniformly  tempered  be- 
fore being  moulded  ;  but  this  advantage  is  partially  neutralized  by 
the  presence  of  sand  seams  (§  56).  Machine-moulded  soft-mud 
bricks  rank  next  in  compactness  and  uniformity  of  texture.  Then 
come  machine-moulded  stiff-mud  bricks,  which  vary  greatly  in 
durability  with  the  kind  of  machine  used  in  their  manufacture. 
By  some  of  the  machines,  the  brick  is  moulded  in  layers  (parallel  to 
any  face,  according  to  the  kind  of  machine),  which  are  not  thor- 
oughly cemented,  and  which  separate  under  the  action  of  the  frost. 
In  compactness,  the  dry-clay  brick  comes  last.  However,  the  rela- 
tive value  of  the  products  made  by  the  different  processes  varies 
with  the  nature  or  the  clay  used. 

3.  The  absorptive  power  is  one  of  the  most  important  elements, 
since  it  greatly  affects  the  durability  of  the  brick,  particularly  its 
resistance  to  the  effect  of  frost  (see  §§  31  and  32).  Very  soft,  un- 
der-burned brick  will  absorb  from  So  to  33  per  cent,  of  their  weight 
of  water.  Weak,  light-red  ones,  such  as  are  frequently  used  in  fill- 
ing in  the  interior  of  walls,  will  absorb  about  20  to  25  per  cent. ; 
while  the  best  brick  will  absorb  only  4  or  5  per  cent.  A  brick  may 
be  called  good  which  will  absorb  not  more  than  10  per  cent.  See 
Table  9  (page  45). 

4.  The  specific  gravity  of  a  brick  does  not  indicate  its  quality^ 
and  depends  mainly  upon  the  amount  of  burning  and  the  kind  of 
fuel  employed.  Over-burned  arch  bricks,  being  both  smaller  and 
heavier  than  the  better  body  bricks,  have  a  considerably  greater 
specific  gravity,  although  inferior  in  quality. 

5.  The  crushing  strength  is  not  a  certain  index  of  the  value  of  a 
brick,  although  it  is  always  one  of  the  3tems  determined  in  testing 
brick — if  a  testing-machine  is  at  hand.  For  any  kind  of  service, 
thfc  durability  of  a  brick  is  of  greater  importance  than  its  ability 
to  resist  crushing, — the  latter  is  only  remotely  connected  with  dura- 


ABSORBIIfG   POWER.  3ft 

bility.  Tests  of  the  crushing  strength  of  individual  bricks  are  use- 
ful only  in  comparing  different  kinds  of  brick,  and  give  no  idea  of 
the  strength  of  walls  built  of  such  bricks  (see  §  246).  Furthermore, 
the  crushing  strength  can  not  be  determined  accurately,  since  it 
varies  greatlv  with  the  size  of  the  specimen  and  with  the  details  of 
the  experiments  (see  §  60). 

6.  Owing  to  both  the  nature  of  the  quality  tested  and  the  facility 
with  which  such  a  test  can  be  made,  the  determination  of  the 
transverse  strength  is  one  of  the  best  means  of  judging  of  the 
quality  of  a  brick.  The  transverse  strength  depends  mainly  upon 
the  toughness  of  the  brick, — a  quality  of  prime  importance  in  bricks 
used  for  paving,  and  also  a  quality  greatly  affecting  the  resistance  to 
frost. 

58.  Absorbing  Power.  The  less  the  amount  of  water  absorbed 
by  a  brick  the  greater,  in  all  probability,  will  be  its  durability. 
The  amount  of  water  absorbed  is,  then,  an  important  consideration 
'n  determining  the  quality  of  a  brick.  There  are  different  methods 
in  use  for  determining  the  amount  of  water  taken  up  by  a  brick, 
and  these  lead  to  slightly  different  results.  Some  experimenters  dry 
the  bricks  in  a  hot-air  chamber,  while  some  dry  them  simj^ly  by  ex- 
posing them  in  a  dry  room;  some  experimenters  immerse  the  bricks 
in  water  in  the  open  air,  while  others  immerse  them  under  the  re- 
ceiver of  an  air-pump;  some  immerse  whole  brick,  and  some  use 
small  pieces;  and,  again,  some  dry  the  surface  with  bibulous  paper, 
while  others  allow  the  surface  to  dry  by  evaporation.  Air-dryiug 
most  nearly  represents  the  conditions  of  actual  exposure  in  ma- 
sonry structures,  since  water  not  expelled  in  that  way  is  in  such  a 
condition  as  not  to  do  any  harm  by  freezing.  Immersion  in  the 
open  air  more  nearly  represents  actual  practice  than  immersion  in 
a  vacuum.  The  conditions  of  actual  practice  are  best  represented 
by  testing  whole  brick,  since  some  kinds  have  a  more  or  less  im- 
pervious skin.  Drying  the  surface  by  evaporation  is  more  accurate 
than  drying  it  with  paper;  however,  neither  process  is  suscei^tible 
ot  mathematical  accuracy. 

The  absorbing  power  given  in  Table  9,  page  45,  was  determined 
by  (1)  drying  whole  brick  in  a  steam-heated  room  for  three  weeks, 
(2)  weighing  and  (3)  immersing  them  in  water  for  forty-four 
hours;  and  then  (4)  drying  for  four  hours — until  all  the  water  on 
the  surface  was  evaporated, — and,  finally,  (5)  again  weighing  them. 


40 


BRICK. 


[chap.  II. 


The  results  in  the  table  represent  the  mean  of  several  observations. 
If  the  brick  had  been  kiln-dried,  or  weighed  before  the  surface 
water  was  entirely  removed,  the  apparent  absorption  would  have 
been  greater. 

Comparing  the  absorbing  power  of  brick  as  given  in  the  table 
on  page  45  with  that  of  stone  on  page  20,  we  see  the  absorbing 
power  of  the  best  brick  is  about  equal  to  that  of  average  lime- 
stone and  sandstone,  and  much  greater  than  marble  and  granite. 
For  a  method  of  rendering  brick  non-absorbent,  see  §§  263-64. 

59.  Transverse  Strength.  The  experiments  necessary  to 
determine  the  transverse  strength  of  brick  are  easily  made  (§  IG), 
give  definite  results,  and  furnish  valuable  information  concerning 
the  practical  value  of  the  brick;  hence  this  test  is  one  of  the  best  in 
use. 

Table  6  gives  the  results  of  experiments  made  by  the  author  on 
Illinois  brick.  The  averages  represent  the  results  of  from  six  to  fifteen 

TABLE  6. 

Transverse  Strength  of  Illinois  Brick. 

(Summarized  from  Table  9,  page  45.) 


Kef. 
No. 

Kind  of  Brick. 

Modulus  op  Rupture  in 
Lbs.  perSq.  In.* 

Co-EFPiciENT  OP  Trans- 
verse Strength.* 

Max. 

Mill. 

Average. 

Max. 

Min. 

Aver. 

1 
2 
3 

4 

Soft  -  clay,   hand  -  moulded, 
— best  50;?  in  kiln 

Soft-clay,    machine-mould- 
ed,—best  50%  in  kiln 

Stiff-clay,   machine-mould- 
ed,—best  50;?  in  kiln 

Dry- clay  (pressed) 

2,233 

2,354 

1,475 

495 

4,348 

846 

1.135 

764 

150 

2,235 

1,409 

1,712 

1,114 

336 

3,217 

124 

142 

82 

27 

241 

47 

63 

42 

8 

124 

78 

95 

62 
19 

5 

Secret  Process 

178 

experiments  on  brick  from  three  localities.  The  ''Max."  and 
*' Min."  columns  contain  the  average  of  the  two  highest  and  the 
two  lowest  results  respectively. 

The  results  in  Table  7  were  obtained  under  the  direction  of  the 
Chief  Engineer  of  the  Lehigh  Valley  E.  B.     Each  result  represents 


*  For  deflnition,  see  §  16. 


CEUSHING   STEENGTH. 


41 


the  mean  of  seven  to  nine  experiments  on  bricks  from  different 
localities.     The  results  in   Table   6  are  considerably  greater  than 

TABLE  7. 
Teansverse  Strength  of  Eastern  Brick. 


Designation  of  Brick. 

Modulus  of  Rupture  in 
Lbs.  per  S(J.  In. 

CO-EFFICIKNT    OF  TRANS- 
VERSE Strength. 

Max. 

Min. 

Average. 

Max. 

Min. 

Average. 

Very  hard 

1 

1 1,796 

944 

645 

:     444 

1,045 
710 
504 
269 

1,352 
805 
597 
373 

100 
52 
36 
25 

58 
39 
28 
15 

75 

Hard 

45 

Medium 

Soft 

32 
21 

those  in  Table  T,  the  difference  being  due  probably  more  to  recent 
improvements  in  the  manufacture  of  brick  and  to  the  method  of 
selection  than  to  locality.  The  brick  from  which  the  results  in 
Table  6  were  derived  were  obtained  from  manufacturers  well  known 
for  the  high  quality  of  their  products. 

60.  Crushing  Strength.  It  has  already  been  explained  (§§  7 
to  14)  that  the  results  for  the  crushing  strength  of  stone  vary 
greatly  with  the  details  of  the  experiments;  but  this  difference  is 
even  greater  in  the  case  of  brick  than  in  that  of  stone.  In  testing 
stone  the  uniform  practice  is  to  test  cubes  (§  10)  whose  faces  are 
carefully  dressed  to  parallel  planes.  In  testing  brick  there  is  no 
settled  custom.  (1)  Some  experimenters  test  half  brick  while  others 
test  whole  ones;  (2)  some  grind  the  pressed  surfaces  accurately  to 
planes,  and  some  level  up  the  surfaces  by  putting  on  a  thin  coat  of 
plaster  of  Paris,  while  others  leave  them  in  the  rough;  and  (3)  some 
test  the  brick  set  on  end,  some  on  the  side,  and  others  laid  flat- 
wise. 

1.  From  a  series  of  experiments  *  on  soft  brick,  the  author  con- 
cludes that  the  crushing  strength  per  square  inch  of  a  quarter  of  a 
brick  is  about  half  that  of  a  whole  one;  and  that  a  half  brick  is 
about  tiuo  thirds,  and  three  quarters  of  a  brick  about ^re  sixths,  as 
strong  per  square  inch  as  a  whole  one  ;  or,  in  other  words,  the 
strength  of  a  quarter,  a  half,  and  three  quarters  of  a  brick,  and  a 


*  Engineering  News,  vol.  xxi.  p. 


42  BRICK.  [chap.  II. 

whole  one,  are  to  each  other  as  3,  4,  5,  and  6  respectively.  The 
reason  for  this  difference  is  apparent  if  a  whole  brick  be  conceived 
as  being  made  up  of  a  number  of  cubes  placed  side  by  side,  in  which 
case  it  is  clear  that  the  interior  cubes  will  be  stronger  than  the 
exterior  ones  because  of  the  side  support  derived  from  the  latter. 
For  experiments  showing  the  marked  effect  of  this  lateral  support, 
see  §  273.  The  quarter  brick  and  the  half  brick  have  less  of  this 
lateral  support  than  the  whole  one,  and  hence  have  correspondingly 
less  crushing  strength. 

2.  The  strength  of  the  specimen  will  vary  greatly  with  the  degree 
of  smoothness  of  its  bed-surfaces.  To  determine  the  difference 
between  reducing  the  pressed  surfaces  to  a  plane  and  leaving  them 
in  the  rough,  the  author  selected  six  bricks  of  regular  form  and 
apparently  of  the  same  strength,  and  tested  three  in  the  rough  and 
the  other  three  after  having  reduced  the  pressed  surfaces  to  j)lanes 
by  laying  on  a  coating  of  plaster  of  Paris,  which,  after  drying,  was 
ground  off  to  a  plane.  The  amount  of  plaster  remaining  on  the 
surfaces  was  just  sufficient  to  fill  up  the  depressions.  Both  sets 
were  tested  in  a  hydraulic  press  between  cast-iron,  parallel  (self- 
adjusting),  pressing  surfaces.  The  average  strength  of  those  that 
were  plastered  was  2.06  times  the  strength  of  those  that  were  not 
plastered.  This  difference  will  vary  Avith  the  relative  strength  of 
the  brick  and  the  plaster.  The  average  strength  of  the  bricks  whose 
surfaces  were  plastered  was  9,170  pounds  per  square  inch,  which  is 
more  than  that  of  the  plaster  used;  and  therefore  it  is  highly 
probable  that  if  the  surfaces  had  been  reduced  to  planes  by  grind- 
ing, the  difference  in  strength  would  have  been  still  greater.  See 
also  the  last  paragraph  of  §  12. 

3.  As  before  stated,  some  experimenters  test  brick  flatwise,  some 
edgewise,  and  some  endwise.  Since  bricks  are  generally  employed 
in  such  a  position  that  the  pressure  is  on  the  broadest  face,  it  seems 
a  little  more  satisfactory  to  lay  the  brick  flatwise  while  testing  it; 
but  since  the  only  object  in  determining  the  crushing  strength  of 
brick  is  to  ascertain  the  relative  strength  of  different  bricks, — the 
crushing  strength  of  the  brick  is  only  remotely  connected  with  the 
crushing  strength  of  the  brick-masonry  (§  246), — the  position  of  the 
brick  while  being  tested  is  not  a  matter  of  vital  importance.  Doubt- 
less the  principal  reason  for  testing  them  on  end  or  edgewise  is  to 
bring  them  within  the  capacity  of  the  testing-machine.     However, 


CRUSHING   STRENGTH.  43 

there  is  one  good  reason  against  testing  brick  flatwise;  viz.,  all 
homogeneous  granular  bodies  fail  under  compression  by  shearing 
along  planes  at  about  45^  with  the  pressed  surfaces,  and  hence  if 
the  height  is  not  sufficient  to  allow  the  shearmg  stresses  to  act 
freely,  an  abnormal  strength  is  developed.     See  also  §  10. 

The  relative  strength  of  brick  tested  m  the  three  positions — flat- 
wise, edgewise,  and  endwise — varies  somewhat  with  the  details  of 
the  experiments;  but  it  is  reasonably  well  settled  that  the  strength 
of  homogeneous  brick  flatwise  between  steel  or  cast-iron  pressing 
surfaces  is  one  and  a  half  to  two  times  as  much  as  when  the  brick  is 
tested  on  end.  A  few  experiments  by  the  author  *  seem  to  indicate 
that  the  strength  edgewise  is  a  little  more  than  a  mean  between  the 
strength  flatwise  and  endwise.  If  the  brick  is  laminated  (see  para- 
graph 2,  §  57),  the  relative  strength  for  the  three  positions — flat- 
wise, edgewise,  and  endwise — will  vary  greatly  with  the  direction  of 
the  grain. 

61.  Comparatively  few  experiments  have  been  made  to  deter- 
mine the  strength  of  brick,  and  they  are  far  from  satisfactory,  since 
the  manner  of  making  the  experiment  is  seldom  recorded.  The 
differences  in  the  details  of  the  experiments,  together  with  the 
differences  in  the  quality  of  the  bricks  themselves,  are  sufficient  to 
cause  a  wide  variation  in  the  results  obtained  by  different  observers. 
The  following  data  are  given  for  reference  and  comparisons. 

The  results  in  Table  8  (page  44)  were  made  with  the  U.  S. 
testing-machine  at  the  Watertown  (Mass.)  Arsenal. f  In  each 
experiment  the  pressed  surfaces  were  "  carefully  ground  flat  and 
set  in  a  thin  facing  of  plaster  of  Paris,  and  then  tested  between  steel 
pressing  surfaces. " 

The  experiments  given  in  Table  9  (page  45)  were  made  by  the 
author,  on  Illinois  brick.  The  bricks  were  crushed  between  self- 
adjusting  cast-iron  pressing  surfaces.  Although  No.  11  shows  an 
average  absorption,  a  moderate  transverse  strength,  and  a  high  crush- 
ing strength,  this  particular  brand  of  brick  disintegrated  rapidly  by 
the  frost.  This  is  characteristic  of  this  class  of  brick,  and  is  caused 
by  the  clay's  being  forced  into  the  moulds  or  through  the  die  in  such  a 
way  as  to  leave  the  brick  in  lamince,  not  well  cemented  together.  A 
critical  examination  of  the  brick  with  the  unaided  eye  gave  no  indi- 

*  Engineering  News,  vol.  xxi.  p.  88. 

+  Compiled  from  the  annual  reports  for  1883-85. 


44 


BEICK. 


[chap  H 


W 

o 


S52 

•-  0.5 

CO  Q, 

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46  BRICK.  [chap.    II. 

cation  of  a  laminated  structure,  and  yet  compressing  the  brick  in 
two  positions — sidewise  and  edgewise — never  failed  to  reveal  such 
structure.  The  crushing  strength  in  the  table  was  obtained  when 
the  pressure  was  applied  to  the  edges  of  the  laminae.  In  experi- 
ments Nos.  12, 13,  and  1-4  the  pressed  surfaces  were  so  nearly  mathe- 
matical planes  that  possibly  these  bricks  stood  more  than  they  would 
have  done  if  their  beds  had  been  plastered.  The  strength  of  No.  15 
was  beyond  the  capacity  of  the  machine;  a  whole  brick,  on  end,  stood 
11,083  lbs.  per  sq.  in.  without  any  cracks  or  snapping  sounds — which 
usually  occur  at  about  half  of  the  ultimate  strength. 

Kankine  says  that  "■  strong  red  brick,  when  set  on  end,  should 
require  at  least  1,100  lbs.  per  sq.  in.  to  crush  them;  weak  red  ones, 
550  to  800  lbs.  per  sq.  in.;  and  fire  bricks,  1,700  lbs.  per.  sq.  in."* 

Experiments  on  the  brick  in  general  use  in  Berlin  gave  for 
*'  ordinary"  brick,  on  edge,  a  strength  of  2,930  lbs.  per  sq.  in.;  and 
for  "  selected"  brick,  3,670  lbs.  per  sq.  in.f 

The  brick  used  in  the  New  York  reservoir,  when  laid  flat  and 
packed  with  sand,  showed  an  average  strength,  for  four  specimens, 
of  2,770  lbs.  per  sq.  in.;  and  two  samples  tested  between  wood 
averaged  2,660  lbs.  per  sq.  in.  J  Prof.  Pike§  tested  half  brick  flat- 
wise between  sheets  of  pasteboard  with  the  following  results:  St. 
Louis  brick,  6,417  lbs.  per  sq.  in.  (the  average  of  six  trials);  and 
pressed  brick,  2,519  lbs.  per  sq.  in.  (the  average  of  thirteen  sam- 
ples from  ten  localities). 

62.  Size  and  Weight.  In  England  the  legal  standard  size  for 
brick  is  8f  X  4f  X  2f  inches.  In  Scotland  the  average  size  is 
about  94^  X  4J  X  3^  inches;  in  Germany,  9|^  X  4|  X  2f  inches;  in 
Austria,  11^  X  5|-  X  2f  inches;  in  Cuba,  11  X  5|  X  2f  inches;  and 
in  South  America,  12f  X  6^  X  2|^  inches. 

In  the  United  States  there  is  no  legal  standard,  and  the  dimen- 
sions vary  with  the  maker.  In  the  Eastern  States  8^  X  4  X  2^ 
inches  is  a  common  size  for  brick,  of  which  23  make  a  cubic  footj 
btit  in  the  West  the  dimensions  are  usually  a  little  smaller.  The 
National  Brick-makers'  Association    in    1887    and    the    National 

*  Civil  Engineering,  pp.  366  and  769. 

t  Van  Nostrand's  Engineering  Magazine,  vol.  xxxiv.  p.  340.  From  abstracts  of 
the  Inst,  of  C.  E. 

X  Jour.  Frank.  Inst.,  vol.  Ixv.  p.  333;  also  Trans.  Am.  Soc.  of  C.  E.,  vol.  ii.  pp. 
185-86. 

§  Jour.  Assoc.  Engineering  Soc,  vol.  iv.  pp.  366-67. 


SIZE,  WEIGHT,    AND    COST.  47 

Traders  and  Builders'  Association  in  1889  adopted  85  x  4  X  2y 
inches  as  the  standard  size  for  common  brick,  and  8|  X  4^  X  ^ 
for  face  brick.  The  price  should  vary  with  the  size.  If,  reckoned 
according  to  cubic  contents,  brick  8x4x3  inches  is  worth  810 
per  thousand,  brick  8y  x  4:^  X  2j  is  worth  $12.33  per  thousand, 
and  8|^  X  4|  X  2^  is  worth  $15  per  thousand.  Further,  where  brick 
is  laid  by  the  thousand,  small  bricks  are  doubly  expensive.  Since 
bricks  shrink  in  burning,  in  proportion  to  the  temperature  to  which 
they  are  exposed,  the  amount  differing  with  the  different  kinds  of 
clays,  it  is  impossible  to  have  the  size  exactly  uniform.  Re-pressed 
and  machine-moulded  bricks  are  more  nearly  uniform  in  size  than 
hand-moulded. 

The  size  of  brick  and  the  thickness  of  the  mortar  joint  should 
be  such  that  brick  may  be  laid  flat,  edgewise,  or  set  vertically,  and 
still  fit  exactly      These  proportions  are  seldom  realized. 

Re-pressed  brick  weighs  about  150  lbs.  per  cu.  ft.  ;  common 
hard  brick,  125  ;  inferior,  soft  brick,  100.  Common  bricks  will 
average  about  4^  lbs.  each. 

63.  Cost.  Brick  is  sold  by  the  thousand.  At  Chicago,  in  1887, 
the  "  best  sewer"  brick  cost  $9  ;  common  brick,  from  $6  to  $7. 


CHAPTER   III. 
LIME   AND  CEMENT. 

64.  Classification.  Considered  as  materials  for  nse  in  the 
builder's  art,  the  prodncts  of  calcination  of  limestone  are  classified 
as  common  lime,  hydraulic  lime,  and  hydraulic  cement.  If  the 
limestone  is  nearly  pure  carbonate  of  lime,  the  product  is  common 
lime,  which  will  slake  upon  the  addition  of  water,  and  mortar  made 
of  it  will  harden  by  absorbing  carbonic  acid  from  the  air,  but  will 
not  harden  under  water.  If  the  limestone  contains  more  impuri- 
ties, the  product  is  hydraulic  lime,  which  will  slake  upon  the  addi- 
tion of  water,  and  mortar  made  of  it  will  harden  either  in  air  or 
under  water  by  the  chemical  action  between  the  hydraulic  lime  and 
the  water  used  in  making  the  mortar.  If  the  limestone  contains 
still  more  impurities,  the  product  is  hydraulic  cement,  which  will 
not  slake  upon  the  addition  of  water  but  must  be  reduced  to  a  paste 
by  grinding,  and  which  will  set  either  in  air  or  under  water  by  the 
chemical  action  between  the  cement  and  the  water  used  in  making 
the  mortar.  Common  lime  is  sometimes  called  air-lime,  because  a 
paste  or  mortar  made  from  it  requires  exposure  to  the  air  to  enable 
it  to  "  set,"  or  harden.  The  hydraulic  limes  and  cements  are  also 
called  water-limes  and  water-cements,  from  their  property  of 
hardening  under  water. 

Common  lime  is  used  in  making  the  mortar  for  most  architect- 
ural masonry,  and  until  recently  it  was  generally  employed  in 
engineering  masonry;  but  the  opinion  is  rapidly  gaining  ground 
that  only  cement  mortar  should  be  employed  in  engineering  struct- 
ures requiring  great  strength  or  being  subject  to  shock.  On  most 
first-class  railroads  hydraulic  cement  mortar  is  used  in  all  masonry 
structures.  This  change  in  practice  is  largely  due  to  the  better 
appreciation  of  the  superiority  of  hydraulic  cement  as  a  building 
material.  Although  it  has  been  manufactured  for  about  fifty 
years,  the  amount  used  was  comparatively  limited  until  within 
recent   years.      At   present   large   quantities   are    imported   from 

48 


ART.  1.]  COMMON"    LIME.  49 

Europe,  and  very  much  more  is  made  in  this  country.     Hydraulic 
lime  is  neither  manufactured  nor  used  in  this  country. 

The  following  discussion  concerning  common  and  hydraulic 
limes  is  given  as  preliminary  to  the  study  of  hydraulic  cements 
rather  than  because  of  the  importance  of  these  materials  in  engineer- 
ing construction 

Art.  1.  Common  Lime. 

65.  Desceiition.  The  limestones  which  furnish  the  common 
lime  are  seldom,  if  ever,  pure;  but  usually  contain,  besides  the  car- 
bonate of  lime,  from  3  to  10  per  cent,  of  impurities, — such  as  silica, 
alumina,  magnesia,  oxide  of  manganese,  and  traces  of  the  alkalies. 
Lime — variously  designated  as  common  lime,  quicklime,  or  caustic 
lame — is  a  protoxide  of  calcium,  and  is  produced  when  marble,  or 
any  other  variety  of  pure  or  nearly  pure  carbonate  of  lime,  is 
calcined  with  a  heat  of  sufficient  intensity  and  duration  to  expel 
the  carbonic  acid.  It  has  a  specific  gravity  of  2.3,  is  amorphous, 
highly  caustic,  has  a  great  avidity  for  water,  and  when  brought  into 
contact  with  it  will  rapidly  absorb  nearly  a  quarter  of  its  weight  of 
that  substance.  This  absorption  is  accompanied  and  followed  by  a 
great  elevation  of  temperature,  by  the  evolution  of  hot  and  slightly 
caustic  vapor,  by  the  bursting  of  the  lime  into  pieces;  and  finally 
the  lime  is  reduced  to  a  powder,  the  volume  of  which  is  from  two 
and  a  half  to  three  and  a  half  times  the  volume  of  the  original  lime 
— the  increase  of  bulk  being  proportional  to  the  purity  of  the  lime- 
stone. In  this  condition  the  lime  is  said  to  be  slaked,  and  is  ready 
for  use  in  making  mortar. 

The  paste  of  common  lime  is  unctuous  and  impalpable  to  sight 
and  touch ;  hence  these  limes  are  sometimes  called  fat  or  rich  limes, 
as  distinguished  from  others  known  as  poor  or  meager  limes.  These 
latter  usually  contain  more  or  less  silica  and  a  greater  proportion  of 
other  impurities  than  the  fat  limes.  In  slaking  they  exhibit  a  more 
moderate  elevation  of  temperature;  evolve  less  vapor;  are  seldom 
reduced  to  an  impalpable  homogeneous  powder;  yield  thin  paste; 
and  expand  less.  They  are  less  valuable  for  mortar  than  the  fat 
limes,  but  are  extensively  employed  as  fertilizers.  When  used  for 
building  purposes  they  should,  if  practicable,  be  reduced  to  powder 
by  grinding,  in  order  to  remove  all  danger  of  subsequent  slaking. 


50  LIME   AND    CEMENT.  [CHAP.  III.. 

66.  Testing.  Grood  lime  may  be  known  by  the  following 
characteristics:  1.  Freedom  from  cinders  and  clinkers,  with  not 
more  than  10  per  cent,  of  other  impurities, — as  silica,  alumina,  etc. 
2.  Chiefly  in  hard  lumps,  with  but  little  dust.  3.  Slakes  readily 
in  water,  forming  a  very  fine  smooth  paste,  without  any  residue. 
4.  Dissolves  in  soft  water,  when  this  is  added  in  sufficient  quanti- 
ties. These  simple  tests  can  be  readily  applied  to  any  sample  of 
lime. 

67.  Preserving.  As  lime  abstracts  water  from  the  atmosphere 
and  is  thereby  slaked,  it  soon  crumbles  into  a  fine  powder,  losing 
all  those  qualities  which  render  it  of  value  for  mortar.  On  this 
account  great  care  must  be  taken  that  the  lime  to  be  used  is  freshly 
burned,  as  may  be  known  by  its  being  in  hard  lumps  rather  than 
in  powder.  Lime  is  shipped  either  in  bulk  or  in  casks.  If  in  bulk, 
it  is  impossible  to  preserve  it  for  any  considerable  time;  if  in  casks, 
it  may  be  preserved  for  some  time  by  storing  in  a  dry  place. 

Common  lime,  when  mixed  to  a  paste  with  water,  may  be  kept 
for  an  indefinite  time  in  that  condition  without  deterioration,  if 
protected  from  contact  with  the  air  so  that  it  will  not  dry  up.  It 
is  customary  to  keep  the  lime  paste  in  casks,  or  in  the  wide,  shallow 
boxes  in  which  it  was  slaked,  or  heaped  up  on  the  ground,  covered 
over  with  the  sand  to  be  subsequently  incorporated  with  it  in  mak- 
ing mortar.  It  is  convenient  for  some  purposes  to  keep  the  slaked 
lime  on  hand  in  a  state  of  powder,  which  may  be  done  in  casks 
under  cover,  or  in  bulk  in  a  room  set  apart  for  that  purpose.  The 
common  limes  contain  impurities  which  prevent  a  thorough, 
uniform,  and  prompt  slaking  of  the  entire  mass,  and  hence  the 
necessity  of  slaking  some  days  before  the  lime  is  to  be  used,  to 
avoid  all  danger  to  the  masonry  by  subsequent  enlargement  of 
volume  and  change  of  condition. 

A  paste  or  mortar  of  common  lime  will  not  harden  under  water, 
nor  in  continuously  damp  places  excluded  from  contact  with  the 
air.  It  will  slowly  harden  in  the  air,  from  the  surface  toward  the 
interior,  by  desiccation  and  the  gradual  absorption  of  carbonic-acid 
gas,  by  which  process  is  formed  a  subcarbonate  with  an  excess  of 
hydrated  base. 

68.  Cost.  Lime  is  sold  by  the  barrel  (about  230  pounds  net), 
or  by  the  bushel  (75  pounds).  At  Chicago  the  average  price,  in 
1898,  was  from  55  to  60  cents  per  barrel. 


art.  2.]  hydraulic  lime.  51 

Art.  2.  Hydraulic  Lime. 

69.  Desceiption.  Hydraulic  lime  is  like  common  lime  in  that 
it  will  slake,  and  differs  from  it  in  that  it  will  harden  nnder  water. 
Hydraulic  lime  may  be  either  argillaceous  or  siliceous.  The  former 
is  derived  from  limestones  containing  from  10  to  '^0  per  cent,  of 
clay,  homogeneously  mixed  with  carbonate  of  lime  as  the  principal 
ingredient;  the  latter  from  siliceous  limestones  containing  from  12 
to  18  per  cent,  of  silica.  Small  percentages  of  oxides  of  iron,  car- 
bonates of  magnesia,  etc.,  are  generally  present. 

During  the  burning,  the  carbonic  acid  is  expelled,  and  the  silica 
and  alumina  entering  into  combination  with  a  portion  of  the  lime 
form  both  the  silicate  and  the  aluminate  of  lime,  leaving  in  the 
burnt  product  an  excess  of  quick  or  caustic  lime,  which  induces 
slaking,  and  becomes  hydrate  of  lime  when  brought  into  contact 
"with  water.  The  product  owes  its  hydraulicity  to  the  crystallizing 
energy  of  the  aluminate  and  the  silicate  of  lime. 

Hydraulic  lime  is  slaked  by  sprinkling  with  just  sufficient  water 
to  slake  the  free  lime.  The  free  lime  has  a  greater  avidity  for  the 
water  than  the  hydraulic  elements,  and  consequently  the  former 
absorbs  the  water,  expands,  and  disintegrates  the  whole  mass  while 
the  hydraulic  ingredients  are  not  affected.  Hydraulic  lime  is 
usually  slaked,  screened,  and  packed  in  sacks  or  barrels  before 
being  sent  to  market.  It  may  be  kept  without  injury  in  this  form 
as  long  as  it  is  protected  from  moisture  and  air. 

No  hydraulic  lime  is  manufactured  in  the  United  States.  It  is 
manufactured  in  several  localities  in  Europe,  notably  at  Teil  and 
Scilly,  in  France,  from  which  places  large  quantities  were  formerly 
brought  to  this  country. 

Art.  3.  Hydraulic  Cement. 

70.  Classification.  Hydraulic  cement  may  be  divided  accord- 
ing to  the  method  of  manufacture  into  three  classes,  viz. :  Portland 
cement,  natural  cement,  and  pozzuolana.  The  first  two  differ  from 
the  third  in  that  the  ingredients  of  which  the  first  two  are  composed 
must  be  roasted  before  they  acqnire  the  property  of  hardening  under 
water,  while  the  ingredients  of  the  third  need  only  to  be  pulverized 
.and  mixed  with  water  to  a  paste. 


S^  LIME    AND    CEMENT.  [CHAP.  Ill, 

71.  Portland.  Portland  cement  is  produced  by  calcining  a 
mixture  containing  from  75  to  80  per  cent,  of  carbonate  of  lime  and 
20  to  23  per  cent,  of  clay,  at  such  a  high  temperature  that  the  silica 
and  alumina  of  the  clay  combines  with  the  lime  of  the  limestone. 
As  the  quantity  of  uncombined  lime  is  not  sufficient  to  cause  the 
mass  to  slake  to  a  powder  upon  the  addition  of  water,  the  cement 
must  be  reduced  to  powder  by  grinding. 

To  secure  a  complete  chemical  combination  of  the  clay  and  the 
lime,  it  is  necessary  that  the  raw  materials  shall  be  reduced  to  a 
powder  and  be  thoroughly  mixed  before  burning,  and  also  necessary 
that  the  calcination  shall  take  place  at  a  high  temperature.  These 
are  the  distinguishing  characteristics  of  the  manufacture  of  Portland 
cement. 

In  a  general  way  Portland  cement  differs  from  natural  cement 
by  being  heavier,  slower  setting,  and  stronger. 

72.  Portland  cement  derives  its  name  from  the  resemblance 
which  hardened  mortar  made  of  it  bears  to  a  stone  found  in  the  isle 
of  Portland,  off  the  south  coast  of  England.  Portland  cement  was 
made  first  in  England  about  1843,  and  in  America  about  1874. 

Until  recent  years  nearly  all  the  Portland  cement  used  in  this 
country  was  imported,  but  at  present  (1898)  about  one  fifth  of  the 
consumption  is  of  domestic  manufacture.  The  best  American 
Portland  is  better  than  the  best  imported,  and  is  sold  equally  cheap. 
In  1896  Portland  cement  was  made  at  twenty-six  places  in  the 
United  States.  Raw  material  suitable  for  the  manufacture  of  Port- 
land cement  exists  in  great  abundance  in  nature,  and  with  proper 
care  a  high-class  Portland  cement  may  be  produced  in  almost  any 
part  of  the  country. 

In  recent  years  the  amount  of  cement  used  in  this  country  has 
greatly  increased,  but  the  proportion  of  Portland  used  has  increased 
at  a  much  more  rapid  rate.  In  1887  only  about  one  fifth  was 
Portland,  while  in  1897  one  third  was  Portland. 

73.  Natural  Cement.  N"atural  cement  is  produced  by  calcining 
at  a  comparatively  low  temperature  either  a  natural  argillaceous 
limestone  or  a  natural  magnesian  limestone  without  pulverization 
or  the  admixture  of  other  materials.  The  stone  is  quarried,  broken 
into  pieces,  and  burned  in  a  kiln.  The  burnt  cement  is  then 
crushed  into  small  fragments,  ground,  packed,  and  sent  to  market. 

In  the  process  of  manufacture  natural  cement  is  distinguished 


ART.  3.]  HYDRAULIC    CEMENT.  53 

from  Portland,  in  nsing  a  natural  instead  of  an  artificial  mixture 
and  in  calcining  at  a  lower  temperature.  As  a  product,  natural 
cement  is  distinguished  from  Portland  in  weighing  less,  being  less 
strong,  and  as  a  rule  setting  more  quickly. 

In  Europe  in  making  this  class  of  cement  argillaceous  limestone 
is  generally  nsed,  and  the  product  is  called  Roman  cement.  In  the 
United  States  magnesian  limestone  is  usually  employed  in  making 
this  cement;  and  formerly  there  was  great  diversity  in  the  term 
nsed  to  designate  the  product,  domestic,  American,  and  natural 
being  employed.  In  the  early  editions  of  this  volume,  the  author 
called  this  class  of  cement  Eosendale,  from  the  place  where  it  was 
first  made  in  this  country — Rosendale,  Ulster  Co.,  N.  Y.  The 
term  natural  is  now  quite  generally  used,  and  on  the  whole  it  seems 
the  best. 

74.  In  1896  natural  cement  was  made  in  sixty-eight  places  in 
seventeen  states  in  this  country,  and  it  may  safely  be  assumed  that 
there  is  no  very  large  area  in  which  a  stone  can  not  be  found  from 
which  some  grade  of  natural  cement  can  be  made. 

Nearly  one  half  of  the  natural  cement  made  in  this  country 
comes  from  Ulster  Co.,  N.  Y.,  and  nearly  half  of  the  remainder 
comes  from  near  Louisville,  Kentucky. 

75.  PozzuoLANA.  Pozzuolana  is  a  term  applied  to  a  combina- 
tion of  silica  and  alumina  which,  when  mixed  with  common  lime 
and  made  into  mortar,  has  the  property  of  hardening  under  water. 
There  are  several  classes  of  materials  possessing  this  property. 

Pozzuolana  proper  is  a  material  of  volcanic  origin,  and  is  the 
first  substance  known  to  possess  the  peculiar  property  of  hydrau- 
licity.  The  discovery  was  made  at  Pozzuoli,  near  the  base  of 
Mount  Vesuvius, — hence  the  name.  Vitruvius  and  Pliny  both 
mention  that  pozzuolana  was  extensively  used  by  the  Romans  before 
their  day;  and  Vitruvius  gives  a  formula  for  its  use  in  monolithic 
masonry,  which  with  slight  variations  has  been  followed  in  Italy 
ever  since.  It  is  as  follows:  "  12  parts  pozzuolana,  well  pulverized; 
6  parts  quartzose  sand,  well  washed;  and  9  parts  rich  lime,  well 
slaked." 

Trass  is  a  volcanic  earth  closely  resembling  pozzuolana,  and  is 
employed  substantially  in  the  same  way.  It  is  found  on  the  Rhine 
between  Mayence  and  Cologne,  and  in  various  localities  in  Holland. 

Arenes  is  a  species  of  ocherous  sand  containing  so  large  a  pro- 


54  LIME    AXD    CEMEN"T.  [CHAP.  III. 

portion  of  clay  that  it  can  be  mixed  into  a  paste  with  water  without 
the  addition  of  lime,  and  used  in  that  state  for  common  mortar. 
Mixed  with  rich  lime  it  yields  hydraulic  mortar  of  considerable 
energy. 

Brick  dust  mixed  with  common  lime  produces  a  feebly  hydraulic 
mortar. 

76.  Slag  Cement.  Slag  cement  is  by  far  the  most  important  of 
the  pozzuolana  cements.  It  is  the  product  obtained  by  mixing 
powdered  slaked  lime  and  finely  pulverized  blast-furnace  slag.  The 
amount  of  slag  cement  manufactured  is  very  small  as  compared 
with  Portland  or  natural  cement,  and  apparently  much  more  is 
manufactured  in  Europe  than  in  America.  Probably  most  of  the 
so-called  pozzuolana  cements  are  slag  cements.  It  is  claimed  that 
slag  cement  mortar  will  not  stain  the  stone  laid  with  it. 

77.  Weight.  Cement  is  generally  sold  by  the  barrel,  although 
not  necessarily  in  a  barrel.  Imported  cement  is  always  sold  in 
barrels,  but  American  cement  is  sold  in  barrels,  or  in  bags,  or  less 
frequently  in  bulk. 

Portland  cement  usually  weighs  400  pounds  per  barrel  gross, 
and  370  to  380  pounds  net.  A  bag  of  Portland  usually  weighs  95 
pounds,  of  which  four  are  counted  a  barrel. 

Natural  cement  made  in  or  near  Eosendale,  N.  Y.,  weighs  318 
pounds  per  barrel  gross,  and  300  net.  Cement  made  in  Akron, 
!N".  Y.,  Milwaukee,  Wis.,  Utica,  111.,  Louisville,  Ky.,  weighs  285 
pounds  per  barrel  gross,  and  2G5  net.  Cloth  bags  usually  contain 
one  third,  and  joaper  bags  one  fourth  of  a  barrel. 

Slag  oenient  weighs  from  325  to  350  pounds  net  per  barrel. 

78.  Cost.  The  price  of  hydraulic  cement  has  decreased  greatly 
in  recent  years,  owing  chiefly  to  the  development  of  the  cement 
industry  in  this  country.  At  present  the  competition  among 
domestic  manufacturers  governs  the  price.  In  1898  the  jDrices  in 
car-load  lots  were  about  as  follows : 

Imjiorted  Portland  cement  at  Atlantic  ports  $1.50  to  $2  per 
barrel  in  wood,  and  at  Chicago  $2  to  $2.o0.  American  Portland  at 
eastern  mills  is  $1.50  to  $1.75  in  wood,  and  in  the  Mississippi  valley 
$1.75  to  $2.  The  price  in  paper  bags  is  about  10  cents  per  barrel 
less  than  in  wood,  and  about  15  cents  per  barrel  cheaper  in  cloth 
bags  than  in  wood — provided  the  cloth  bags  are  returned  to  the 
mill,  freight  prepaid. 


ART.  4.]  TESTS   OF   CEMEK-T.  55 

Xaiural  cement  in  the  Rosendale  (X.  Y.)  district  costs  f.  o.  b. 
mills  50  cents  per  barrel  (300  pounds  net)  in  bulk,  60  cents  in 
paper,  and  70  cents  in  wood.  The  price  at  the  western  mills  in 
recent  years  was  50  cents  per  barrel  (2G5  pounds  net)  in  cloth  (the 
sacks  to  be  returned,  freight  prepaid),  55  Cents  in  paper,  and  60 
cents  in  wood. 

Slag  cement  is  made  in  this  country  only  at  Chicago,  where  it 
sells  at  prices  but  little  below  those  of  similar  grades  of  Portland 
cements.  The  imported  pozzuolana  sells  substantially  the  same  as 
similar  grades  of  Portland. 

Art.  4.  Tests  of  Cement. 

79.  The  value  of  a  cement  varies  greatly  with  the  chemical 
composition,  the  temperature  of  calcination,  the  fineness  of  grind- 
ing, etc. ;  and  a  slight  variation  in  any  one  of  these  items  may 
greatly  affect  the  physical  properties  of  the  product.  Unless  the 
process  of  manufacture  is  conducted  with  the  utmost  care,  two  lots 
of  cement  of  the  same  brand  are  liable  to  differ  considerably  in 
physical  properties.  Therefore  the  testing  of  cement  to  determine 
its  fitness  for  the  use  proposed  is  a  matter  of  very  great  imj^ortance. 
The  properties  of  a  cement  which  are  examined  to  determine  its 
€onstructive  value  are:  (1)  color,  (2)  thoroughness  of  burning,  (3) 
activity,  (4)  soundness,  (5)  fineness,  (6)  strength. 

80.  Color.  The  color  of  the  cement  powder  indicates  but 
little,  since  it  is  chiefly  due  to  oxides  of  iron  and  manganese,  which 
in  no  way  affect  the  cementitious  value;  but  for  any  given  brand, 
variations  in  shade  may  indicate  differences  in  the  character  of  the 
rock  or  in  the  degree  of  burning. 

With  Portland  cement,  gray  or  greenish  gray  is  generally  con- 
sidered best;  bluish  gray  indicates  a  probable  excess  of  lime,  and 
brown  an  excess  of  clay.  An  undue  proportion  of  under-burned 
material  is  generally  indicated  by  a  yellowish  shade,  with  a  marked 
difference  between  the  color  of  the  hard-burned,  nnground  particles 
retained  by  a  fine  sieve  and  the  finer  cement  which  passes  through 
the  sieve. 

Natural  cements  are  usually  brown,  but  vary  from  very  light  to 
very  dark. 

Slag  cement  has  a  mauve  tint — a  delicate  lilac. 


56  IIME    AND   CEMEXT.  [CHAP.  III. 

81.  Thoroughness  of  Burning.  The  higher  the  temperature 
of  burning  the  greater  the  weight  of  the  clinker  (the  unground 
cement).  Two  methods  have  been  employed  in  utilizing  this  prin- 
ciple as  a  test  of  the  thoroughness  of  burning,  viz. :  (1)  determine 
the  weight  of  a  unit  of  volume  of  the  ground  cement,  and  (2) 
determine  the  specific  gravity  of  the  cement. 

82.  "Weight.  For  any  particular  cement  the  weight  varies  with 
the  temperature  of  burning,  the  degree  of  fineness  in  grinding,  and 
the  density  of  packing.  Other  things  being  the  same,  the  harder- 
burned  varieties  are  the  heavier.  The  finer  a  cement  is  ground  the 
more  bulky  it  becomes,  and  consequently  the  less  it  weighs.  Hence 
light  weight  may  be  caused  by  laudable  fine  grinding  or  by  objec- 
tionable under-burning. 

The  weight  per  unit  of  volume  is  usually  determined  by  sifting 
the  cement  into  a  measure,  and  striking  the  top  level  with  a  straight- 
edge. In  careful  work  the  height  of  fall  and  the  size  of  the  meas- 
uring vessel  are  specified.  The  weight  per  cubic  foot  is  neither 
exactly  constant,  nor  can  it  be  determined  precisely;  and  is  of  very 
little  service  in  determining  the  value  of  a  cement.  However,  it  is 
often  specified  as  one  of  the  requirements  to  be  fulfilled.  The  fol- 
lowing values,  determined  by  sifting  the  cement  with  a  fall  of  three 
feet  into  a  box  having  a  capacity  of  one  tenth  of  a  cubic  foot,  may 
be  taken  as  fair  averages  for  ordinary  cements.  The  difference  in 
weight  for  any  particular  kind  is  mainly  due  to  a  difference  in  fine- 
ness: 

Portland  75  to  90  lbs.  per  cubic  foot,  or  94  to  112  lbs.  per  bushel. 
Natural   50  to  56  lbs.  per  cubic  foot,  or  62  to    70  lbs.  per  bushel. 

Specifications  for  the  reception  of  cement  frequently  specify  the 
net  weight  per  barrel ;  but  this  is  a  specification  for  quantity  and 
not  quality. 

83.  Specific  Gravity.  The  determination  of  the  specific  gravity 
of  a  cement  is  the  only  real  test  of  the  thoroughness  of  burning. 
The  specific  gravity  is  determined  by  immersing  a  known  weight 
of  the  cement  in  a  liquid  which  will  not  act  upon  it  (usually  turpen- 
tine or  benzine),  and  obtaining  the  volume  of  the  liquid  displaced. 
The  specific  gravity  is  equal  to  the  weight  of  the  cement  (in 
grammes)  divided  by  the  displaced  volume  (in  cubic  centimetres). 

A  variety  of  forms  of  apparatus  for  use  in  making  this  test  are 


ART.  4.]  TESTS    OF   CEMENT.  57 

upon  the  market,  but  as  several  of  the  volumeters  in  ordinary  use 
in  chemical  and  physical  laboratories  are  suitable  for  this  purpose, 
it  is  unnecessary  to  describe  any  of  them  here.  As  a  slight  differ- 
ence in  specific  gravity  is  frequently  accompanied  by  a  considerable 
difference  in  the  quality  of  the  cement,  great  care  is  necessary  in 
making  the  test.  It  is  necessary  that  all  the  air-bubbles  contained 
in  the  cement  powder  be  eliminated,  so  that  the  volume  obtained 
be  that  of  the  cement  particles  only.  The  cement  should  be  passed 
through  a  sieve,  say  Xo.  80,  to  eliminate  the  lumps.  The  tempera- 
ture of  the  liquid  should  not  be  above  60°  Fahr.,  and  should  not 
change  during  the  test.  A  change  of  1°  C.  in  the  turpentine 
between  the  readings  of  the  volumeter  will  make  a  difference  of 
0.08  in  the  resalting  specific  gravity. 

The  specific  gravity  of  Portland  cement  varies  from  3.00  to  3.25, 
usually  between  3.05  and  3.17.  Natural  cement  varies  from  2.75  to 
3.05,  and  is  usually  between  2.80  and  3.00.  Slag  cement  has  a 
specific  gravity  of  2.72  to  2.76.  The  specific  gravity  of  cement 
decreases  with  age  owing  to  the  absorption  of  water  and  carbonic  acid 
from  the  air. 

German  authorities  state  that  the  specific  gravity  of  fresh  Port- 
land cement  is  between  3.12  and  3.25.  English  specifications  re- 
quire 3.10  for  fresh  Portland  and  3.07  for  cement  3  months  old. 
By  the  specifications  of  the  Canadian  Society  of  Civil  Engineers 
the  minimum  for  fresh  Portland  is  3.09.  Many  specifications  fix 
3.00  or  3.05  for  the  lower  limit. 

84.  Activity.  When  cement  powder  is  mixed  with  water  to  a 
plastic  condition  and  allowed  to  stand,  the  cement  chemically  com- 
bines with  the  water  and  the  entire  mass  gradually  becomes  firm 
and  hard.  This  process  of  solidifying  is  called  setting.  Cements 
differ  very  widely  in  their  rate  and  manner  of  setting.  Some 
occupy  but  a  few  minutes  in  the  operation,  while  others  require 
several  hours.  Some  begin  to  set  comparatively  early  and  take 
considerable  time  to  complete  the  process,  while  others  stand  con- 
siderable time  without  apparent  change  and  then  set  very  quickly. 

A  knowledge  of  the  activity  of  a  cement  is  of  importance  both 
in  testing  and  in  using  a  cement,  since  its  strength  is  seriously 
impaired  if  the  mortar  is  disturbed  after  it  has  begun  to  set. 
Ordinarily  the  moderately  slow-setting  cements  are  preferable,  since 
they  need  not  be  handled  so  rapidly  and  may  be  mixed  in  larger 


58  LIME   AND    CEMEN"T.  [CHAP.  III. 

quantities;  but  in  some  cases  it  is  necessary  to  use  a  rapid-setting 
cement,  as  for  example  when  an  inflow  of  water  is  to  be  prevented. 

To  determine  the  rate  of  setting,  points  have  been  arbitrarily 
fixed  where  the  set  is  said  to  begin  and  to  end.  It  is  very  difficult 
to  determine  these  points  with  exactness,  particularly  the  latter; 
but  an  exact  determination  is  not  necessary  to  judge  of  the  fitness 
of  a  cement  for  a  particular  use.  For  this  purpose  it  is  ordinarily 
sufficient  to  say  that  a  mortar  has  begun  to  set  when  it  has  lost  its 
plasticity,  i.e.,  when  its  form  cannot  be  altered  without  producing 
a  fracture;  and  that  it  has  set  hard  when  it  will  resist  a  slight 
pressure  of  the  thumb-nail.  Cements  will  increase  in  hardness  long 
after  they  can  not  be  indented  with  the  thumb-nail. 

For  an  accurate  determination  of  rate  of  set  two  standards  are 
in  use,  viz. :  Clillmore's  and  the  German. 

85.  Gillmore's  Test.  Mix  the  cement  with  water  to  a  stiff 
plastic  mortar  (see  §§  103—4),  and  make  a  cake  or  pat  2  or  3  inches 
in  diameter  and  about  I  iuch  thick.  The  mortar  is  said  to  have 
begun  to  set  when  it  will  just  support  a  wire  yV-inch  in  diameter 
weighing  ^  pound,  and  to  have  "  set  hard  "  when  it  will  bear  a  -/j- 
inch  wire  weighing  1  poand.  A  loaded  wire  used  for  this  purpose 
is  frequently  called  a  Vicat  needle,  after  Vicat,  its  inventor.  The 
interval  between  the  time  of  adding  the  water  and  the  time  when 
the  light  wire  is  just  supported  is  the  time  of  beginning  to  set,  and 
the  interval  between  the  time  the  light  wire  is  supported  and  the 
time  when  the  heavy  one  is  just  supported  is  the  time  of  setting. 

86.  German  Test.*  "  A  slow-setting  cement  (one  setting  in  not 
less  than  two  hours)  shall  be  mixed  three  minutes,  and  a  quick- 
setting  cement  (one  setting  in  less  than  two  hours)  one  minute,  with 
water  to  a  stiff  paste.  The  consistency  of  the  cement  paste  for  this 
cake  shall  be  such  that,  when  wrought  with  a  trowel  on  the  plate, 
the  paste  will  only  begin  to  run  towards  the  edge  of  the  same  after 
the  paste  has  been  repeatedly  jarred.  As  a  rule,  27  to  30  per  cent, 
of  water  will  suffice  to  give  the  necessary  consistency  to  a  Portland 
cement  paste,  f 

"  For  the  exact  determination  of  the  time  of  beginning  to  set, 
and  for  determining  the  time  of  setting,  a  standard  needle  300 

♦Specifications  of  the  Prussian  Minister  of  Public  Worlts,  July  28,  1887. 
t  Apparently  this  mortar  is  more  moist  than  the  "  plastic  mortar "  ordinarily 
•employed  in  this  country  (see  §§  103-4). 


ART.  4.]  TESTS   OF   CEMENT.  59 

grammes  (11  oz.)  in  weight  and  1  sqnare  millimetre  (0.0006  square 
inch)  in  cross-section  is  used.  A  metal  ring  4  centimetres  (1.575 
inches)  in  height  and  8  centimetres  (3.15  inches)  clear  diameter 
(inside)  is  placed  on  a  glass  plate,  filled  with  cement  paste  of  the 
above  consistency,  and  brought  under  the  needle,*  The  moment 
at  which  the  needle  is  no  longer  capable  of  completely  penetrating 
the  cement  cake  is  considered  the  beginning  of  the  time  of  setting. 
The  time  elapsing  between  this  and  the  moment  when  the  standard 
needle  no  longer  leaves  an  appreciable  imj^ression  on  the  hardened 
cake  is  considered  the  time  of  setting." 

To  facilitate  the  making  of  this  test,  an  apparatus  is  provided 
which  consists  of  a  light  rod  freely  sliding  through  an  arm;  and 
carrying  in  its  lower  end  the  penetrating  needle.  The  amount  of 
penetration  is  read  by  an  index  moving  over  a  graduated  scale. 

87.  Elements  Affecting  Rate  of  Set.  The  amount  of  water 
emjiloyed  is  important.  For  data  as  to  the  amount  of  water  to  be 
used,  see  §§  103—4.     The  less  the  water,  the  more  rapid  the  set. 

It  is  usually  specified  that  the  temperature  of  the  water  and  air 
shall  be  from  60°  to  65°  F.  The  higher  the  temperature,  the  more 
rapid  the  set.  To  prevent  the  surface  of  the  test  specimen  from 
hardening  by  drying,  it  is  specified  that  the  pat  shall  be  immersed 
in  water  at  60°  to  65°  F.  The  setting  under  water  is  much  slower 
than  in  air  even  though  the  air  be  saturated  with  moisture  and  be 
at  the  same  temperature  as  the  water,  due  to  the  mechanical  action 
of  the  water. 

Other  things  being  the  same,  the  finer  the  cement  is  ground  the 
quicker  it  sets. 

Cements  usually  become  slower  setting  with  age,  particularly  if 
exposed  to  the  air — Portlands  usually  but  slightly. 

The  standard  tests  for  activity  are  usually  made  on  neat  cement 
on  account  of  the  interference  of  the  sand  grains  with  the  descent 
of  the  needle.  The  rate  of  setting  of  neat  mortar  gives  but  little 
indication  of  what  the  action  may  be  with  sand.  Sand  increases 
the  time  of  setting — but  very  differently  for  different  cements. 
With  some  cements  a  mortar  composed  of  one  part  cement  to  three 
parts  sand  will  require  twice  as  long  to  set  as  a  neat  mortar,  while 
with  other  cements  the  time  will  be  eight  or  ten  times  as  long. 

*  For  an  illustration  of  the  apparatus  see  Trans.  Amer.  Soc.  of  C.  E.,  vol.  zzx. 
p.  11. 


60  LIME   AND    CEMENT.  [CHAP.  III. 

Sulphate  of  lime  (plaster  of  Paris)  greatly  influences  the  rate  of 
setting  of  Portland  cements.  The  addition  of  1  or  2  per  cent,  is 
sufficient  to  change  the  time  of  setting  from  a  few  minutes  to  several 
hours.  Cement  which  has  been  made  slow-setting  by  the  addition  of 
sulphate  of  lime,  usually  becomes  quick-setting  again  after  exposure 
to  the  air;  cement  which  has  not  had  its  time  of  setting  changed 
by  the  addition  of  sulphate  of  lime,  usually  becomes  slower  setting 
with  age  and  may  finally  lose  the  power  of  setting.  Cement  which 
has  become  slow-setting  by  the  addition  of  sulphate  of  lime  will 
become  quick-setting  if  mixed  with  a  solution  of  carbonate  of 
soda. 

A  weak  solution  of  chloride  of  lime  usually  causes  the  cement 
to  set  more  slowly;  while  a  strong  solution  usually  accelerates  the 
rate  of  setting. 

88.  Time  of  Set.  A  few  of  the  quickest  natural  cements  when 
tested  neat  with  the  minimum  of  water  will  begin  to  set  in  5  to  10 
minutes,  and  set  hard  in  15  to  20  minutes;  while  the  majority  will 
begin  to  set  in  20  to  30  minutes  and  will  set  hard  in  40  to  60 
minutes;  and  a  few  of  the  slowest  will  not  begin  to  set  under  60 
minutes. 

The  quickest  of  the  Portlands  will  begin  to  set  in  20  to  40  min- 
utes; but  the  majority  will  not  begin  to  set  under  2  or  3  hours, 
and  will  not  set  hard  under  6  or  8  hours.  The  1887  standard 
German  specifications  reject  a  Portland  cement  which  begins  to 
set  in  less  than  30  minutes  or  which  sets  hard  in  less  than 
3  hours. 

89.  Soundness.  Soundness  refers  to  cue  ability  of  a  cement  to 
retain  its  strength  and  form  unimpaired  for  an  indefinite  period. 
Soundness  is  a  most  important  element;  since  if  a  cement  ultimately 
loses  its  strength  it  is  worthless,  and  if  it  finally  expands  it  becomes 
a  destructive  agent.  A  cement  may  be  unsound  because  of  the 
presence  in  it  of  some  active  elements  which  cause  the  mortar  to 
expand  or  contract  in  setting,  or  the  unsoundness  may  be  due  to 
exterior  agencies  which  act  upon  the  ingredients  of  the  cement. 
Most  unsound  cements  fail  by  swelling  and  cracking  under  the 
action  of  expansives;  but  sometimes  the  mortar  fails  by  a  gradual 
softening  of  the  mass  without  material  change  of  form.  The  ex- 
pansive action  is  usually  due  to  free  lime  or  free  magnesia  in  the 
cement,  but  may  be  caused  by  sulphur  compounds.     The  principal 


ART.  4.]  TESTS    OF    CEMENT.  61 

exterior  agencies  acting  npon  a  cemeut  are  air,  sea-water,  and 
•extremes  of  heat  and  cold. 

The  presence  of  small  quantities  of  free  lime  in  the  cement  is  a 
frequent  cause  of  unsoundness.  The  lime  slakes,  and  causes  the 
jnortar  to  swell  and  crack — and  perhaps  finally  disintegrate.  The 
degree  of  heat  employed  in  the  burning,  and  the  fineness,  modify 
the  eSect  of  the  free  lime.  Lime  burned  at  a  high  heat  slakes 
more  slowly  than  when  burned  at  a  low  temperature,  and  is  there- 
fore more  likely  to  be  injurious.  Finely  ground  lime  slakes  more 
quickly  than  coarsely  ground,  and  hence  with  fine  cement  the  lime 
may  slake  before  the  cement  has  set,  and  therefore  do  no  harm. 
The  lime  in  finely  ground  cements  will  air-slake  sooner  than  that  in 
coarsely  ground. 

Free  magnesia  in  cement  acts  very  much  like  free  lime.  The 
action  of  the  magnesia  is  much  slower  than  that  of  lime,  and  hence 
its  presence  is  a  more  serious  defect,  since  it  is  less  likely  to  be 
detected  before  the  cement  is  used.  The  effect  of  magnesia  in 
cement  is  not  thoroughly  understood,  but  seems  to  vary  with  the 
composition  of  the  cement,  the  degree  of  burning,  and  the  amount 
of  water  used  in  mixing.  It  was  formerly  held  that  1^  or  2  per 
cent,  of  magnesia  in  Portland  cement  was  dangerous;  but  it  is  now 
known  that  5  per  cent,  is  not  injurious,  while  8  per  cent,  may  pro- 
duce expansion.  Since  many  of  the  natural  cements  are  made  of 
magnesium  limestone,  they  contain  much  more  magnesia  than 
Portland  cements;  but  chemists  are  not  agreed  as  to  the  manner  in 
which  the  different  constituents  are  combined,  and  consequently  are 
not  agreed  either  as  to  the  amount  or  effect  of  free  magnesia  in  such 
a  cement.  Fortunately,  it  is  not  necessary  to  resort  to  a  chemical 
analysis  to  determine  the  amount  of  lime  or  magnesia  present,  for 
a  cement  which  successfully  stands  the  ordinary  test  for  soundness 
(§  92)  for  7,  or  at  most  28  days,  may  be  used  with  reasonable  con- 
fidence. 

The  effect  of  lime  and  magnesia  seems  to  be  more  serious 
in  water  than  in  air,  and  greater  in  sea-water  than  in  fresh 
water. 

90.  The  action  of  sulphur  in  a  cemeut  is  extremely  variable, 
depending  upon  the  state  in  which  it  may  exist  and  upon  the 
nature  of  the  cement.  Sulphur  may  occur  naturally  in  the  cement 
or  may  be  added  in  the  form  of  sulphate  of  lime  (plaster  of  Paris) 


62  LIME    AND    CEMENT.  [CHAP.   III. 

to  retard  the  time  of  set  (§  87).  Under  certain  conditions  the 
sulphur  may  form  sulphides,  which  on  exposure  to  the  air  oxidize 
and  form  sulphates  and  cause  the  mortar  to  decrease  in  strength. 
Many,  if  not  all,  of  the  slag  cements  contain  an  excess  of  sulphides, 
and  are  therefore  unfit  for  use  in  the  air,  particularly  a  very  dry 
atmosphere,  although  under  water  they  may  give  satisfactory  results 
and  compare  favorably  with  Portland  cement. 

91.  Tests  of  Soundness.  Several  methods  of  testing  soundness 
have  been  recommended.  Of  those  mentioned  below,  the  first  two 
are  called  cold  tests,  since  the  mortar  is  tested  at  ordinary  tempera- 
tures; and  the  others  accelerated  or  hot  tests. 

92.  The  Pat  Test.  The  ordinary  method  of  testing  soundness 
is  to  make  small  cakes  or  pats  of  neat  mortar  3  or  4  inches  in 
diameter,  about  half  an  inch  thick  and  having  thin  edges,  upon  a 
sheet  of  glass,  and  examine  from  day  to  day,  for  28  days  (if 
possible),  to  see  if  they  show  any  cracks  or  signs  of  distortion. 
The  amount  of  water  used  in  mixing  (see  §  104)  within  reason- 
able limits  seems  to  have  no  material  effect  on  the  result.  The 
German  standard  specifications  require  the  cake  to  be  kept  24 
hours  in  a  closed  box  or  under  a  damp  cloth,  and  then  stored  in 
water.  The  French,  to  make  sure  that  the  pats  do  not  get  dry 
before  immersion,  recommend  that  the  cakes  be  immersed  immedi- 
ately after  mixing  without  waiting  for  the  mortar  to  set.  Some 
really  sound  natural  cements  will  disintegrate  if  immersed  before 
setting  has  begun. 

The  first  evidence  of  bad  quality  is  the  loosening  of  the  pat  from 
the  glass,  which  generally  takes  place,  if  at  all,  within  one  or  two 
days.  Good  cement  will  remain  firmly  attached  to  the  glass  for  two 
weeks  at  least.  The  cracks  due  to  expansion  occur  usually  at  the 
edges  of  the  pat,  and  radiate  from  the  center.  These  cracks  should 
not  be  confused  with  irregular  hair-like  shrinkage  cracks,  which 
appear  over  the  entire  surface  when  the  pats  are  made  too  wet  and 
dry  out  too  much  while  setting. 

93.  A  cement  high  in  sulphides,  as  for  example  one  made  of 
blast-furnace  slag,  will  successfully  pass  the  above,  the  usual,  test 
for  soundness;  and  still  the  mortar  when  exposed  in  the  air  will 
show  a  marked  decrease  in  strength  and  perhaps  finally  dis- 
integrate. The  presence  of  an  excess  of  sulphides  may  be  sus- 
pected  in   any   cement   made   from    blast-furnace   slag.     A    slag 


AKT.  4.]  TESTS   OF   CEMENT.  63 

cement  is  indicated  by  a  manve  or  delicate  lilac  tint  of  the  dry- 
powder. 

Therefore,  in  making  the  pat  test,  it  is  wise  to  expose  a  pat  in 
the  air  as  well  as  one  under  water.  Any  sulphides  in  the  cement 
will  be  revealed  by  brown  or  yellowish  blotches  on  the  pat  exposed 
in  air,  and  also  by  a  greenish  color  of  the  interior  of  the  pat  exposed 
under  water.  The  pat  in  air  is  not  as  good  a  test  of  expansives  as 
the  pat  under  water,  owing  to  a  possible  deficiency  of  water  and  to 
greater  shrinkage  cracks. 

If  there  are  any  considerable  indications  of  sulphides,  before 
accepting  the  cement  a  chemical  analysis  should  be  made  to  deter- 
mine the  sulphur  and  the  probable  ultimate  action  of  the  cement. 
Any  cement  containing  sulphides  in  appreciable  quantities  is  at 
least  doubtful  and  should  probably  be  rejected.  Slag  cements 
usually  contain  1  to  1.5  per  cent,  of  sulphides. 

Another  excellent  method  of  examining  for  the  presence  of  sul- 
phides is,  in  making  the  test  for  tensile  strength  (§g  99-111^),  to 
store  part  of  the  briquettes  in  air  and  part  in  water.  Any  material 
difference  in  strength  between  the  two  lots  is  sufficient  ground  for 
rejecting  the  cement  for  use  in  a  dry  place.  Of  course  due  con- 
sideration should  be  given  to  the  possible  effect  of  evajaoration  of 
water  from  the  briquettes  stored  in  air. 

94.  Expansion  Test.  Various  experimenters  test  the  soundness 
of  cement  by  measuring  the  expansion  of  a  bar  of  cement  mortar. 
The  French  Commission  recommend  the  measurement  of  the  expan- 
sion of  a  bar  32  inches  long  by  \  inch  square,  or  the  measurement 
of  the  increase  of  circumference  of  a  cylinder.  The  German 
standard  tests  require  the  measurement  of  the  increase  in  length  of 
a  prism  4  inches  long  by  2  inches  square.  The  apparatus  for 
making  these  tests  can  be  had  in  the  market.  The  tests  require 
very  delicate  manipulation  to  secure  reliable  results. 

95.  Accelerated  Tests.  The  ordinary  tests  extending  over  a 
reasonable  period,  sometimes  fail  to  detect  unsoundness;  and  many 
efforts  have  been  made  to  utilize  heat  to  accelerate  the  action,  with 
a  view  of  determining  from  the  effect  of  heat  during  a  short  time 
what  would  be  the  action  in  a  longer  period  under  normal  condi- 
tions. Some  of  these  tests  have  been  fairly  successful,  but  none 
have  been  extensively  employed.  It  is  difficult  to  interpret  the 
tests,  as  the  results  vary  with  the  per  cent,  of  lime,  magnesia,  sul- 


64  LIME   AND    CEMENT.  [CHAP.  III. 

phates,  etc.,  present,  and  with  their  proportions  relative  to  each 
other  and  to  the  whole.  There  is  a  great  diversity  as  to  the  value 
of  accelerated  tests.  Many  natural  cements  which  go  all  to  pieces 
in  the  accelerated  tests,  particularly  the  boiling  test,  still  stand 
well  in  actual  service.  This  is  a  strong  argument  against  drawing 
adverse  conclusions  from  accelerated  tests  when  applied  to  Portland 
■-cement. 

The  warm-water  test,  proposed  by  Mr.  Faija,*  a  British 
authority,  is  made  with  a  covered  vessel  partly  full  of  water 
maintained  at  a  temperature  of  100°  to  115°  F.,  in  the  upper 
part  of  which  the  pat  is  placed  until  set.  When  the  pat  is 
set,  it  is  placed  in  the  water  for  24  hours.  If  the  cement  remains 
firmly  attached  to  the  glass  and  shows  no  cracks,  it  is  very  probably 
sound. 

The  liot-water  test,  proposed  by  Mr.  Maclay,f  an  American 
authority,  is  substantially  like  Faija's  test  above,  except  that 
Maclay  recommends  195°  to  200°  F. 

The  hoiliiig  test,  suggested  by  Professor  Tetmajer,  the  Swiss 
authority,  consists  in  placing  the  mortar  in  cold  water  immediately 
after  mixing,  then  gradually  raising  the  temperature  to  boiling 
after  about  an  hour,  and  boiling  for  three  hours.  The  test 
specimen  consists  of  a  small  ball  of  such  a  consistency  that  when 
flattened  to  half  its  diameter  it  neither  cracks  nor  runs  at  the 
-edges. 

The  kiln  tests  consist  of  exposing  a  small  cake  of  cement 
mortar,  after  it  has  set,  to  a  temperature  of  110°  to  120°  C. 
(166°  to  248°  F.)  in  a  drying  oven  until  all  the  water  is  driven  off. 
If  no  edge  cracks  appear,  the  cement  is  considered  of  constant 
volume. 

The  Jiarne  test  is  made  by  placing  a  ball  of  the  cement  paste, 
about  2  inches  in  diameter,  on  a  wire  gauge  and  applying  the 
flame  of  a  Bunsen  burner  gradually  until  at  the  end  of  an  hour  the 
temperature  is  about  90°  C.  (194°  F.).  The  heat  is  then  in- 
creased until  the  lower  part  of  the  ball  becomes  red-hot.  The 
appearance  of  cracks  probably  indicates  the  presence  of  an  expansive 
element. 


♦Trans.  Am.  Soe.  of  C.  E.,  vol.  xvii.  p.  223;  vol.  xxx.  p.  57. 
f  Trans.  Am.  Soc.  of  C.  E.,  vol.  xxvii.  p.  412. 


ART.  4. J  TESTS    OF    CEMENT.  65 

The  cliloride-of-Ume  test  is  to  mix  the  paste  for  the  cakes  with 
a  solution  of  40  grammes  of  calcium  chloride  per  liter  of  water, 
allow  to  set,  immerse  in  the  same  solution  for  24  hours,  and  then 
examine  for  checking  and  softening.  The  chloride  of  lime  accel- 
erates the  hydration  of  the  free  lime.  The  chloride  in  the  solution 
used  in  mixing  causes  the  slaking  before  setting  of  only  so  much  of 
the  free  lime  as  is  not  objectionable  in  the  cement.  The  chloride 
of  calcium  has  no  effect  upon  free  magnesia. 

96.  Fineness.  The  question  of  fineness  is  wholly  a  matter  of 
economy.  Cement  until  ground  is  a  mass  of  partially  vitrified 
clinker,  which  is  not  affected  by  water,  and  which  has  no  setting 
power.  It  is  only  after  it  is  ground  that  the  addition  of  water 
induces  crystallization.  Consequently  the  coarse  particles  in  a 
cement  have  no  setting  power  whatever,  and  may  for  practical 
purposes  be  considered  as  so  much  sand  and  essentially  an  adul- 
terant. 

There  is  another  reason  why  cement  should  be  well  ground.  A 
mortar  or  concrete  being  composed  of  a  certain  quantity  of  inert 
material  bound  together  by  cement,  it  is  evident  that  to  secure  a 
strong  mortar  or  concrete  it  is  essential  that  each  piece  of  aggregate 
shall  be  entirely  surrounded  by  the  cementing  material,  so  that  no 
two  pieces  are  in  actual  contact.  Obviously,  then,  the  finer  a 
cement  the  greater  surface  will  a  given  weight  cover,  and  the  more 
economy  will  there  be  in  its  use. 

Fine  cement  can  be  produced  by  the  manufacturers  in  three 
ways:  1,  by  supplying  the  mill  with  comparatively  soft,  under-burnt 
rock,  which  is  easily  reduced  to  powder;  2,  by  more  thorough 
grinding;  or  3,  by  bolting  through  a  sieve  and  returning  the 
ungronnd  particles  to  the  mill.  The  first  process  produces  an  in- 
ferior quality  of  cement,  while  the  second  and  third  add  to  the  cost 
of  manufacture. 

It  is  possible  to  reduce  a  cement  to  an  impalpable  powder,  but 
the  proper  degree  of  fineness  is  reached  when  it  becomes  cheaper  to 
use  more  cement  in  proportion  to  the  aggregate  than  to  pay  the 
extra  cost  of  additional  grinding. 

97.  Measuring  Fineness.  The  degree  of  fineness  is  determined 
by  weighing  the  per  cent,  which  will  not  pass  through  sieves  of  a 
specified  number  of  meshes  per  square  inch.  In  the  past,  three 
sieves  have  been  used  for  this  purpose,  viz.,  sieves  having  50,  75, 


66  LIME   AND    CEMENT.  [CHAP.  III.. 

and  100  meshes  per  linear  inch,  or  2,500,  5,625,  and  10,000  meshes 
per  square  inch  respectively.  These  sieves  are  usually  referred  to 
by  the  number  of  meshes  per  linear  inch,  the  first  being  known  as 
No.  50,  the  second  as  No.  75,  and  the  third  as  No.  100.  In  each 
case  the  diameter  of  the  mesh  is  about  equal  to  that  of  the  wire. 
The  per  cent,  left  on  the  coarser  sieves  has  no  special  significance, 
and  hence  the  use  of  more  than  one  sieve  has  been  almost  aban- 
doned. More  recently  in  this  country  a  No.  120  sieve  (14,400 
meshes  per  square  inch)  has  been  employed,  and  sometimes  a 
No.  200.  On  the  continent  of  Europe  the  sieve  generally  nsed  has 
70  meshes  per  linear  centimetre,  corresponding  to  175  meshes  per 
linear  inch  (30,625  per  square  inch). 

98.  Data  on  Fineness.  Nearly  all  Portland  cements  are  so 
ground  as  not  to  leave  more  than  20  per  cent,  on  a  No.  100  sieve, 
and  many  of  them  will  not  leave  more  than  10  per  cent,  on  a 
No.  100  sieve  or  more  than  20  per  cent,  on  a  No.  200  sieve;  and 
some  manufacturers  claim  less  than  10  per  cent,  on  a  No.  200  sieve. 
As  a  rule,  American  Portlands  are  finer  ground  than  German,  and 
German  finer  than  English. 

Most  of  the  natural  cements  are  usually  ground  so  as  to  give 
not  more  than  20  per  cent,  on  the  No.  100  sieve,  and  many  of  them 
will  not  leave  more  than  10  per  cent,  on  the  No.  100  sieve,  and  a 
few  will  leave  only  10  per  cent,  on  the  No.  200  sieve. 

A  common  sj^ecification  is  that  not  more  than  10  per  cent,  shall 
be  left  on  a  No.  50  sieve.  Such  a  test  simply  prevents  the  adultera- 
tion of  the  cement  with  very  coarse  particles,  but  does  not  insure 
any  considerable  proportion  of  impalpable  powder  (approximately 
that  which  will  pass  a  No.  200  sieve),  which  alone  gives  value  to 
the  cement.* 

Since  the  natural  cement  is  not  so  hard  burned  as  the  Portland, 
there  is  more  impalpable  powder  in  proportion  to  the  per  cent,  left 
on  the  test  sieve  than  with  the  Portland;  and  consequently  a  severe 
test   for   fineness   is   not  as  important  for  natural  cement  as  for 

♦There  has  recently  been  introduced  an  article  called  sand-cement,  which  is 
made  by  mixing  cement  and  silica  sand  and  grinding  the  mixture.  The  grind  ■ 
ing  of  the  mixture  greatly  increases  the  fineness  of  the  cement.  A  mixture  of 
1  part  cement  and  3  parts  silica  sand  when  reground  will  carry  nearly  as  much 
sand  as  the  original  pure  cement,  which  shows  the  striking  effect  of  the  very 
flue  grinding  of  the  cement. 


.ART.  4.]  TESTS   OF    CEMENT.  67 

Portland.  Farther,  since  natural  cement  is  much  cheaper  than 
Portland,  it  is  more  economical  to  use  more  cement  than  to 
require  extra  fineness.  Again,  since  natural  cement  is  weaker, 
it  is  not  ordinarily  used  with  as  large  a  proportion  of  sand  as 
Portland,  and  hence  fineness  is  not  as  important  with  natural  as  with 
Portland. 

Por  various  specifications  for  fineness,  see  Art.  5,  pages  7Sd- 
78h,  particularly  Tables  10c  and  lOd,  jjages  78/",  7Sg. 

99.  Tensile  Strength.  The  strength  of  cement  mortar  is 
usually  determined  by  submitting  a  specimen  having  a  cross  section 
•of  1  square  inch  to  a  tensile  stress.  The  reason  for  adopting  tensile 
tests  instead  of  compressive  is  the  greater  ease  of  making  the  former 
and  the  less  variation  in  the  results.  Mortar  is  eight  to  ten  times 
as  strong  in  compression  as  in  tension. 

The  accurate  determination  of  the  tensile  strength  of  cement  is 
a  much  less  simple  process  than  at  first  appears.  Many  things, 
apparently  of  minor  importance,  exert  such  a  marked  influence  upon 
the  results  that  it  is  only  by  the  greatest  care  that  trustworthy  tests 
can  be  made.  The  variations  in  the  results  of  different  experienced 
operators  working  by  the  same  method  and  upon  the  same  material 
are  frequently  very  large.  In  one  particular  test  case,*  the  lowest 
of  nine  results  was  but  30  per  cent,  of  the  highest,  the  remainder 
being  evenly  distributed  between  the  two  extremes.  Similar  varia- 
tions are  not  at  all  unusual.  The  variation  is  chiefly  due  to  differ- 
ences in  making  the  test  specimen.  Unfortunately,  there  is  at 
present  no  detailed  standard  method  of  procedure  in  making  the 
tests,  and  consequently  all  that  can  be  done  is  to  observe  with  the 
most  conscientious  care  the  rules  that  have  been  formulated,  and 
draw  the  specifications  in  accordance  with  the  personal  equation  of 
the  one  to  make  the  tests. 

100.  Neat  vs.  Sand  Tests,  It  is  very  common  to  test  neat-cement 
mortar,  but  there  are  two  serious  objections  to  this  practice.  First, 
most  neat  cements  decrease  in  tensile  strength  after  a  time.  This 
decrease  seems  to  be  due  to  a  change  in  the  molecular  structure  of 
the  cement,  the  crystals  growing  larger  with  increase  of  age,  thus 
producing  a  crowding  which  results  in  a  decrease  of  the  tensile 
strength.    This  decrease  is  most  marked  with  high-grade  Portlands 


*  Engineen'ing  Xeics,  vol.  xxxv.  pp.  150-51. 


68  LIME    AND    CEMENT.  [CHAP.  III. 

which  attain  their  strength  rapidly,  and  usually  occurs  between 
three  months  and  a  year.  A  second  objection  to  neat  tests  is  that 
coarsely-ground  cements  show  greater  strength  than  finely-ground 
cements,  although  the  latter  mixed  with  the  usual  proportion  of 
sand  will  give  the  greater  strength. 

On  the  other  hand,  more  skill  is  required  to  secure  uniform 
results  with  sand  than  with  neat  cement. 

101.  The  Sand.  The  quality  of  the  sand  employed  is  of  great 
importance,  for  sands  looking  alike  and  sifted  through  the  same 
sieve  give  results  varying  30  to  40  per  cent. 

The  standard  sand  employed  in  the  official  German  tests  is  a 
natural  quartz  sand  obtained  at  Freienwalde  on  the  Oder,  passing 
a  sieve  of  GO  meshes  per  square  centimetre  (20  per  linear  inch)  and 
caught  upon  a  sieve  of  120  meshes  per  square  centimetre  (2S  per 
linear  inch).  The  standard  "sand"  recommended  by  the  Com- 
mittee of  the  American  Society  of  Civil  Engineers  is  crushed  quartz, 
used  in  the  manufacture  of  sand-paper,  which  passes  a  No.  20  sieve 
(wire  No.  28  Stubs's  gauge)  and  is  caught  on  a  No.  30  sieve  (wire 
No.  30  Stubs's  gauge),  the  grains  being  from  0.03  to  0.02  inch  in. 
diameter. 

The  crushed  quartz  consists  of  sharp,  glossy  splinters,  while  the 
standard  German  sand  is  composed  of  nearly  spherical  grains  having 
a  rough  surface  like  ground  glass.  The  quartz  contains  about  50 
per  cent,  of  voids,  while  the  German  standard  sand  contains  only 
about  40  (see  Table  10^,  page  79r.)  The  crushed  quartz  will  give 
less  strength  than  standard  sand.  Ordinarily  common  building  sand 
will  give  a  higher  strength  than  standard  sand,  since  usually  the 
former  consists  of  grains  having  a  greater  variety  of  sizes,  and  con- 
sequently there  are  fewer  voids  to  be  filled  by  the  cement  (see  Table 
lOg,  page  79i.) 

102.  The  Amount  of  Water.  The  amount  of  water  necessary 
to  make  the  strongest  mortar  varies  with  each  cement.  It  is  com- 
monly expressed  in  per  cents,  by  weight,  although  in  part  at  least 
it  depends  upon  volume.  The  variation  in  the  amount  of  water 
required  depends  upon  the  degree  of  fineness,  the  specific  gravity, 
the  weight  per  unit  of  volume,  and  the  chemical  composition.  If 
the  cement  is  coarsely  ground,  the  voids  are  less,  and  consequently 
the  volume  of  water  required  is  less.  If  the  specific  gravity  of  one 
cement  is  greater  than  that  of  another,  equal  volumes  of  cement 


ART.  4.]  TESTS   OF    CEMENT,  69 

will  require  different  volumes  of  water.  The  chemical  compositioa 
has  the  greatest  inflneuce  upon  the  amount  of  water  necessary. 
Part  of  the  water  is  required  to  combine  chemically  with  the  cement, 
and  part  acts  physically  in  reducing  the  cement  to  a  plastic  mass; 
and  the  portion  required  for  each  of  these  effects  differs  with  differ- 
ent cements.  The  dryness  and  porosity  of  the  sand  may  also 
appreciably  affect  the  quantity  of  water  required.  The  finer  the 
sand,  the  greater  the  amount  of  water  required.  Again,  the  same 
consistency  may  be  arrived  at  in  two  ways — by  using  a  small  quan- 
tity of  water  and  working  thoroughly,  or  by  using  a  larger 
quantity  and  working  less.  (For  instructions  concerning  mixing, 
see  §  106). 

Attempts  have  been  made  to  establish  a  standard  consistency, 
but  there  is  no  constant  relation  between  the  consistency  and  the 
maximum  strength.  With  one  cement  a  particular  consistency  may 
give  maximum  strength,  while  with  another  cement  a  different  con- 
sistency may  be  required  to  develop  the  greatest  strength.  The 
relationship  between  consistency  and  strength  will  vary  also  with 
the  details  of  the  experiment.  In  reporting  the  results  of  tests  the 
quantity  of  water  employed  should  be  stated. 

There  are  two  distinct  standards  of  consistency  for  the  mortar 
employed  in  testing  cements, — the  plastic  and  the  dry. 

103.  Plastic  Mortar.  This  grade  of  mortar  is  that  com- 
monly employed  in  the  United  States  and  England,  and  is  fre- 
quently used  in  France.*  There  are  two  methods  of  identifying 
this  degree  of  consistency,  viz. :  the  Tetmajer  method  and  the 
Boulogne  method.  The  Tetmajer  method  requires  more  water 
than  the  Boulogne  method — for  Portland  this  excess  is  about  3 
per  cent,  of  the  weight  of  the  cement,  and  for  natural  about  5 
per  cent. 

The  Tetmajer  method  is  much  used  on  the  continent  of  Europe. 
It  is  as  follows:  The  plasticity  shall  be  such  that  a  rod  0.4  of  an 
inch  in  diameter  and  weighing  0.G6  pounds  will  penetrate  1.25 
inches  into  a  box  3  inches  in  diameter  and  1.57  inches  deep,  filled 
with  the  mortar,  f 

The  Boulogne  method  is  frequently  used  in  France.     It  is  as 

*  See  foot  note,  page  71. 

t  For  an  illustration  of  the  apparatus,  see  Trans,  Amer.  Soc,  of  C.  E.,  vol.  xxx. 
p,ll. 


70  LIME   AND    CEME]srT.  [CHAP.  III. 

follows:  *  "  The  quantity  of  water  is  ascertained  by  a  preliminary 
experiment.  It  is  recommended  to  commence  with  a  rather  smaller 
quantity  of  water  than  may  be  ultimately  required,  and  then  to 
make  fresh  mixings  with  a  slight  additional  quantity  of  water. 
The  mortar  is  to  be  vigorously  worked  for  five  minutes  with  a  trowel 
on  a  marble  slab  to  bring  it  to  the  required  consistency,  after  which 
the  four  following  tests  are  to  be  applied  to  determine  whether  the 
proportion  of  water  is  correct:  1.  The  consistency  of  the  mortar 
should  not  change  if  it  be  ganged  for  an  additional  period  of  three 
miuutes  after  the  initial  five  minutes.  2.  A  small  quantity  of  the 
mortar  dropped  from  the  trowel  upon  the  marble  slab  from  a  height 
of  about  0.50  metres  (20  inches)  should  leave  the  trowel  clean,  and 
retain  its  form  approximately  without  cracking.  3.  A  small  quan- 
tity of  the  mortar  worked  gently  in  the  hands  should  be  easily 
moulded  into  a  ball,  on  the  surface  of  which  water  should  appear. 
When  this  ball  is  dropped  from  a  height  of  0.50  metres  (20  inches), 
it  should  retain  a  rounded  shaj^e  without  cracking.  4.  If  a  slightly 
smaller  quantity  of  water  be  used,  the  mortar  should  be  crumbly, 
and  crack  when  dropped  upon  the  slab.  On  the  other  hand,  the 
addition  of  a  further  quantity  of  water — 1  to  2  per  cent,  of  the 
weight  of  the  cement — would  soften  the  mortar,  rendering  it  more 
sticky,  and  preventing  it  from  retaining  its  form  when  allowed  to 
fall  upon  the  slab." 

104.  With  any  particular  cement  the  exact  amount  of  water  to 
produce  the  above  degree  of  plasticity  can  be  determined  only  by 
trial,  but  as  a  rule  the  quantity  required  by  the  Boulogne  method 
will  be  about  as  follows: 

For  neat  cement:  Portland,  23  to  25  per  cent.;  natural,  from 
30  to  40,  usually  from  32  to  36  per  cent. 

For  1  part  cement  to  1  part  sand:  Portland  cement,  13  to  15 
per  cent,  of  the  total  weight  of  cement  and  sand;  natural,  17  to 
20,  usually  18  to  19  per  cent. 

For  1  part  cement  to  2  parts  sand:  Portland,  12  to  13  per  cent, 
of  the  total  weight  of  the  sand  and  cement;  natural,  12  to  16, 
usually  13  to  15  per  cent. 

For  1  part  cement  to  3  parts  sand:  Portland,  11  to  12  per  cent. 

*  rrom  abstracts  of  lust,  of  C.  E. 


ART.  4.]  TESTS   OF   CEMENT.  71 

of  the  total  weight  of  the  sand  and  cement;  natural,  12  to  13  per 
cent. 

105.  Dry  Mortar.  This  grade  of  mortar  is  employed  in  the 
German  and  French  *  governmental  tests  of  tensile  strength.  The 
rules  for  the  identification  of  this  degree  of  consistency  are  not  very 
specific.  "  Dry  mortars  "  are  usually  described  as  being  "  as  damp 
as  moist  earth." 

The  German  government  does  not  recognize  tensile  tests  of  neat 
cement  mortar;  but  for  1  to  3  sand  mortars  specifies  that  the  weight 
of  water  irsed  for  Portland  cement  shall  be  equal  to  10  per  cent,  of 
the  total  weight  of  the  sand  and  cement. 

The  French  Commission  gives  a  rule  f  for  1  to  2,  1  to  3,  and 
1  to  5  mortars,  with  either  Portland  or  natural  cement,  which  is 
equivalent  to  the  following  formula: 

w  =  ^WR  +  45, 

in  which  w  =  the  weight,  in  grammes,  of  water  required  for 
1,000  grammes  of  the  sand  and  cement; 

W  =  the  weight,  in  grammes,  of  water  required  to  re- 
duce 1,000  grammes  of  neat  cement  to  plastic 
mortar  (see  §  104); 

H  =  the  ratio  of  the  weight  of  the  cement  to  the  weight 
of  the  sand  and  cement. 

For  a  1  to  3  mortar  the  preceding  formula  gives  8.5  per  cent., 
which  seems  to  show  that  the  French  standard  requires  less  water 
than  the  German. 

The  cement  laboratory  of  the  city  of  Philadelphia  employs  the 
above  formula,  but  uses  GO  for  the  constant  instead  of  45.  For  a 
1  to  3  mortar,  the  Philadelphia  formula  gives  10  per  cent.,  which 
agrees  with  the  German  standard. 

106.  Mixing  the  Mortar.  The  sand  and  cement  should  be 
thoroughly  mixed  dry,  and  the  water  required  to  reduce  the  mass 
to  the  proper  consistency  should  be  added  all  at  once.     The  mixing 

*  The  French  Commission  recommends  dry  mortar  for  tensile  tests  only ;  and 
also  recommends  that,  after  an  international  agreement  to  that  effect,  plastlo 
mortars  be  employed  for  all  tests  to  the  exclusion  of  dry  mortars. 

t  Carter  and  Gieseler's  Conclusions  adopted  by  the  French  Commission  iu 
reference  to  Tests  of  Cements,  p.  21. 


72 


LIME    AND    CEMENT. 


[chap.  III. 


should  be  prompt  and  tliorougli.  The  mass  should  not  be  simply 
turned,  but  the  mortar  should  be  rubbed  against  the  top  of  the 
slate  or  glass  mixing-table  with  a  trowel,  or  in  a  mortar  with  a 
pestle.  Insufficient  working  greatly  decreases  the  strength  of  the 
mortar — frequently  one  half.  The  inexperienced  operator  is  very 
liable  to  use  too  much  water  and  too  little  labor.  With  a  slow- 
setting  cement  a  kilogramme  of  the  dry  materials  should  be  strongly 
and  rapidly  rubbed  for  not  less  than  5  minutes,  when  the  consist- 
ency should  be  such  that  it  will  not  be  changed  by  an  additional 
mixing  for  3  minutes. 

Usually  the  mortar  is  mixed  with  a  trowel  on  a  stone  slab;  but 
when  many  batches  are  required,  there  is  a  decided  advantage  in 
mixing  the  mortar  with  a  hoe  in  a  short  Y-shaped  trough  on  the 
floor.  Various  machines  have  been  devised  with  which  to  mix  the 
mortar.  The  jig  mixer*  is  an  apparatus  in  which  the  materials  are 
placed  in  a  covered  cup,  and  shaken  rapidly  up  and  down.  The 
Faija   mixer  f    consists  of   a   cylindrical    pan    in    which    a    mixer 

formed    of   four   blades   revolves.      The 

rr^-j^ZSlTT *'  latter  seems  to  give  the   better   result, 

bnt  neither  are  used  to  any  considerable 
extent. 

107.  The  Form  of  Briquette.  The 
)riqnette  recommended  by  the  Committee 
of  the  American  Society  of  Civil  En- 
gineers, Fig.  2,  is  the  form  ordinarily 
used  in  this  country  and  in  England. 
The  form  generally  emjiloyed  in  con- 
tinental Europe  is  somewhat  similar  to 
the  above,  except  that  the  section  is  5 
square  centimetres  (0.8  square  inch)  and 
the  reduction  to  produce  the  minimum 
^^  section    is    by  very  much  more   abrupt 

curves.  J     The  latter  form  gives  only  70 
to  80  per  cent,  as  much  strength  as  the  former. 


*  For  illustrated  description,  see  Trans.  Anaer.  Soc.  of  C.  E.,  vol.  xxv.  p.  300-1. 

t  For  British  form,  see  Trans.  Am.  Soc.  of  C.  E.,  vol.  xvii.  p.  223 ;  and  for  the 
American  form,  see  catalogue  of  Eiehle  Bros.  Testing  Machine  Co.,  Philadelphia. 

J  For  an  elaborate  discussion  of  the  best  form  of  briquette,  see  Johnson's 
Materials  of  Construction,  p.  432-38. 


ART.  4.]  TESTS   OF   CEMENT.  73 

The  moulds  are  made  of  brass  and  are  single  or  multiple,  the 
latter  being  preferred  where  a  great  number  of  briquettes  is  required. 
The  moulds  are  in  two  parts,  to  facilitate  removal  from  the 
briquette  without  breaking  it. 

108.  Moulding  the  Briquette.  In  moulding  the  briquette  there 
are  two  general  methods  employed,  corresponding  to  the  two  stand- 
ard consistencies  of  the  mortar. 

109.  Plastic  Mortar.  The  rules  of  this  section  (109)  apply  to 
liand-moulding . 

The  Committee  of  the  American  Society  of  Civil  Engineers' 
recommendations  are  as  follows:  "  The  moulds  while  being  charged 
should  be  laid  directly  on  glass,  slate,  or  some  non-absorbing 
material.  The  mortar  should  be  firmly  pressed  into  the  moulds 
with  a  trowel,  without  ramming,  and  struck  off  level.  The  mould- 
ing must  be  completed  before  incipient  setting  begins.  As  soon  as 
the  briquettes  are  hard  enough  to  bear  it,  they  should  be  taken 
from  the  moulds  and  kept  covered  with  a  damp  cloth  until  they 
are  immersed." 

The  French  Commission  recommends  the  following  method :  * 
"  The  moulds  are  placed  upon  a  plate  of  marble  or  polished  metal 
which  has  been  well  cleaned  and  rubbed  with  an  oiled  cloth.  Six 
moulds  are  filled  from  each  ganging  if  the  cement  be  slow-setting, 
and  four  if  it  be  quick-setting.  Sufficient  material  is  at  once  placed 
in  each  mould  to  more  than  fill  it.  The  mortar  is  pressed  into  the 
mould  with  the  fingers  so  as  to  leave  no  voids,  and  the  side  of  the 
mould  tapped  several  times  with  the  trowel  to  assist  in  disengaging 
the  bubbles  of  air.  The  excess  of  mortar  is  then  removed  by  slid- 
ing a  knife-blade  over  the  top  of  the  mould  so  as  to  produce  no 
compression  upon  the  mortar.  The  briquettes  are  removed  from 
the  mould  when  sufficiently  firm,  and  are  allowed  to  remain  for  24 
hours  npon  the  plate  in  a  moist  atmosphere,  protected  from  currents 
of  air  or  the  direct  rays  of  the  sun,  and  at  a  nearly  constant  tem- 
perature of  15°  to  18°  C.  (59°  to  64.4°  F.)." 

110.  Various  machines  have  been  devised  for  moulding  bri- 
quettes of  plastic  mortar,  but  none  are  used  to  any  considerable 
extent,  t 

*  Carter  and  Gieseler's  Conclusions   adopted  by  the  French  Commission  in 
reference  to  Tests  of  Cements,  p.  23. 

+  For  an  illustrated  description  of  Russell's  lever  machine,  see  Trans.  Amer.  Soc. 


74  LIME   AND    CEMENT.  [CHAP.  III. 

In  Canada,  and  to  some  extent  in  England,  the  briquettes  are 
moulded  by  applying  a  pressure  of  20  pounds  per  square  inch  on  the 
snrface  of  the  briquette.*  Some  advocate  a  pressure  of  1,000  to 
1,500  pounds  upon  the  upper  face  of  the  briquette,  f 

111.  Dry  Mortar.  The  rules  of  this  section  (111)  are  for  liand- 
moulding. 

The  German  standard  rules  are:  \  "On  a  metal  or  thick  glass 
plate  five  sheets  of  blotting-paper  soaked  in  water  are  laid,  and  on 
these  are  placed  five  moulds  wetted  with  water.  250  grammes 
(8.75  oz.)  of  cement  and  750  grammes  (2G.25  oz.)  of  standard  sand 
are  weighed,  and  thoroughly  mixed  dry  in  a  vessel.  Then  100 
cubic  centimetres  (100  grammes  or  3.5  oz.)  of  fresh  water  are  added, 
and  the  whole  mass  thoroughly  mixed  for  five  minutes.  With  the 
mortar  so  obtained,  the  moulds  are  at  once  filled,  with  one  filling, 
so  high  as  to  be  rounded  on  top,  the  mortar  being  well  pressed  in. 
By  means  of  an  iron  trowel  5  to  8  centimetres  (1.96  inches  to  3.14 
inches)  wide,  35  centimetres  (13.79  inches)  long,  and  weighing 
about  250  grammes  (8.75  oz.),  the  projecting  mortar  is  pounded, 
first  gently  and  from  the  side,  then  harder  into  the  moulds,  until 
the  mortar  grows  elastic  and  water  flushes  to  the  surface.  A 
pounding  of  at  least  one  minute  is  absolutely  essential.  An  addi- 
tional filling  and  pounding  in  of  the  mortar  is  not  admissible,  since 
the  test  pieces  of  the  same  cement  should  have  the  same  densities 
at  the  different  testing  stations.  The  mass  projecting  over  the 
mould  is  now  cut  off  with  a  knife,  and  the  surface  smoothed.  The 
mould  is  carefully  taken  off  and  the  test  piece  placed  in  a  box  lined 
with  zinc,  which  is  to  be  provided  with  a  cover,  to  prevent  a  non- 
uniform drying  of  tlie  test  j^ieces  at  different  temperatures. 
Twenty-four  hours  after  being  made,  tlie  test  pieces  are  placed 
under  water,  and  care  must  be  taken  that  they  remain  under  water 
during  the  whole  period  of  hardening." 

The  French   Commission  recommend   the  following  for  sand 


of  C.  E.,  vol.  xxvii.  p.  441 ;  ditto  of  Jamieson's  lever  machine,  see  The  Transit 
(Iowa  State  University),  December,  1889,  or  Enginee7'ing  News,  vol.  xxv.  p.  138,  or 
Trans.  Amer.  Soc.  of  C.  E.,  vol.  xxv.  p.  302. 

*  Trans.  Canadian  Soc.  of  C.  E.,  vol.  ix.  p.  56,  "  Final  Report  of  the  Committee  on 
a  Standard  Method  of  Testing  Cements." 

f  Spalding's  Hydraulic  Cement,  p.  135. 

J  Engineering  News,  vol.  xvi.  p.  316. 


ART.  4.]  TESTS    OF    CEMENT.  75 

mortars:  "  Sufficient  mortar  is  gauged  at  once  to  make  six 
briquettes,  requiring  250  grammes  of  cement  and  750  grammes  of 
normal  sand.  The  mould  is  placed  upon  a  metal  plate,  and  upon 
top  of  it  is  fitted  a  guide  having  the  same  section  as  the  mould  and 
a  height  of  125  millimetres  (5  inches),  180  grammes  of  the  mortar 
are  introduced  and  rouglily  distributed  in  the  mould  and  guide  with 
a  rod.  By  means  of  a  metallic  pestle  weighing  1  kilogramme,  and 
having  a  base  of  the  form  of  the  briquette  but  of  slightly  less 
dimensions,  the  mortar  is  pounded  softly  at  first,  then  stronger  and 
stronger  until  a  little  water  escapes  under  the  bottom  of  the  mould. 
The  pestle  and  guide  are  then  removed  and  the  mortar  cut  off  level 
with  the  top  of  the  mould." 

Ilia.  The  Bohme  hammer  apparatus  is  much  used,  particularly 
in  Germany.  It  consists  of  an  arrangement  by  which  the  mortar 
is  compacted  in  the  mould  by  a  succession  of  blows  of  a  hammer 
weighing  2  kilogrammes  (4.4  pounds)  upon  a  plunger  sliding  in  a 
guide  placed  upon  top  of  the  mould.  The  machine  is  arranged  to 
lock  after  striking  150  blows.  A  high  degree  of  density  is  thus 
produced,  and  more  regular  results  are  obtained  than  by  hand. 
The  apparatus  is  slow.* 

The  Tetmajer  apparatus  f  is  similar  in  character  to  the  Bobme 
hammer.  "  It  consists  of  an  iron  rod  carrying  a  weight  upon  its 
lower  end,  which  is  raised  through  a  given  height  and  dropped  upon 
the  mortar  in  the  mould.  The  ram  weighs  3  kilogrammes.  This 
machine  is  used  in  the  Zurich  laboratory,  and  Prof.  Tetrtiajer  regu- 
lates the  number  of  blows  by  requiring  a  certain  amount  of  work 
to  be  done  upon  a  unit  volume  of  mortar, — 0.3  kilogrammetre  of 
work  per  gramme  of  dry  material  of  which  the  mortar  is  composed. 
This  apparatus  is  subject  to  the  same  limitations  in  practice  as  the 
Bohme  hammer,  in  being  very  slow  in  use  and  somewhat  expensive 
in  first  cost." 

lllh.  Storing  the  Briquettes.  It  is  usual  to  store  the  briquettes 
under  a  damp  cloth  or  in  a  moist  chamber  for  24  hours,  and  then 
immerse  in  water  at  a  temperature  of  60°  to  65°  F.  For  one-day 
tests,  the  briquettes  are  removed  from  the  moulds  and  immersed  as 

*  For  an  illustrated  description,  see  Engineering  News,  vol.  xvii.  p.  200 ;  Trans. 
Amer.  Soc.  of  C.  E.,  vol.  xxx.  p.  24. 

f  French  Commission's  Report,  vol.  i.  p.  287. 


76  LIME   AND    CEMENT.  [CHAP.  III. 

soon  as  they  have  began  to  set.  The  volume  of  the  water  should 
be  at  least  four  times  the  volnme  of  the  immersed  briquettes,  and 
the  water  should  be  renewed  every  seven  days. 

The  briquettes  should  be  labeled  or  numbered  to  preserve  their 
identity.  Neat-cement  briquettes  may  be  stamped  with  steel  dies, 
as  may  also  sand  briquettes,  provided  a  thin  layer  of  neat  cement  is 
spread  over  one  end  in  which  to  stamp  the  number. 

111c.  Age  when  Tested.  Since  in  many  cases  it  is  impracticable 
to  extend  the  tests  over  a  longer  time,  it  has  become  customary  to 
break  the  briquettes  at  one  and  seven  days.  This  practice,  together 
with  a  demand  for  high  tensile  strength,  has  led  manufacturers  to 
increase  the  proportion  of  lime  in  their  cements  to  the  highest 
possible  limit,  which  brings  them  near  the  danger-line  of  unsound- 
ness. A  high  strength  at  1  or  7  days  is  usually  followed  by  a 
decrease  in  strength  at  28  days.  Steadily  increasing  strength  at 
long  periods  is  better  proof  of  good  quality  than  high  results  during 
the  first  few  days.  The  German  standard  test  recognizes  only 
breaks  at  28  days.  The  French  standard  permits,  for  slow-setting 
cements,  tests  at  7  and  28  days,  and  3  and  6  months,  and  1,  2, 
etc.,  years;  and  for  rapid-setting  cements,  from  3  to  24  hours  for 
neat  mortar  and  24  hours  for  sand  mortars.  In  all  cases  the  time 
is  counted  from  the  instant  of  adding  the  water  when  mixing  the 
briquette.  The  briquettes  should  be  tested  as  soon  as  taken  from 
the  water. 

111(7.  The  Testing  Machine.  There  are  two  types  in  common 
use.  In  one  the  weight  is  applied  by  a  stream  of  shot,  which  runs 
from  a  reservoir  into  a  pail  suspended  at  the  end  of  the  steelyard 
arm;  when  the  briquette  breaks  the  arm  falls,  automatically  cutting 
off  the  flow  of  shot.  In  the  other  type,  a  heavy  weight  is  slowly 
drawn  along  a  graduated  beam  by  a  cord  wound  on  a  wheel  turned 
by  the  operator.  The  first  is  made  by  Fairbanks  Scale  Co. ,  and  the 
second  by  Riehle  Bros.,  and  also  by  Tinius  Olsen,  both  of  Phila- 
delphia. 

Fig.  3  represents  a  cement-testing  machine  which  can  be 
made  by  an  ordinary  mechanic  at  an  expense  of  only  a  few 
dollars.  Athough  it  does  not  have  the  conveniences  and  is  not 
as  accurate  as  the  more  elaborate  machines,  it  is  valuable  where 
the  quantity  of  work  will  not  warrant  a  more  expensive  one,  and 
in  many  cases  is  amply  sufficient.     It  was  devised  by  F.  W.  Bruce 


ART.  4.] 


TESTS   OF    CEMENT. 


for  use  at  Fort  Marion,  St.  Aagastine,  Fla.,  and  reported  to  the 
Engineering  News  (vol.  v.  pp.  104:-96)  by  Lientenant  "W.  M.  Black, 
U.  S.  A. 

The  machine  consists  essentially  of  a  counterpoised  wooden  lever 
10  feet  long,  working  on  a  horizontal  pin  between  two  broad 
uprights  20  inches  from  one  end.  Along  the  top  of  the  long  arm 
runs  a  grooved  wheel  carrying  a  weight.  The  distances  from  the 
fulcrum  in  feet  and  inches  are  marked  on  the  surface  of  the  lever. 
The  clamp  for  holding  the  briquette  for  tensile  tests  is  suspended 
from  the  short  arm,  18  inches  from  the  fulcrum.  Pressure  for 
shearing  and  compressive  stresses  is  communicated  through  a  loose 
upright,  set  under  the  long  arm  at  any  desired  distance  (generally 
6  or  12  inches)  from  the  fulcrum.  The  lower  clip  for  tensile  strains 
is  fastened  to  the  bed-plate.     On  this  plate  the  cube  to  be  crushed 


TF,  fixed  weight, 
block  for  crushing. 


W,  rolling  weight.     TI' 
C,  tensile  strain  clips. 


,  counterpoise.     B\  block  for  shearing.     B, 


rests  between  blocks  of  wood,  and  to  it  is  fastened  an  upright  with 
a  square  mortise  at  the  proper  height  for  blocks  to  be  sheared.  The 
rail  on  which  the  wheel  runs  is  a  piece  of  light  T-iron  fastened  on 
top  of  the  lever.  The  pin  is  iron  and  the  pin-holes  are  reinforced  by 
iron  washers.  The  clamps  are  wood,  and  are  fastened  by  clevis 
joints  to  the  lever  arm  and  bed-plate  respectively.  When  great 
stresses  are  desired,  extra  Aveights  are  hung  on  the  end  of  the  long 
arm.  Pressures  of  3,000  pounds  have  been  developed  with  this 
machine.  , 

For  detailed  drawings  of  a  more  elaborate  home-made  cement- 
testing  machine,  see  Proceedings  Engineers'  Club  of  Philadelphia, 
vol.  V.  p.  194,  or  Engineering  Neius,  vol.  xv.  p.  310. 

llle.  The  Clips.  The  most  important  part  of  the  testing 
machine  are  the  clips,  by  means  of  which  the  stress  is  applied 
to  the    briquette.     1.   The   form    must   be   such  as  to  grasp   the 


78 


LIME    AND    CEMENT. 


[chap.  III. 


briquette  on  four  symmetrical  surfaces.  2.  The  surface  of  con- 
tact must  be  large  enough  to  prevent  the 
briquette  from  being  crushed  between  the 
points  of  contact.  3.  The  clip  must  turn 
without  appreciable  friction  when  under 
stress.  4.  The  clip  must  not  spread  ap- 
preciably while  subjected  to  the  maximum 
load. 

The  form  of  clip  recommended  by  the 
Committee  of  the  American  Society  of 
Civil  Engineers  is  shown  in  Fig.  4.  This 
■^  form  does  not  offer  sufficient  bearing  sur- 
face, and  the  briquette  is  frequently  crushed 
at  the  point  of  contact.  The  difficulty  is 
remedied  somewhat  by  the  use  of  rubber- 
tipi^ed  clips. 

Whatever  the  form  of  the  machine  or 
clips,  great  care  should  be  taken  to  center 
the  briquette  in  the  machine. 

111/.  The  Speed.  The  rate  at  which 
the  stress  is  applied  makes  a  material 
difference  in  the  strength.  The  following 
data  are  given  by  H.  Faija,*  an  English 

authority,  as  showing  the  effect  of  a  variation  in  the  speed  of 

applying  the  stress : 

Rate.  Tensile  Strength. 

100  pounds  in  120  seconds 400  pounds. 

100        "         "     60       "  415 

100        "         "     30       "  430 

100        "         "     15       "  450 

100        "         ■■       1       <■  493 

The  French  and  German  standard  specifications  require  660 
pounds  per  minute.  The  American  Society  of  Civil  Engineers 
recommends  400  pounds  per  minute  for  strong  mixtures,  and  half 
this  speed  for  weak  mixtures.  The  Canadian  Society  of  Civil 
Engineers  recommends  200  pounds  per  minute. 

111^.  Data  on  Tensile  Strength.     Owing  to  the  great  variation 


Fig.  4. 


*  Trans.  Amer.  Soc.  of  C.  E.,  vol.  xvii.  p.  227. 


AKT.  4.] 


TESTS   OF    CEMENT. 


78a 


in  the  manner  of  making  the  tests,  it  is  not  possible  to  give  any  very 
valuable  data  on  the  strength  that  good  cement  should  show.  In 
1885  a  Committee  of  the  American  Society  of  Civil  Engineers 
recommended  the  values  given  in  Table  10  below.  At  least  the 
minimum  values  there  given  are  required  in  ordinary  specifications, 
and  the  maximum  values  are  sometimes  employed.     Many  of  the 


TABLE  10. 
Tensile  Strength  op  Cement  Mortaks. 


Age  of  Mortar  when  Tested. 

Average  Tensile  Strength 
IN  Pounds  per  Square  Inch. 

Portland. 

Natural. 

Clear  Cement. 

Min. 

Max. 

Min. 

Max. 

1  day— 1  hour,  or  until  set,  in  air,  the  remainder 
of  the  time  in  water 

100 

140 

40 

80 

1  week— 1  day  in  air,  the  remainder  of  the  time 
in  water 

250 

550 

60 

100 

4  weeks— 1  day  in  air,  the  remainder  of  the  time 
in  water 

350 

700 

100 

150 

1  year — 1  day  in  air,  the  remainder  of  the  time 
in  water 

450 

800 

300 

400 

1  Part  Cement  to  1  Part  Sand. 

1  week— 1  day  in  air,  the  remainder  of  the  time 
in  water 

30 

50 

200 

50 

4  weeks— 1  day  in  air,  the  remainder  of  the  time 
in  water 

80 

1  year — 1  day  in  air,  the  remainder  of  the  time 
in  water 

300 

1  Part  Cement  to  3  Parts  Sand. 

(  week — 1  day  in  air,  the  remainder  of  the  time 
in  water 

80 
100 
200 

125 
200 
350 

i  weeks — 1  day  in  air,  the  remainder  of  the  time 
in  water 

t  year — 1  day  in  air,  the  remainder  of  the  time 
in  water 

Tfeetter  cements  commonly  give  results  above  the  maximum  values 
in  the  table.  Natural  cement,  neat  plastic  mortar,  will  generally 
show  50  to  75  pounds  per  square  inch  in  7  days,  and  100  to  200  in 


LIME    AND    CEMENT. 


[chap.  III. 


:28  days.  Good  Portland  cement,  neat  plastic  mortar,  will  show 
100  to  200  pounds  per  sqaare  inch  in  one  day,  400  to  600  in 
7  days,  and  600  to  800  in  28  days.  With  3  parts  sand,  Portland 
cement,  plastic  mortar,  will  give  at  least  100  pounds  per  square 
inch  in  7  days,  and  200  in  28  days.  Of  course  the  strength  varies 
greatly  with  the  method  of  testing.  In  consulting  authorities  on 
this  subject,  it  should  be  borne  in  mind  that  the  strength  of  cement, 
particularly  Portland,  has  greatly  increased  in  the  past  10  years. 
The  specifications  should  be  drawn  to  correspond  with  the  personal 
equation  of  the  one  who  is  to  test  the  cement. 

For  various  specifications  for  tensile  strength,  see  Art.  5,  pages 
78c-7Sh,  particularly  Tables  10c  and  lOd,  pages  78/,  78^. 

For  additional  data  on  the  strength  of  mortars  composed  of 
different  proportions  of  cement  and  sand,  see  Fig.  5,  page  91. 

lllh.  Equating  the  Results.  It  not  infrequently  occurs  that 
several  samples  of  cement  are  submitted,  and  it  is  required  to 
determine  which  is  the  most  economical.  One  may  be  high-priced 
and  have  great  strength;   another  may  show  great  strength  neat 

TABLE   10a. 
Relative  Economy  op  Cements  Tested  Neat  at  7  Days. 


A 
B 
C 
D 
E 


Fin 

ENESS. 

Tensile  Strength. 

Cheapness. 

.a 

c8 

m  ® 

> 

$ 

o 

o    . 

d 

S  3 

Is 

4^ 

2h 

(2«= 

^ 

a 

$2.30 

100.0 

90.0 

98.1 

628 

81.5 

88.0 

95.9 

771 

100.0 

2.34 

98.3 

88.7 

96.6 

477 

61.9 

2.40 

95.8 

91.8 

100.0 

391 

50.7 

2.45 

93.8 

81.5 

88.8 

660 

85.6 

2.47 

93.1 

Relative 
Economy. 


OJ  «  u  jj 


79.95 
94.26 

57.28 
47.55 
70.79 


and   be   coarsely   ground.      If   the   cement   is   tested   neat,   then 
strength,  fineness,  and  cost  should  be  considered ;  but  if  the  cement 


ART.  5.] 


SPECIFICATIONS   FOR   CEMENT. 


78c 


is  tested  with  the  proportion  of  sand  usually  employed  in  practice, 
then  only  strength  and  cost  need  to  be  considered. 

Table  10a  (page  78b)  shows  the  method  of  deducing  the  relative 
economy  when  the  cement  is  tested  neat;  and  Table  10b  shows  the 

TABLE   106. 
Relative  Economy  of  Cements  Tested  with  Sand  at  7  Days. 


Tensile  Strength 

1  C.  TO  3  s. 

Cheapness. 

Relative  Economy. 

Cements. 

Pounds  per 
Square 
Inch. 

Relative. 

Cost  per 
Barrel. 

Relative. 

Product  of 

Relative 

Strength 

and  Relative 

Cost. 

Rank. 

A 
B 
C 
D 
E 

168 
176 
166 
135 
135 

95.4 

100.0 

94.3 

76.7 
76.7 

$2.30 
2.34 
2.40 
2.45 
2.47 

100.0 
98.3 
95.8 
93.8 
93.1 

95.40 
98.30 
90.33 
71.94 
71.40 

2 
1 
3 
4 
5 

method  when  the  cement  is  tested  with  sand.  The  data  are  from 
actual  practice,  and  the  cements  are  the  same  in  both  tables. 
Results  similar  to  the  above  could  be  deduced  for  any  other 
age;  the  circumstances  under  which  the  cement  is  to  be  used 
should  determine  the  age  for  which  the  comparison  should  be 
made. 

The  above  method  of  equating  the  results  gives  the  advantage 
to  a  cement  which  gains  its  strength  rapidly  and  which  is  liable  to 
be  unsound.  Therefore  this  method  should  be  used  with  discretion, 
particularly  with  short-time  tests. 


Art.  5.  Specifications  for  Cement. 

llli.  Cement  is  so  variable  in  quality  and  intrinsic  value  that 
no  considerable  quantity  should  be  accepted  without  testing  it  to 
see  that  it  conforms  to  a  specified  standard.  A  careful  study  of 
Art.  4,  preceding,  will  enable  any  one  to  prepare  such  specifications 
as  will  suit  the  special  requirements,  and  also  give  the  instructions 


78(?  LIME   AND    CEMENT.  [CHAP.  III. 


necessary  for  applying  the  tests.     A  few  specifications  will  be  given 
to  serve  as  guides  in  preparing  others. 

SPECIFICATIONS    FOR    QUALITY. 

my.  (jERMAN  Portland.  The  following  are  the  most  im- 
portant paragraphs  from  the  standard  specifications  of  the  German 
government  as  given  in  the  oflicial  circular  issued  by  the  Minister 
of  Public  Works  of  Prussia  under  date  of  July  28,  1887:* 

"  Time  of  Setting.  According  to  the  purpose  for  which  it  is  intended, 
quick  or  slow-setting  Portland  cement  may  be  demanded.  Slow-setting  cements 
are  those  that  set  in  about  two  hours."  The  test  is  made  as  described  in 
§86. 

"  Constancy  of  Volume.  The  volume  of  Portland  cement  should  remain 
constant.  The  decisive  test  of  this  should  be  that  a  cake  of  cement,  made  on  a 
glass  plate,  protected  from  sudden  drying  and  placed  under  water  after  24 
hours,  should  show,  even  after  long  submersion,  no  signs  of  crumbling  or  of 
cracking  at  the  edges."     For  method  of  making  the  test,  see  §  92. 

"  Fineness  of  Grinding.  Portland  cement  must  be  ground  so  fine  that  no 
more  than  10  per  cent,  of  a  sample  shall  be  left  on  a  sieve  of  900  meshes  per 
square  centimetre  (5,800  per  square  inch).  The  thickness  of  the  wires  of  the 
sieve  to  be  one-half  the  width  of  the  meshes."  Notice  that  a  sieve  having  900' 
meshes  per  square  centimetre  (5,800  per  sq.  in.)  is  the  standard,  although 
sieves  of  5,000  meshes  per  square  centimetre  (32,000  per  sq.  in.)  are  frequently 
used. 

"  Tests  of  Strength.  The  binding  strength  of  Portland  cement  is  to  be 
determined  by  testing  a  mi.\ture  of  cement  and  sand.  The  test  is  to  be  con- 
ducted for  tensile  and  compressive  strength  according  to  a  uniform  method, 
and  is  to  be  performed  upon  test  specimens  of  like  form,  like  cross  section,  and 
with  like  apparatus.  It  is  recommended,  besides,  to  determine  the  strength  of 
neat  cement.  The  tests  for  tension  are  to  be  made  upon  briquettes  of  5  sq. 
cm.  (0.78  sq.  in.)  cross  section  at  the  place  of  rupture,  the  tests  for  compression 
upon  cubes  of  50  sq.  cm.  (7.8  sq.  in.)  area." 

"  Tensile  and  Compressive  Strength.  Slow-setting  Portland  cement,  when 
mixed  with  standard  sand  in  the  proportion  of  1  part  of  cement  to  3  of  sand, 
by  weight,  28  days  after  being  mixed — one  day  in  air  and  27  in  water — must 
,  possess  a  tensile  strength  of  not  less  than  16  kilog.  per  square  centimetre  (225 
lbs.  per  square  inch),  and  a  maximum  compressive  strength  of  160  kilog.  per 
square  centimetre  (2,250  lbs.  persq.  in.).  Qtiick-setting  cements  generally  show 
a  lower  strength  after  28  days  than  that  given  above.  The  time  of  setting 
must,  therefore,  be  given  when  stating  figures  relative  to  strength."  The  test  is 
made  as  described  in  the  second  paragraph  of  §  111  or  the  first  paragraph  of 
§  lllo. 

*  Translation  from  Trans.  Amer.  Soc.  of  C.  E.,  vol.  yxx.  pp.  10-21. 


J\.RT.  5.J  SPECIFICATIONS   FOR   CEMENT.  78« 

11 11-.  English  Portland.  In  Great  Britain  there  are  no 
•official  specifications,  but  the  following  proposed  *  by  Mr.  Henry 
Faija  are  much  used : 

"  Fineness  to  be  such  that  the  cement  will  all  puss  through  a  sieve  having 
625  holes  (25')  to  the  square  inch,  and  leave  only  10  per  cent,  residue  when 
sifted  through  a  sieve  having  2,500  holes  (50')  to  the  square  inch. 

"  Expansion  or  Contraction,  A  pat  made  and  submitted  to  moist  heat  and 
warm  water  at  a  temperature  of  100°  to  115°  F.,  shall  show  no  sign  of  expan- 
sion or  contraction  (blowing)  in  twenty-four  hours. 

"  Tensile  Strength.  Briquettes  of  slow-setting  Portland,  which  have  been 
gauged,  treated,  and  tested  in  the  prescribed  manner,  to  carry  an  average  ten- 
sile strain,  witliout  fracture,  of  at  least  176  lbs.  per  sq.  in.  at  the  expiration  of 
■3  days  from  gauging;  and  those  tested  at  the  expiration  of  7  days,  to  show  an 
increase  of  at  least  50  per  cent,  over  the  strength  of  those  at  3  days,  but  to 
carry  a  minimum  of  350  lbs.  per  sq.  in. 

"For  quick-setting  Portland,  afleast  176  lbs.  pcf  sq.  in.  at  3  days,  and  an 
increase  at  7  days  of  30  to  25  per  cent.,  but  a  minimum  of  400  lbs.  per  sq.  In. 
Very  high  tensile  strengths  at  early  dates  generally  indicate  a  cement  verging 
on  an  unsound  one." 

111^.  French  Portland.  The  following  are  the  requirements 
of  the  Services  Maritimes  des  Fonts  et  Chaussees,f  and  are  fre- 
quently employed  in  France: 

"  Density.  A  liter  measure  is  loosely  filled  with  cement,  previously 
screened  through  a  sieve  of  180  meshes  to  the  linear  inch,  and  weighed.  This 
test  is  used  for  comparison  of  diflfereut  lots  of  the  same  cement,  the  weight  of 
1  liter  of  which  must  exceed  a  certain  figure  determined  for  the  cement  in 
question.     No  general  requirement  as  to  density  is  made." 

"  Chemical  Composition.  Cement  containing  more  than  1  per  cent,  of  sul- 
phuric anhydride  (=1.7  per  cent,  sulphate  of  lime)  is  rejected,  while  that  con- 
taining more  than  4  per  cent,  of  oxide  of  iron  is  declared  suspicious.  Cement 
containing  less  than  44  parts  of  silica  and  alumina  to  100  of  lime  is  also  con- 
sidered suspicious." 

Tim,e  of  Setting.  The  test  for  time  of  setting  is  made  as  described  in  §  86 
{page  58).  "  Cement  which  begins  to  set  in  less  than  30  minutes  or  sets  com- 
pletely in  less  than  3  hours  is  refused." 

"  Constancy  of  Volume.  Pats  on  glass  are  immersed  in  sea-water  kept 
at  a  temperature  of  59°  to  65°  F.,  and  examined  for  cracking  or  change  of 
form." 

"  Tensile  Strength.     The  amount  of  water  to  be  employed  is  determined  as 

*  Trans.  Amer.  Soc,  of  C.  E.,  vol.  xvii.  p.  225 ;  vol.  xxx.  (1893)  p.  60. 
f  Candlot's  "Ciments  and  Chaux  HydrauUcs,"  Paris,  1891,  pp.  150-61. 


78/ 


LIME    AND    CEMENT. 


[chap.  III. 


in  third  paragraph  of  §  103  (page  69).  The  briquettes  are  moulded  as  de- 
scribed in  in  the  third  paragraph  of  §  109,  page  73. 

"  For  neat  cement,  the  tensile  strength  at  7  days  must  be  at  least  20  kilog. 
per  sq.  cm.  (284  lbs.  per  sq.  in.);  at  28  days,  35  kilog.  per  sq.  cm.  (497  lbs. 
per  sq.  in.);  at  12  weeks,  45  kilog.  per  sq.  cm.  (639  lbs.  per  sq.  in.).  The 
tensile  strength  at  28  days  must  exceed  that  at  7  days  by  at  least  5  kilog.  per 
sq.  cm.  (71  lbs.  per  sq.  in.).  The  tensile  strength  at  12  weeks  must  be  greater 
than  that  at  28  days  unless  the  latter  shall  be  at  least  55  kilog.  per  sq.  cm. 
(781  lbs.  per  .sq.  in.). 

"For  3  parts  crushed  quartz  to  1  part  cement,  with  12  per  cent,  water 
[moulded  as  described  in  the  third  paragraph  of  §111],  the  tensile  strength 
must  be  at  7  days  at  least  8  kilog.  per  sq.  cm.  (114  lbs.  per  sq.  in.);  at  28  days 
at  least  15  kilog.  per  sq.  era.  (213  lbs.  per  sq.  in.);  and  at  12  weeks,  18  kilog. 
per  sq.  cm.  (256  lbs.  per  sq.  in.).  The  strength  at  12  weeks  must  in  all  cases 
be  greater  than  that  at  28  days." 

lllwi.  American  Practice.  Tables  10c  and  10c?  give  the  average 
requirements  for  fineness  and  tensile  strength  of  Portland  and 
natural  cements,  for  various  classes  of  work  in  the  United  States. 
These  values  may  be  regarded  as  representative  of  the  average 
American  practice: 

TABLE  10c. 

American  Requirements  for  Fineness  and  Strength  of  Portland 

Cement. 


AvKBAGE  American  Pkactick 
AS  Represented  by 


38  U.  S.  A.  Engineers. 
10  Cities 

6  Railways 

6  Bridges 

3  Aqueducts 

81  Specifications 


Per  cent. 
Passing 

Sieve. 
No. 


50       100 


95 
97 
95 
97 


96 


84 
89 
80 
88 
80 
85 


Tensilk  Strength,  Lbs.  per  S<j.  In. 


Neat  Cement. 


1 


131 
161 
115 
119 
110 
134 


Mortar. 


lto2. 


1  to  3. 


Age  when  Tested,  Days. 


402 
388 
319 
347 
333 
384 


28 


547 
534 
483 
487 
400 
528 


150 
145 
123 
142 
125 
146 


28 


200 
200 
175 
250 
200 
216 


119 
132 
108 
132 
132 
118 


189 
197 
150 
200 
225 
189 


Airf.  5.] 


SPECIFICATIONS    FOR   CEMENT. 


:8g 


TABLE    Wd. 

Amekican  Requikements  for  Fineness  and  Strength  of  Natural 

Cement. 


Average  American  Practice  as 
Represented  by 


24  U.  S.  A.  Engineers 

10  Cities 

4  Railways 

2  Bridges 

3  Aqueducts 

51  Specifications 


Per  cent. 

Passing 

Sieve. 

No. 


50 

100 

91 

72 

93 

81 

95 

95 

92 


79 


Tensile  Strength. 
Lbs.  per  Sq.  In. 


Neat  Cement. 


1  to  2 
Mortar. 


Age  when  Tested.    Days. 


38 
70 
65 
55 
60 
63 


102 
134 
105 

87 
117 
109 


164 
237 
162 

185 
200 

178 


34 
40 
47 
35 
50 
40 


28 
77 

80 
70 
75 

77 


llln.  Philadelphia:  Natural  and  Portland,  The  follow- 
ing is  an  abstract  of  the  specifications  used  in  1897  by  the  Depart- 
ment of  Public  Works  of  the  City  of  Philadelphia.  These 
specifications  are  inserted  as  showing  the  extreme  of  American 
practice  in  the  high  degree  of  fineness  and  great  strength  required. 
Compare  these  results  with  those  in  Tables  10c  and  lOd.  The 
Philadelphia  specifications  are  not  included  in  these  tables. 


NATURAL    CEMENT. 

"  Specific  Gravity.     The  specific  gravity  shall  not  be  less  than  2.7. 

"  Fineness.  The  residue  shall  not  leave  more  than  2  per  cent  on  a  No.  50 
sieve,  nor  15  on  a  Xo.  100  sieve,  nor  35  ou  a  No.  200  sieve,  the  sieves  having 
2,400, 10,200,  and  35,700  meshes  per  square  inch  and  the  diameter  of  the  wire 
being  0.0090,  0.0045,  and  0.0020  of  an  inch  respectivel}-. 

"  Constancy  of  Volume.  Pats  of  neat  cement  one  half  inch  thick  with 
thin  edges,  immersed  in  water  after  hard  set,  shall  show  no  signs  of  checking 
or  disintegration. 

"  Time  of  Setting.  It  shall  begin  to  set  in  not  less  than  10  minutes,  and  set 
hard  in  less  than  30  minutes. 

"  Tensile  Strength.  The  tensile  strength  of  dry  mortar  [see  last  paragraph 
of  g  105]  shall  not  be  less  than  in  the  accompanying  table : 


ISh 


LIME    AND    CEMENT. 


[chap.  III. 


Tensile  Strength. 
Pounds  per  Square  Inch. 

Neat. 

1  Cement  to 
~'  Quartz. 

24  hours  (in  water  after  set  hard) 

100 
200 
300 

7  days  (1  day  in  air,  6  days  in  water) 

125 

28  days  (1  day  in  air,  27  days  in  water) 

200 

PORTLAND    CEMENT. 

"  Specific  Oravity.     The  specific  gravity  shall  not  be  less  than  3.0. 

"  Fineness.  The  residue  shall  not  be  less  than  1  per  cent,  on  a  No.  50  sieve, 
10  on  a  No.  100  sieve,  aud  30  on  a  No.  200  sieve. 

"  Constancy  of  Volume.     Same  as  for  natural  cement  above. 

"  Time  of  Setting.  The  cement  shall  not  develop  initial  set  in  less  than  30 
minutes. 

"  Ti-nsile  Strength.  The  tensile  strength  of  dry  mortar  [see  last  paragragh 
of  ^  105]  shall  not  be  less  than  in  the  accompanying  table  : 


Tensile  Strength. 
Pounds  per  Square  Inch. 

Neat. 

1  Cement  to 
2  Quartz. 

24  hours  (in  water  after  hard  set) 

175 
500 
600 

7  days  (1  day  in  air,  6  days  in  water) 

170 

28  days  (1  day  in  air,  27  days  in  water) 

240 

SPECIFICATIONS    FOR    DELIVERY    AND    STORAGE. 

lllo.  The  preceding  specifications  prescribe  the  quality  of  the 
cement;  and  the  following  refer  to  the  quantity,  the  sampling,  and 
the  storage.  The  tests  under  the  former  are  made  in  the  laboratory, 
those  under  the  latter  on  the  work. 

Package.     The  cement  shall  be  delivered  in  strong  barrels*  lined  with 


*  It  is  customary  to  specify  that  the  cement,  particularly  Portland,  shall  be 
delivered  iu  barrels.  The  only  reason  for  shipping  in  barrels  is  that  the  cement  is 
better  protected  from  the  weather  in  barrels  than  in  bags.    The  arguments  in 


^RT.  5.]  SPECIFICATIONS   FOR   CEMENT.  78 i 

paper  so  as  to  be  reasonably  protected  from  the  air  and  dampness.  Each 
package  shall  be  labeled  with  the  brand,  the  manufacturer's  name,  and  the 
gross  weight. 

WeigJU.  The  net  weight  of  a  barrel  of  cement  shall  be  understood  to  be 
375  pounds  of  Portland,  and  300  pounds  of  Eastern  natural  or  265  pounds  of 
Western  natural  [see  §  77,  page  54];  and  bags  shall  contain  an  aliquot  part  of 
a  barrel.  A  variation  of  2  per  cent,  is  allowable  in  the  weight  of  individual 
packages.  Any  broken  barrel  or  torn  bag  may  be  rejected  or  accepted  at  half 
its  original  weight, — at  the  option  of  the  Inspector. 

Time  of  Delivery.  The  inspection  and  tests  will  occupy  at  least  ten  *  days, 
and  the  Contractor  shall  submit  the  cement  for  sampling  at  least  ten  *  days 
before  desiring  to  use  it.  The  Inspector  shall  be  promptly  notified  upon  the 
receipt  of  each  shipment. 

Sampling.  The  cement  from  which  to  test  the  quality  shall  be  selected  by 
taking,  from  the  interior  of  each  of  six  \  well-distributed  barrels  or  bags  in 
each  car-load,  suflicient  cement  to  make  from  five  to  ten  briquettes.  These 
sixf  portions,  after/being  thrown  together  and  thoroughly  .mixed,  will  be 
assumad  to  represent  the  average  of  the  whole  car-load. 

Storage.  All  cement  when  delivered  shall  be  fully  protected  from  the 
weather  ;  and  shall  not  be  placed  upon  the  ground  without  proper  blocking 
under  it.  Accepted  cement  may  be  re-inspected  at  any  time  ;  and  if  found 
to  be  damaged,  it  shall  be  rejected.  Any  cement  damaged  by  water  to  such 
an  extent  as  to  show  upon  the  outside  of  the  barrel  will  be  rejected. 

Iiispectiori  Marks.  Cement  which  has  been  accepted  may  be  so  labeled  by 
the  Inspector  ;  and  the  Contractor  shall  preserve  these  labels  from  deface- 
ment and  shall  prevent  their  imitation.  Rejected  cement  shall  be  so  marked  ; 
and  the  Contractor  shall  promptly  remove  such  cement. 

Basis  for  Rejecting.  Each  shipment  of  cement  shall  be  tested  for  quantity 
and  quality.  If  the  average  weight  of  the  barrels  or  bags  tested  is  less  than 
the  weight  specified,  a  corresponding  deduction  shall  be  made  in  the  price  ; 
and  if  ten  per  cent,  fails  to  conform  to  the  requirements  for  quality,  the  entire 

favor  of  shipping  in  bags  are :  1.  The  cost  is  less,  since  the  cost  of  the  barrel  is 
eliminated.  2.  The  cement  is  more  easily  handled,  since  the  weight  of  a  unit  is 
less.  3.  In  cloth  bags  the  cement  improves  by  seasoning,  i.e.,  the  contact  with 
the  air  hydrates  any  free  lime  due  to  improper  chemical  combination  or  imperfect 
calcination.  4.  The  practice  of  shipping  in  barrels  is  only  a  survival  from  the  time 
when  the  best  cement  was  of  European  manufacture,  which  of  necessity  was 
shipped  in  barrels  because  of  the  excessive  moisture  in  the  holds  of  vessels.  5.  In 
Europe  Portland  cement  is  usually  shipped  in  cotton-ducli  bags. 

*  For  important  work  this  time  is  usually  made  thirty  days. 

f  This  is  the  number  specified  by  the  Pennsylvania  Railroad,  a  road  noted  for 
careful  and  thorough  work.  It  is  frequently  specified  that  ten  samples  shall  be 
taken ;  and  in  important  workjwhero  a  single  barrel  of  poor  cement  may  materially 
affect  the  strength  of  the  work,  it  is  sometimes  specified  that  each  and  every 
barrel  shall  be  tested. 


TSy  LIME    AXD    CEMENT.  [CHAP.  III. 

shipment  may  be  rejected.  The  failure  of  a  shipment  to  meet  the  specifica- 
tions for  quality  may  prohibit  further  use  of  that  brand  on  the  work. 

Barrels  containing  a  large  proportion  of  lumps  shall  be  rejected. 

Refusal  to  Test.  The  Engineer  reserves  the  right  to  refuse  to  test  any 
brand  which  in  his  judgment  is  unsuitable  for  the  work.*  A  barrel  or  bag 
which  is  not  plainly  labeled  with  the  brand  and  maker's  name  shall  not  be 
tested,  and  shall  be  immediately  removed. 

*  This  provision  is  sometimes  inserted  to  avoid  the  trouble  and  delay  of  testing 
any  brand  which  the  Engineer  is  reasonably  certain  is  unfit  for  the  work  owing  to 
its  general  reputation  for  poor  quality  or  lack  of  uniformity. 


CHAPTER  nil 
SAND,   GRAVEL,   AND  BROKEN  STONE. 

112.  Sand  is  used  in  making  mortar;  and  gravel,  or  sand  and 
broken  stone,  in  making  concrete.  The  qualities  of  the  sand  and 
broken  stone  have  an  important  effect  upon  the  strength  and  cost 
of  the  mortar  and  the  concrete.  The  effect  of  the  variation  in  these 
materials  is  generally  overlooked,  even  though  the  cement  is  subject 
to  rigid  specifications. 

Art.  1.     Sand. 

113.  Sand  is  mixed  with  lime  or  cement  to  reduce  the  cost  of 
the  mortar;  and  is  added  to  lime  also  to  prevent  the  cracking  which 
would  occur  if  lime  were  used  alone.  Any  material  may  be  used  to 
dilute  the  mortar,  provided  it  has  no  effect  upon  the  durability  of 
the  cementing  material  and  is  not  itself  liable  to  decay.  Pulverized 
stone,  powdered  brick,  slag,  or  coal  cinders  may  be  used  ;  but 
natural  sand  is  by  far  the  most  common,  although  fine  crushed 
stone,  or  "stone  screenings,"  are  sometimes  employed  and  are  in 
some  respects  better  than  natural  sand. 

In  testing  cement  a  standard  natural  sand  or  crushed  quartz  is 
employed;  but  in  the  execution  of  actual  work  usually  local  natural 
sand  must  be  employed  for  economic  reasons.  Before  commencing 
any  considerable  work,  all  available  natural  sands  and  possible  sub- 
stitutes should  be  examined  to  determine  their  values  for  use  in 
mortar. 

114.  Requisites  for  Good  Sand.  To  be  suitable  for  use  in 
mortar,  the  sand  should  be  sharp,  clean,  and  coarse;  and  the  grains 
should  be  composed  of  durable  minerals,  and  the  size  of  the  grains 
should  be  such  as  to  give  a  minimum  of  voids,  i.e.,  interstices 
between  the  grains. 

The  usual  specifications  are  simply:  "  The  sand  shall  be  sharp, 
clean,  and  coarse. "" 

114a.  Durability.     As  a  rule  ocean  and  lake  sands  are  more 

79a 


79J  SAND.  [chap.  Ilia. 

durable  than  glacial  sands.  The  latter  are  rock  meal  ground  in  the 
geological  mill,  and  usually  consist  of  silica  with  a  considerable  ad- 
mixture of  mica,  hornblende,  feldspar,  carbonate  of  lime,  etc.  The 
silica  is  hard  and  durable ;  but  the  mica,  hornblende,  feldspar,  and 
carbonate  of  lime  are  soft  and  friable,  and  are  easily  decomposed 
by  the  gases  of  the  atmosphere  and  the  acids  of  rain-water.  The 
lake  and  ocean  sands  are  older  geologically  ;  and  therefore  are 
usually  nearly  pure  quartz,  since  the  action  of  the  elements  has 
eliminated  the  softer  and  more  easily  decomposed  constituents. 
Some  ocean  sands  are  nearly  pure  carbonate  of  lime,  which  is  soft 
and  friable,  and  are  therefore  entirely  unfit  for  use  in  mortar. 
These  are  known  as  calcareous  sands. 

The  glacial  sands  frequently  contain  so  large  a  proportion  of 
soft  and  easily  decomposed  constituents  as  to  render  them  unfit  for 
use  in  exposed  work,  as  for  example  in  cement  sidewalks.  Instead 
of  constructing  exposed  work  with  poor  drift  sand,  it  is  better  either 
to  ship  natural  silica  sand  a  considerable  distance  or  to  secure 
crushed  quartz.  Crushed  granite  is  frequently  used  instead  of  sand 
in  cement  sidewalk  construction;  but  granite  frequently  contains 
mica,  hornblende,  and  feldspar  which  render  it  unsuitable  for  this 
kind  of  work. 

However,  as  a  rule  the  physical  condition  of  the  sand  is  of  more 
importance  than  its  chemical  composition. 

114Z).  Sharpness.  Sharp  sand,  i.e.,  sand  with  angular  grains, 
is  preferred  to  that  with  rounded  grains  because  (1)  the  angular 
grains  are  rougher  and  therefore  the  cement  will  adhere  better;  and 
(2)  the  angular  grains  offer  greater  resistance  to  moving  one  on  the 
other  under  compression.  On  the  other  hand,  the  sharper  the  sand 
the  greater  the  proportion  of  the  interstices  between  the  grains 
(compare  line  4  of  Table  10^,  page  79i,  with  the  preceding  lines  of 
the  table) ;  and  consequently  the  greater  the  amount  of  cement 
required  to  produce  a  given  strength  or  density.  But  a  high 
degree  of  sharpness  is  more  important  than  a  small  per  cent,  of 
voids. 

The  sharpness  of  sand  can  be  determined  approximately  by 
rubbing  a  few  grains  in  the  hand,  or  by  crushing  it  near  the  ear 
and  noting  if  a  grating  sound  is  produced;  but  an  examination 
through  a  small  lens  is  better.  Sharp  sand  is  often  difficult  to 
obtain,  and  the  requirement  that  "  the  sand  shall  be  sharp"  is 
practically  a  dead  letter  in  most  specifications. 


ART.   1.]  CLEANKESS.  79C 

114c.  Cleanness.  Clean  sand  is  necessary  for  the  strongest 
mortar,  since  an  envelop  of  loam  or  organic  matter  aboat  the  sand 
grains  will  prevent  the  adherence  of  the  cement.  The  cleanness  of 
sand  may  be  judged  by  pressing  it  together  in  the  hand  while  it  is 
damp;  if  the  sand  sticks  together  when  the  pressure  is  removed,  it 
is  entirely  unfit  for  mortar  purposes.  The  cleanness  may  also  be 
tested  by  rubbing  a  little  of  the  dry  sand  in  the  palm  of  the  hand; 
if  the  hand  is  nearly  or  quite  clean  after  throwing  the  sand  out,  it 
is  probably  clean  enough  for  mortar.  The  cleanness  of  the  sand 
may  be  tested  quantitatively  by  agitating  a  quantity  of  sand  with 
water  in  a  graduated  glass  flask;  after  allowing  the  mixture  to 
settle,  the  amount  of  precipitate  and  of  sand  may  be  read  from  the 
graduation.  Care  should  be  taken  that  the  precipitate  has  fully 
settled,  since  it  will  condense  considerably  after  its  upper  surface  is 
clearly  marked. 

Sand  is  sometimes  washed.  This  may  be  done  by  placing  it  on 
a  wire  screen  and  playing  upon  it  with  a  hose;  or  by  placing  it  in 
an  inclined  revolving  screen  and  drenching  with  water.  When 
only  comparatively  small  quantities  of  clean  sand  are  required,  it 
can  be  washed  by  shoveling  into  the  upper  end  of  an  inclined 
V-shaped  trough  and  playing  upon  it  with  a  hose,  the  clay  and 
lighter  organic  matter  floating  away  and  leaving  the  clean  sand  in 
the  lower  portion  of  the  trough,  from  which  it  can  be  drawn  off  by 
removing  for  a  short  time  plugs  in  the  sides  of  the  trough.  Sand 
can  be  washed  fairly  clean  by  this  method  at  an  expense  of  about 
10  cents  per  cubic  yard  exclusive  of  the  cost  of  the  water.  For  a 
sketch  and  description  of  an  elaborate  machine  for  washing  sand 
by  paddles  revolving  in  a  box,  see  Engineering  Xe^vs^  vol.  xli. 
page  111  (Feb.  16,  1899).  By  this  method  the  cost  of  thoroughly 
washing  dirty  sand  is  about  15  cents  per  cubic  yard. 

Although  it  is  customary  to  require  that  only  clean  sand  shall 
be  used  in  making  mortar,  a  small  quantity  of  very  finely  powdered 
clay  will  not  materially  decrease  the  strength  of  the  mortar.  In 
some  instances  clay  to  the  amount  of  10  per  cent,  of  the  sand  seems 
not  to  decrease  the  strength  of  the  mortar.*  Mortar  containmg 
■tonsiderable  clay  is  much  more  dense,  plastic,  and  water-tight;  and 
IS  occasionally  convenient  for  plastering  surfaces  and  stopping  leaky 
joints.     Such  mortar  is  not  affected  by  the  presence  of  water. 

*  Report  of  Chief  of  Engineers,  U.  S.  A.,  1894,  pp.  3001-10;  and  Trans.  Amer.  Sor*. 
of  C.  E.,  vol.  xiv.  p.  164. 


79d  SAND.  [chap.  ma. 

Ill  eugiaeering  literature  but  few  definite  specifications  for  the 
cleanness  of  sand  can  be  found,  a  diligent  search  revealing  only  the 
following:  For  bridge  work  on  the  New  York  Central  and  Hudson 
Eiver  E.  E.,  the  specifications  required  that  the  sand  shall  be  so 
clean  as  not  to  soil  white  paper  when  rubbed  on  it.  For  the  retain- 
ing walls  on  the  Chicago  Sanitary  Canal,  the  suspended  matter 
when  shaken  with  water  was  limited  to  0.5  per  cent.  For  the  dam 
on  the  Monongahela  Eiver,  built  under  the  direction  of  the 
U.  S.  A.  engineers,  the  suspended  matter  was  limited  to  1  per 
cent.  For  the  dam  at  Portage,  N.  Y.,  built  by  the  State  Engineer, 
the  "  aggregate  of  the  impurities  "  was  limited  to  5  to  8  per  cent. 
The  contamination  permissible  in  any  particular  case  depends  upon 
the  cleanness  of  the  sand  available  and  upon  the  difficulty  of 
obtaining  perfectly  clean  sand.  Sand  employed  in  masonry  con- 
struction frequently  contains  5,  and  sometimes  10,  per  cent,  of 
suspended  matter. 

114fZ.  Fineness.  Coarse  sand  is  preferable  to  fine,  since  (1)  the 
former  has  less  surface  to  be  covered  and  hence  requires  less 
cement;  and  (2)  coarse  sand  requires  less  labor  to  fill  the  interstices 
with  the  cement.  The  sand  should  be  screened  to  remove  the 
pebbles,  the  fineness  of  the  screen  depending  upon  the  kind  of  work 
in  which  the  mortar  is  to  be  used.  The  coarser  the  sand  the 
better,  even  if  it  may  properly  be  designated  fine  gravel,  provided 
the  diameter  of  the  largest  pebble  is  not  too  nearly  equal  to  the 
thickness  of  the  mortar  joint. 

Table  lOe  gives  the  results  of  a  series  of  experiments  to  deter- 
mine the  effect  of  the  size  of  grains  of  sand  upon  the  tensile 
strength  of  cement  mortar.  The  briquettes  were  all  made  at  the 
same  time  by  the  same  person  from  the  same  cement  and  sand,  the 
only  difference  being  in  the  fineness  of  the  sand.  The  table  clearly 
shows  that  coarse  sand  is  better  than  fine.  Notice  that  the  results 
in  line  4  of  the  table  are  larger  than  those  in  line  3.  This  is 
probably  due  to  the  fact  that  the  sand  for  line  4  has  a  greater  range 
of  sizes  and  consequently  fewer  voids.  If  this  explanation  is  true, 
then  since  the  sand  in  each  line  of  the  lower  half  of  the  table  has 
greater  variety  of  sizes  than  those  in  the  upper  half,  the  coarse  sand 
is  relatively  better  than  appears  from  Table  lOe. 

Table  10/  shows  the  fineness  of  natural  sands  employed  in 
actual  construction;  and  as  the  sands  were  to  all  appearances  of 
the  same  character,  this  table  also  shows  at  least  approximately  the 


ART.  1.] 


FINENESS. 


79e 


TABLE    lOe. 

Effect  of  Fineness  of  Sand  upon  the  Tensile  Stkength  of  1 : 2 
Cement  Mortak. 


Ref. 
No. 

Sand  caught  between 
the  two  sieves  stated 

BELOW. 

Tensile  Strength 

,  IN  POUNDS  PER  SQUARE  INCH, 
AFTER 

7  Days. 

IMo. 

3  Mos. 

6  Mos. 

12  Mos. 

1 

No.  4  and  No.  8 

243 

442 

539 

470 

665 

2 

"   8  "  "  16 

269 

345 

473 

512 

572 

3 

"  16  "  "  20 

186 

250 

313 

397 

396 

4 

"  20  "  "   30 

211 

281 

322 

402 

440 

5 

"  30  "  "  50 

149 

205 

238 

275 

318 

6 

"  50  "  "  75 

122 

214 

260 

275 

308 

7 

"  75  "  "  100 

98 

153 

211 

208 

253 

8 

Passing  No.  100 

98 

155 

161 

229 

271 

TABLE   10/. 

Tensile  Strength  of  a  1  : 3  Cement  Mortar  with  Natural  Sands 
differing  chiefly  in  Fineness. 


Fineness. 

Ref. 

No. 

Per  Cent.,  by 

weight,  caught 

on  Sieve 

No. 

Per 

Cent 

PASSING 

No. 
100. 

4 

8 

16 

20 

30 

50 

75 

100 

5  2 
z  o 

1 

0 

26 

21 

16 

11 

9 

8 

7 

2 

700 

2 

0 

29 

29 

13 

10 

12 

5 

1 

447 

3 

0 

22 

21 

11 

17 

20 

8 

1 

370 

4 

0 

13 

15 

10 

19 

33 

6 

1 

341 

5 

0 

9 

10 

6 

11 

45 

15 

2 

332 

6 

0 

13 

15 

7 

8 

38 

15 

4 

^ 

309 

7 

0 

0 

0 

0 

1 

6 

69 

23 

2 

246 

8 

0 

0 

0 

0 

0 

0 

0 

6 

94 

200 

9 

0 

0 

0 

2 

3 

15 

45 

30 

5 

189- 

79/  SAND.  [chap.  uia. 

effect  of  fineness  upon  tensile  strength.  This  table  agrees  with 
the  preceding  in  showing  that  the  coarser  sand  makes  the  stronger 
mortar.     This  conclusion  is  perfectly  general. 

If  the  voids  are  filled  with  cement,  uniform  coarse  grains  give 
greater  strength  than  coarse  and  fine  mixed ;  or,  in  other  words, 
for  rich  mortar  coarse  grains  are  more  important  than  small  voids. 
But  if  the  voids  are  not  filled,  then  coarse  and  fine  sand  mixed  give 
greater  strength  than  uniform  coarse  grains;  or,  in  other  words, 
for  lean  mortar  a  small  proportion  of  voids  is  more  important  than 
coarse  grains.* 

As  a  rale,  the  sand  ordinarily  employed  in  making  cement 
mortar  is  mucli  too  fine  to  give  maximum  strength  or  to  permit 
the  use  of  a  minimum  amount  of  cement.  For  example,  the  sands 
in  lines  13  and  14  of  Table  10^  (page  79i)  are  much  used  in  actual 
work,  and  have  approximately  the  same  degree  of  fineness  as  the 
sands  in  the  last  three  lines  of  Table  10/,  which  give  a  much  weaker 
mortar  than  the  preceding  sands  of  Table  10/ 

114e.  Specifications  seldom  contain  any  numerical  requirement 
for  the  fineness  of  the  sand.  The  two  following  are  all  that  can 
be  found.  For  the  retaining- wall  masonry  on  the  Chicago  Sanitary 
Canal  the  requirements  were  that  not  more  than  50  per  cen^  shall 
pass  a  No.  50  sieve,  and  not  more  than  12  per  cent,  shall  pass  a 
No.  80  sieve.  For  the  Portage  Dam  on  the  Genesee  Kiver,  built 
by  the  New  York  State  Engineer,  the  specifications  were  that  at 
least  75  per  cent,  should  pass  a  No.  20  sieve  and  be  caught  on  a 
No.  40. 

The  fineness  of  the  sand  employed  in  several  noted  works  is  as 
follows,  the  larger  figures  being  the  number  of  the  sieve,  and  the 
smaller  figures  preceding  the  number  of  a  sieve  being  the  per  cent, 
retained  by  that  sieve,  and  the  small  number  after  the  last  sieve 
number  being  the  per  cent,  passing  that  sieve:  Poe  Lock, 
St.  Mary's  Fall  Canal,  ^  20 '' 30  ^  40  *';  concrete  for  pavement 
foundations  in  the  City  of  Washington,  D.  C,  »  3  '  G  «  8  '^  10  ^  30  '^ 
40^60^80^;  Genesee  (N.  Y.)  Storage  Dam,  "20*30^50^ 
100  *;  Rough  River  (Ky.)  Improvement,  "  20  "  30  ^^  50  '^;  St.  Regis 
sand,   Soulanges    Canal,    Canada,  ^^  20 -^^  30  "  50  " ;    Grand   Coteau 


*  Report  of  Chief  of  Engineers,  U.  S.  A.,  1896,  p.  2862,  or  Jour.  West.  Soe.  of 
Engrs.,  vol.  ii.  p.  519 ;  and  Report  of  Operations  of  the  Engineering  Department  of 
the  District  of  Columbia,  1896,  p.  195. 


ART.  1.]  VOIDS.  79^ 

sand,*  Soulanges  Canal,  Canada,  "  20  '"  30  ^  50  ^.  Tables  10/  and 
10^  show  the  fineness  of  a  number  of  natural  sands  employed  in 
actual  work, 

114/.  Voids.  The  smaller  the  proportion  of  voids,  i.e.,  the 
interstices  between  the  grains  of  the  sand,  the  less  the  amount  of 
cement  required,  and  consequently  the  more  economical  the  saud. 

The  proportion  of  voids  may  be  determined  by  filling  a  vessel 
with  sand  and  then  determining  the  amount  of  water  that  can  be 
put  into  the  vessel  with  the  sand.  This  quantity  of  water  divided 
by  the  amount  of  water  alone  which  the  vessel  will  contain  is  the 
proportion  of  voids  in  the  sand.  The  quantities  of  water  as  above 
may  be  determined  by  volumes  or  by  weight.  The  proportion  of 
voids  may  be  determined  for  the  sand  loose  or  rammed,  the  latter 
being  the  more  appropriate,  since  the  mortar  is  either  compressed 
or  rammed  when  used.  In  either  case  it  is  more  accurate  to  drop 
the  sand  through  the  water  than  to  pour  the  water  upon  the  sand, 
since  with  the  latter  method  it  is  difficult  to  eliminate  the  air- 
bubbles, — particularly  if  the  sand  be  first  rammed.  If  the  sand  is 
dirty  and  the  water  is  poured  upon  it,  there  is  liability  of  the  clay's 
being  washed  down  and  puddling  a  stratum  Avhich  will  prevent  the 
water  penetrating  to  the  bottom.  If  the  bubbles  are  not  excluded, 
or  if  the  water  does  not  penetrate  to  the  bottom,  the  result  obtained 
is  less  than  the  true  proportion  of  voids.  Again,  if  the  sand  is 
dropped  through  a  considerable  depth  of  water,  there  is  liability 
that  the  sand  may  become  separated  into  strata  having  a  single  size 
of  grains  in  each,  in  which  case  the  voids  will  be  greater  than  if 
the  several  sizes  were  thoroughly  mixed. 

The  per  cent,  of  voids  varies  with  the  moisture  of  the  sand. 
A  small  per  cent,  of  moisture  has  a  surprising  effect  upon  the 
volume  and  consequently  upon  the  per  cent,  of  voids.  For 
example,  fine  sand  containing  2  per  cent,  of  moisture  uniformly 
distributed  has  nearly  20  per  cent,  greater  volume  than  the  same 
sand  when  perfectly  dry.  This  effect  of  moisture  increases  with 
the  fineness  of  the  sand  and  decreases  with  the  amount  of  water 
present. 

114^.  Table  10^7,  page  79^,  shows  the  voids  of  a  number  of  both 
artificial  and  natural  sands.     An  examination  of  the  table  shows 

*  A  1  to  2  mortar  ■with  this  sand  was  only  79  per  cent,  as  strong  as  the  pre- 
ceding; and  with  a  1  to  3  mortar  only  71  per  cent. — Trans.  Can.  Soc.  of  C.  E.,  vol.  ix. 
p.  297. 


79A  SAND.  [chap.  Ilia. 

that  the  voids  of  natural  sand  when  rammed  vary  from  30  to  37 
per  cent.  Sands  Nos.  10,  11,  and  12  are  very  good;  but  Xos.  13 
and  14  are  very  poor.  All  five  are  frequently  employed  in  actual 
work.  Compare  the  fineness  of  these  sands  with  those  in  Table 
10/,  page  79e. 

114/i.  The  following  observations  may  be  useful  in  investigating 
the  relative  merits  of  different  sands: 

The  proportion  of  voids  is  independent  of  the  size  of  the 
grains,  but  depends  upon  the  gradation  of  the  sizes;  and  varies 
with  the  form  of  the  grains  and  the  roughness  of  the  surface.  A 
mass  of  perfectly  smooth  spheres  of  uniform  size  would  have  the 
same  proportion  of  voids,  whether  the  spheres  be  large  or  small. 
A  mass  of  perfectly  smooth  spheres  packed  as  closely  as  possible 
would  have  26  per  cent,  of  voids;  but  if  the  sj^heres  are  packed  as 
loosely  as  possible  the  voids  would  be  48  per  cent.  A  promiscuous 
mass  of  bird-shot  has  about  36  per  cent,  of  voids.  The  difference 
between  this  and  the  theoretical  minimum  per  cent,  for  perfectly 
smooth  spheres  is  due  to  the  variation  in  size,  to  roughness  of  the 
surface,  and  to  not  securing  in  all  parts  of  the  mass  the  arrangement 
of  the  shot  necessary  for  minimum  voids.  German  standard  sand 
has  grains  nearly  spherical  and  nearly  uniform  in  size,  having 
slightly  rough  surface,  and  has  41  per  cent,  voids  loose  (see  line  2, 
Table  10^).  The  difference  in  the  per  cent,  of  voids  between  this 
sand  and  a  mass  of  spheres  uniform  in  size  and  perfectly  spherical 
is  due  to  irregularities  in  form  and  to  roughness  of  surface  of  the 
sand  grains,  and  to  not  securing  the  arrangement  of  the  grains 
necessary  for  minimum  voids.  Crushed  stone  retained  between  the 
same  sieves  as  German  standard  sand  has  55  per  cent,  of  voids  (see 
lines  1  to  3  of  Table  10^),  the  excess  of  this  over  German  standard 
sand  being  due  to  the  rough  surfaces  and  sharp  corners  preventing 
the  grains  from  fitting  closely  together. 

If  the  mass  consists  of  a  mixture  of  two  sizes  such  that  the 
smaller  grains  can  occupy  the  voids  between  the  larger,  then  the 
proportion  of  voids  may  be  very  much  smaller  than  with  a  single 
size  of  grains.  For  this  reason  a  mixture  of  two  grades  of  sand  of 
widely  different  sizes  has  a  smaller  per  cent,  of  voids  than  any  one 
size  alone, — compare  lines  1  to  9  with  the  remainder  of  Table  10^^'. 

The  best  sand  is  that  which  has  grains  of  several  sizes  such  tliat 
the  smaller  grains  fit  into  the  voids  of  the  larger,  the  j)roportion  of 
any  particular  size  being  only  sufficient  to  fill  the  voids  between 


ART.  1.1 


VOIDS. 


79i 


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79;  SAND.  [chap.  ma. 

the  grains  of  the  next  larger  size.  If  the  grains  are  spherical  and 
the  diameter  of  the  smaller  is  about  one  fifth  of  the  diameter  of  the 
larger,  the  smaller  grains  will  just  fit  into  the  interstice  between 
the  larger  ones.  The  smaller  the  voids  the  greater  the  economy, 
and  the  denser  and  stronger  the  mortar. 

The  finer  the  sand  the  more  nearly  uniform  the  size  of  the 
grains,  and  consequently  the  greater  the  proportion  of  voids.  Also 
the  finer  the  sand  the  less  sharp  it  is,  and  consequently  the  greater 
the  surface  to  be  covered.  Therefore  a  coarse  sand  is  to  be  preferred 
to  a  very  fine  one — see  Tables  lOe  and  10/,  page  ?9e.  Further,  the 
advantage  of  coarse  sand  over  fine  increases  as  the  proportion  of 
cement  decreases,  since  with  the  smaller  proportions  of  cement  the 
Toids  are  not  filled. 

114?'.  Conclusion.  An  examination  of  the  preceding  data  shows 
that  very  fine  sand  makes  a  much  weaker  mortar  than  coarse  sand, 
and  also  that  natural  sands  vary  considerably  in  the  proportion  of 
voids  and  consequently  differ  in  the  amount  of  cement  required  to 
produce  any  particular  strength.  Therefore  before  adopting  a  sand 
for  a  work  of  any  considerable  magnitude,  all  available  sands  should 
be  carefully  examined  with  reference  to  (1)  their  effects  upon  the 
strength  of  the  mortar,  (2)  their  per  cent,  of  voids  or  the  amount 
of  cement  required  with  each,  and  (3)  their  cost.  If  mortar  of  any 
particular  strength  is  desired,  the  proportion  of  cement  should  be 
adjusted  according  to  the  fineness  and  voids  of  the  best  available 
sand. 

114/.  Stone  Sceeenings.  The  finer  particles  screened  out  of 
crushed  stone  are  sometimes  used  instead  of  sand.  For  the 
physical  characteristics  of  stone  screenings  see  Nos.  16  and  17, 
page  7 9 1. 

Experiments  show  that  sandstone  screenings  give  a  slightly 
stronger  mortar  than  natural  sand,  probably  because  of  the  greater 
sharpness  of  the  grains.  Crushed  limestone  usually  makes  a  con- 
siderably stronger  mortar,  in  both  tension  and  compression,  than 
natural  sand,  and  this  difference  seems  to  increase  with  the  age  of 
the  mortar.*  Part  of  the  greater  strength  is  unquestionably  due 
to  the  greater  sharpness  of  the  limestone  screenings,  and  the  part 

*  Annual  Report  of  Chief  of  Engineers,  U.  S.  A.,  1893,  Part  3,  p.  3015  ;  do.  1894, 
Part  4,  p.  2321;  do.  1895,  Part  4,  p.  2953;  Jour.  West.  Soc.  of  Engrs.,  vol.  ii,  pp.  394 
and  400. 


ART.  2.]  GRAVEL.  79^ 

that  increases  with  the  age  of  the  mortar  seems  to  be  due  to  some 
■chemical  action  between  the  limestone  and  the  cement. 

114^-.  Cost  and  Weight  of  Sand.  The  price  of  reasonably  good 
sand  varies  from  40  cents  to  81.  GO  per  yard,  according  to  locality. 

Sand  is  sometimes  sold  by  the  ton.  It  weighs,  when  dry,  from 
80  to  115  pounds  per  cubic  foot  (see  Table  10^^,  page  79t),  or  about 
1  to  1^  tons  per  cubic  yard. 

Art.  2.    Gravel  and  Broken  Stone. 

115.  The  term  gravel  is  sometimes  used  as  meaning  a  mixture 
of  coarse  pebbles  and  sand,  and  sometimes  as  meaning  pebbles  with- 
out sand.  In  this  volume,  gravel  will  be  understood  as  a  mixture 
of  coarse  pebbles  and  sand. 

115r/.  Gravel  and  broken  stone  are  mixed  with  cement  mortar 
to  make  an  artificial  stone  called  concrete  (Art.  2,  Chap.  IV).  The 
quality  of  the  concrete  varies  greatly  with  the  condition  of  the 
gravel  or  broken  stone,  but  unfortunately  too  little  attention  is 
given  to  the  character  of  this  component. 

1155.  Gkavel.  To  be  suitable  for  use  in  making  concrete, 
gravel  should  be  clean,  and  it  should  be  composed  of  durable 
minerals,  and  the  size  of  the  pebbles  and  grains  should  be  such  as 
to  give  minimum  voids. 

The  investigation  of  the  suitability  of  gravel  for  use  in  concrete 
is  essentially  the  same  as  that  of  sand,  which  has  been  fully  con- 
sidered in  the  preceding  article. 

The  physical  characteristics  of  pebbles  and  gravel  are  given  near 
the  foot  of  Table  lOh,  page  80.  Judging  from  the  little  data  that 
•can  be  found  in  engineering  literature  and  from  all  the  information 
gathered  by  an  extensive  correspondence,  gravels  No.  16  and  No. 
17  of  the  table  are  representative  of  the  gravels  employed  in  actual 
work. 

Concerning  No.  18  notice  that  65  per  cent,  passed  a  No.  5 
screen ;  and  therefore  this  mixture  could  more  properly  be  called 
gravelly  sand.  If  one  fifth  of  the  material  passing  the  No.  5  sieve 
be  omitted,  the  voids  of  the  remainder  will  be  only  15  per  cent, 
when  rammed  ;  in  other  words,  if  one-tenth  of  this  gravel  were 
sifted  on  a  No.  5  sieve  and  tliat  portion  I'etained  on  the  sieve  were 
mixed  with  the  remainder  of  the  original,  the  voids  would  be 
reduced  to  15  per  cent.,  which  would  improve  the  quality  of  the 


791  BROKEN    STONE.  [CHAP.  lUa. 

gravel  for  making  concrete.  This  is  a  valuable  hint  as  to  the  pos- 
sible advantage  of  sifting  even  a  portion  of  the  gravel. 

115c.  Broken  Stone.  Any  hard  and  durable  stone  is  suitable 
for  use  in  making  concrete.  It  is  usual  to  specify  that  the  stone 
for  concrete  shall  be  broken  to  pass,  every  way,  through  a  2-inch 
ring,  although  it  is  sometimes  broken  to  pass  a  1-inch  ring.  The 
stone  should  be  broken  small  enough  to  be  conveniently  handled 
and  easily  incorporated  with  the  mortar.  The  finer  the  stone  is 
broken  the  greater  its  cost,  and  the  greater  the  surface  to  be  coated ; 
and  consequently  the  greater  the  amount  of  cement  required. 
Approximately  cubical  pieces  are  preferable  to  long,  thin,  splintery 
fragments,  since  the  latter  are  liable  to  break  under  pressure  or 
while  being  rammed  into  place,  and  thus  leave  two  uncemented 
surfaces. 

115d.  Voids.  The  proportion  of  voids,  i.e.,  interstices  between 
the  fragments,  may  be  determined  in  either  of  two  ways  as  follows : 

1.  The  voids  may  be  found  by  filling  a  vessel  with  the  aggregate, 
and  then  pouring  in  water  until  the  vessel  is  full.  The  amount  of 
water  required  to  fill  the  voids  divided  by  the  amount  of  water 
alone  the  vessel  will  contain  is  the  proportion  of  voids  in  the 
aggregate.  The  amount  of  water  in  each  case  may  be  determined 
by  weight  or  by  volume. 

For  some  precautions  applicable  in  this  case,  particularly  in 
determining  the  voids  of  broken  stone  containing  considerable  fine 
material,  see  §  114/.  If  the  material  is  porous,  it  is  best  to  wet  it, 
so  as  to  determine  the  voids  exterior  to  the  fragments.  The  water 
absorbed  by  the  material  should  not  be  included  in  the  voids,  since 
when  the  concrete  is  mixed  the  aggregate  is  usually  dampened, 
particularly  if  it  is  porous.  Of  course  in  wetting  the  aggregate 
before  determining  the  voids  no  loose  water  should  remain  in  the 
pile.  The  voids  may  be  determined  for  the  material  either  loose 
or  compacted.  The  proportion  of  the  voids  is  found  to  determine 
the  amount  of  mortar  required  to  fill  the  voids  of  the  concrete  in 
place ;  and  therefore  it  is  better  to  determine  the  voids  in  the  com- 
pacted mass,  since  the  concrete  is  usually  rammed  when  laid.  The 
compacting  may  be  done  by  shaking  or  by  ramming,  the  latter 
being  the  better  since  it  more  nearly  agrees  with  the  conditions 
under  which  the  concrete  is  used,  and  further  since  in  compacting 
by  shaking  the  smaller  pieces  work  to  the  bottom  and  the  larger  to 
the  top,  which  separation  increases  the  voids. 


ART.  2.]  VOIDS.  79w> 

This  method  usually  gives  results  slightly  too  small,  owing  to 
the  difficulty  of  excluding  all  the  air-bubbles.  However,  a  high 
degree  of  accuracy  can  not  be  expected,  since  the  material  is  neither 
uniform  in  composition  nor  uniformly  mixed. 

2.  To  find  the  voids  determine  the  specific  gravity  of  a  frag- 
ment of  the  material  (§  7) ,  and  from  that  the  weight  of  a  unit  of 
volume  of  the  solid;  and  also  weigh  a  unit  of  volume  of  the  aggre- 
gate. The  difference  between  these  weights  divided  by  the  first 
gives  the  proportion  of  voids. 

115e.  Table  lOA,  page  80,  shows  the  per  cent,  of  voids  in 
various  grades  of  broken  stones  used  in  making  concrete. 

The  per  cent,  of  voids  in  broken  stone  varies  with  the  hardness 
of  the  stone,  the  form  of  the  fragments,  and  the  relative  propor- 
tions of  the  several  sizes  present.  The  last  is  the  most  important. 
If  broken  stone  jDassing  a  3^-inch  ring  and  not  a  ^-inch  screen 
be  separated  into  three  sizes,  any  one  size  will  give  from  52  to  54 
per  cent,  of  voids  loose,  while  equal  parts  of  any  two  of  the  three 
sizes  will  give  48  to  50  j)er  cent.,  and  a  mixture  in  which  the 
volume  of  the  smallest  size  is  equal  to  the  sum  of  the  other  two 
gives  a  trifle  less  than  48  per  cent.  Notice,  however,  that  un- 
screened crushed  stone  has  only  32  to  35  per  cent,  voids — see  lines 
7  and  11  of  Table  lOh.  This  is  a  very  excellent  reason  for  not 
screening  the  broken  stone  to  be  used  in  making  concrete. 

A  mass  of  pebbles  has  only  about  three  fourths  as  many  voids 
as  a  mass  of  broken  etone  having  pieces  retained  between  the  same 
screens.  Notice,  however,  that  gravel,  i.e.  pebbles  and  sand,  has  a 
less  proportion  of  voids  than  pebbles  alone. 

115/.  Cost  and  Weight.  The  cost  of  breaking  stone  for  con- 
crete varies  from  50  to  75  cents  per  cubic  yard  according  to  kind 
of  stone  and  size  of  plant.*  The  original  cost  of  the  stone  and 
transportation  expenses  are  too  variable  to  attempt  to  generalize. 
Ordinarily  the  cost  of  broken  stone  is  not  more  than  11.50  to  $2.00 
per  cubic  yard  f.  o.  b.  cars  at  destination. 

The  weight  of  broken  stone  varies  from  85  to  120  lbs.  per  cubic 
foot  (see  Table  lOh,  page  80) ;  or  about  2200  to  3200  pounds  per 
cubic  yard. 

*  For  additional  datii,  see  Supplemental  Notes,  No.  5,  p.  546. 


80 


BROKEN   STONE. 


[chap.  Ilia. 


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PART  II. 

METHODS  OF  PREPAKING  AND  USING  THE 
MATERIALS. 


CHAPTEE  IV. 
MORTAR,   CONCRETE,   AND   ARTIFICIAL  STONE. 

Art.  1.     Mortar. 

116.  Mortar  is  a  mixture  of  the  paste  of  cement  or  lime  with 
Band.  In  common  mortar,  the  cementing  snbstance  is  ordinary 
lime;  in  hydraulic  mortar,  it  is  hydraulic  cement. 

117.  Common  Lime  Mortar.  Mortar  made  of  the  paste  of 
common  or  fat  lime  is  extensively  used  on  account  of  (1)  its  intrin- 
sic cheapness,  (2)  its  great  economic  advantage  owing  to  its  great 
increase  of  volume  in  slaking,  and  (3)  the  simplicity  attending  the 
mixing  of  the  mortar.  On  account  of  the  augmentation  of  volume, 
the  paste  of  fat  lime  shrinks  in  hardening,  to  such  an  extent  that 
it  can  not  be  employed  as  mortar  without  a  large  dose  of  sand. 

As  a  paste  of  common  lime  sets  or  hardens  very  slowly,  even  in 
the  open  air,  unless  it  be  subdivided  into  small  particles  or  thin 
films,  it  is  important  that  the  volume  of  lime  paste  in  common 
mortar  should  be  but  slightly  in  excess  of  what  is  sufficient  to  coat 
all  the  grains  of  sand  and  to  fill  the  voids  between  them.  If  this 
limit  be  exceeded,  the  strength  of  the  mortar  will  be  impaired. 
With  most  sands  the  proper  proportions  will  be  from  2.5  to  3 
volumes  of  sand  to  1  volume  of  lime  paste.  Generally,  if  either 
less  or  more  sand  than  this  be  used,  the  mortar  will  be  injured, — 
in  the  former  case  from  excess  of  lime  paste,  and  in  the  latter  from 

81 


82  MORTAR.  [chap.  IV. 

porosity.     Notice  that  the  volume  of  the  resulting  mortar  is  about 
equal  to  the  volume  of  the  sand  alone. 

118.  The  ordinary  method  of  slaking  lime  consists  in  placing 
the  lumps  in  a  layer  6  or  8  inches  deep  in  either  a  water-tight  box, 
or  a  basin  formed  in  the  sand  to  be  used  in  mixing  the  mortar,  and 
pouring  upon  the  lumps  a  quantity  of  water  2^  to  3  times  the 
volume  of  the  lime. 

This  process  is  liable  to  great  abuse  at  the  hands  of  the  work- 
men. They  are  apt  either  to  use  too  much  water,  which  reduces 
the  slaked  lime  to  a  semi-fluid  condition  and  thereby  injures  its 
binding  qualities;  or,  not  having  used  enough  water  in  the  first 
place,  to  seek  to  remedy  the  error  by  adding  more  after  the  slaking 
has  well  progressed  and  a  portion  of  the  lime  is  already  reduced  to 
powder,  thus  suddenly  depressing  the  temperature  and  chilling  the 
lime,  which  renders  it  granular  and  lumpy.  It  is  also  very  im- 
portant that  the  lime  should  not  be  stirred  while  slaking.  The 
essential  point  is  to  secure  the  reduction  of  all  the  lumps.  Cover- 
ing the  bed  of  lime  with  a  tarpaulin  or  with  a  layer  of  sand  retains 
the  heat  and  accelerates  the  slaking.  All  the  lime  necessary  for 
any  required  quantity  of  mortar  should  be  slaked  at  least  one  day 
before  it  is  incorporated  with  the  sand. 

After  the  lime  is  slaked  the  sand  is  spread  evenly  over  the  paste/ 
and  the  ingredients  are  thoroughly  mixed  with  a  shovel  or  hoe,  a 
little  water  being  added  occasionally  if  the  mortar  is  too  stiff. 

119.  Mortar  composed  of  common  lime  and  sand  is  not  fit  for 
thick  walls,  because  it  depends  upon  the  slow  action  of  the  atmos- 
phere for  hardening  it;  and,  being  excluded  from  the  air  by  the 
surrounding  masonry,  the  mortar  in  the  interior  of  the  mass 
hardens  only  after  the  lapse  of  years,  or  perhaps  never.*  The 
mortar  of  cement,  if  of  good  quality,  sets  immediately ;  and,  as  far 
as  is  known,  continues  forever  to  harden  without  contact  with  the 
air.  Cement  mortar  is  the  only  material  whose  strength  increases 
with  age.  Owing  to  its  not  setting  when  excluded  from  the  air, 
common  lime  mortar  should  never  be  used  for  masonry  construction 
under  water,  or  in  soil  that  is  constantly  wet;  and,  owing  to  its 
weakness,  it  is  unsuitable  for  structures  requiring  great  strength,  or 

*Lime  mortar  taken  from  the  walls  of  ancient  buildings  has  been  found  to  be 
only  50  to  80  per  cent,  saturated  with  carbonic  acid  after  nearly  2,000  years  of  ex- 
posure. Lime  mortar  2,000  years  old  has  been  found  in  subterranean  vaults,  in 
exactly  the  condition,  except  for  a  thin  crust  on  top,  of  freshly  mixed  mortar. 


AET.  l.J  METHODS    OF    PROPOKTIONING.  83 


subject  to  shock.  Its  use  iu  engineering  masonry  has  been  aban- 
doned on  all  first-class  railroads.  Cement  is  so  cheap  that  it  could 
profitably  be  substituted  for  lime  in  the  mortar  for  ordinary 
masonry. 

120.  Hydraulic  Lime  Mortar.  With  mortars  of  hydraulic 
lime  the  volume  of  sand  sliould  not  be  less  than  1.8  times  that  of 
the  lime  paste,  in  order  to  secure  the  best  results  regardless  of  cost. 
The  usual  proportions  are,  however,  for  ordinary  work,  the  same 
as  in  common  mortars,  care  being  taken  to  incorporate  sufficient 
paste  to  coat  all  the  grains  of  sand  and  to  fill  up  the  voids  between 
them. 

121.  Hydraulic  Cement  Mortar.  Hydraulic  cement  mortar 
hardens  simultaneously  and  uniformly  throughout  the  mass,  and  if 
the  cement  is  good  continues  to  gain  in  hardness  with  age, — the 
slow-setting  cements  for  a  longer  time  than  the  quick-setting.  For 
the  best  results  the  cement  paste  should  be  just  sufficient  to  coat 
the  grains  and  fill  the  voids  of  the  sand.  More  cement  than  this 
adds  to  the  cost  and  weakens  the  mortar  (see  §  100).  If  the  amount 
of  cement  is  not  sufficient  to  coat  all  the  grains  and  fill  the  voids, 
the  mortar  will  be  weak  and  porous,  and  hence  will  not  be  durable. 
A  dense,  impervious  mortar  is  particularly  desirable  for  masonry 
exposed  to  sea-water,  to  exclude  the  water  from  the  interior  of  the 
mass  and  prevent  its  chemical  as  well  as  physical  action  upon  the 
cement. 

122.  Methods  of  Proportioning.  In  laboratory  work  the  propor- 
tions of  the  cement  and  sand  are  uniformly  determined  by  weigh- 
ing; but  there  is  no  uniform  practice  of  measuring  the  proportions 
on  the  work.  One  of  the  three  following  methods  is  generally 
employed. 

1.  By  Weight.  The  most  accurate  but  least  common  method 
is  to  weigh  the  ingredients  for  each  batch.  This  method  is  incon- 
venient in  practice,  and  adds  somewhat  to  the  cost  of  the  work; 
and  therefore  occasionally  the  weight  of  a  unit  of  volume  of  the 
sand  and  of  the  cement  is  determined,  and  the  relative  volumes  of 
the  ingredients  are  fixed  accordingly,  the  actual  proportioning  being 
done  by  volumes.  Cement  is  bought  and  sold  by  weight,  and 
hence  it  is  very  appropriate  to  proportion  the  mortar  by  weight. 

2.  Packed  Cement  and  Loose  Sa^id.  A  commercial  barrel  of 
cement  is  mixed  with  one  or  more  barrels  of  loose  sand,  i.e.,  the 
proportioning  is  done  by  mixing  one  volume  of  packed  cement  with 


84  MORTAR.  [chap.  IV. 

one  or  more  Yolames  of  loose  sand.  This  method  is  frequently 
used.  As  far  as  the  cement  is  concerned,  it  is  as  accurate  as  the 
first,  since  the  weight  and  volume  of  a  barrel  of  cement  may  readily 
be  known  when  only  whole  barrels  are  used, — as  is  usually  the  case. 
Even  though  the  cement  is  received  in  bags,  the  barrel  of  packed 
cement  is  still  a  convenient  unit,  for  an  integral  number  of  bags, 
usually  three  or  four,  are  equal  in  weight  to  a  barrel.  As  far  as 
the  sand  is  concerned  this  method  is  not  as  accurate  as  the  first. 
The  weight  of  the  sand  is  affected  by  the  amount  of  moisture 
present;  but  a  small  amount  of  moisture  affects  the  volume  in  a 
greater  proportion  than  the  weight.  For  example,  the  addition  of 
2  per  cent,  of  water  (by  weight)  thoroughly  mixed  with  dry  sand 
increases  the  volume  of  the  sand  nearly  20  per  cent.*  Therefore  if 
the  mortar  is  proportioned  by  volumes,  damp  sand  will  give  a  richer 
mortar  than  dry  sand.  The  effect  of  moisture  on  the  volume  is 
greater  the  finer  the  sand,  and  decreases  as  the  amount  of  moisture 
increases.  Measuring  the  sand  by  volumes  is  inaccurate  also  owing 
to  the  packing  of  the  saud. 

Except  for  the  inaccuracies  in  measuring  the  sand,  this  method 
gives  practically  the  same  results  for  Portland  as  the  first  method, 
since  ordinarily  a  unit  of  volume  of  packed  cement  and  of  sand 
weighs  substantially  the  same;  viz.,  100  pounds  per  cubic  foot. 
Since  natural  cement  when  packed  in  barrels  usually  weighs  about 
75  pounds  per  cubic  foot,  a  mortar  of  1  part  natural  cement  to 
1  part  sand  by  weight  is  equivalent  to  1^  parts  cement  to  1  part 
sand  by  volumes  of  j^acked  cement  and  loose  sand. 

3.  Loose  Cement  and  Loose  Sajid.  A  volume  of  loose  cement  is 
mixed  with  one  or  more  volumes  of  loose  sand.  The  actual  propor- 
tioning is  usually  done  by  emptying  a  bag  or  fractional  part  of  a 
barrel  of  cement  into  a  wheelbarrow,  and  filling  one  or  more  wheel- 
barrows equally  full  of  sand.  As  far  as  the  sand  is  concerned,  this 
metiiod  is  as  inaccurate  as  the  second;  and  it  is  also  subject  to  great 
variations  owing  to  differences  in  specific  gravity,  fineness  and 
packing  of  the  cement.  Even  though  inaccurate,  it  is  very  fre- 
quently employed.  It  is  the  most  convenient  method  when  the 
cement  is  shipped  in  bulk, — which  is  only  rarely. 

Occasionally  the  actual  proportioning  is  done  by  throwing  into 

*  Feret,  Chief  of  Laboratory  Fonts  et  Chauss^es,  Id  Engineering  News,  vol. 
xxvii.  p.  310.  For  similar  data  see  Eeport  of  Chief  of  Engineers,  U.  8.  A.,  1895, 
p.  2935. 


ART.   1.]  MIXING    THE   MORTAR.  85 

the  mortar-box  one  shovelful  of  cement  to  one  or  more  shovelfuls 
of  sand.     This  is  very  crude,  and  should  never  be  permitted. 

Since  a  commercial  barrel  of  Portland  will  make  1.1  to  1.4 
barrels  if  measured  loose,  a  mortar  composed  of  1  part  Portland 
cement  to  1  part  sand,  by  weight,  is  equivalent  to  0.7  to  0.8  parts 
cement  to  1  part  sand  by  volumes  of  loose  cement  and  loose  sand ; 
and  a  mortar  composed  of  1  part  natural  cement  to  1  part  sand, 
by  weight,  is  equivalent  to  0.50  to  0.T5  parts  cement  to  1  part  of 
sand  by  volumes  of  loose  cement  and  loose  sand. 

122^.  For  a  tabular  statement  incidentally  showing  the  relative 
amounts  of  cement  required  by  the  three  methods  of  proportioning, 
see  Table  11,  page  88. 

123.  Proportions  in  Practice.  The  proportions  commonly  used 
in  practice  are :  for  Portland  cement,  1  volume  of  cement  to  2  or  3 
volumes  of  sand;  and  for  natural  cement,  1  volume  of  cement  to  1 
or  2  volumes  of  sand.  The  specifications  are  usually  defective  in 
not  defining  which  method  is  to  be  employed  in  proportioning. 
This  is  a  matter  of  great  importance.  Compared  with  the  second 
method  of  proportioning  in  §  129,  the  third  requires  for  Portland 
only  0.7  to  0.8  as  much  cement,  and  for  natural  cement  only  0.4 
to  0.5  as  much. 

124.  Mixing  the  Mortar.  When  the  mortar  is  required  in  small 
quantities,  as  for  use  in  ordinary  masonry,  it  is  mixed  as  follows : 
About  half  the  sand  to  be  used  in  a  batch  of  mortar  is  spread  evenly 
over  the  bed  of  the  mortar-box,  then  the  dry  cement  is  spread 
evenly  over  the  sand,  and  finally  the  remainder  of  the  sand  is 
spread  on  top.  The  sand  and  cement  are  then  mixed  with  a  hoe 
or  by  turning  and  re-turning  with  a  shovel.  The  mixing  can  be 
done  more  economically  with  a  shovel  than  with  a  hoe;  but  the 
effectiveness  of  the  shovel  varies  greatly  with  the  manner  of  using 
it.  It  is  not  sufficient  to  simply  turn  the  mass;  but  the  sand  and 
cement  should  be  allowed  to  run  off  from  the  shovel  in  such  a 
manner  as  to  thoroughly  mix  them.  Owing  to  the  difficulty  of 
getting  laborers  to  do  this,  the  hoe  is  sometimes  prescribed.  If 
skillfully  done,  twice  turning  with  a  shovel  will  thoroughly  mix 
the  dry  ingredients;  although  fonr  turnings  are  sometimes  specified, 
and  occasionally  as  high  as  six  (see  §  2G0).  It  is  very  important 
that  the  sand  and  cement  be  thoroughly  mixed.  When  thoroughly 
mixed  it  will  have  a  uniform  color. 

The  dampness  of  the  sand  is  a  matter  of  some  importance.     If 


86  MOKTAR.  [chap.  IY. 

the  sand  is  very  damp  when  it  is  mixed  with  the  cement,  suflBcient 
moisture  may  be  given  off  to  cause  the  cement  to  set  partially, 
which  may  materially  decrease  its  strength.  This  is  particularly 
noticeable  with  quick-setting  cements. 

The  dry  mixture  is  then  shoveled  to  one  end  of  the  box,  and 
water  is  poured  into  the  other.  The  sand  and  cement  are  then 
drawn  down  with  a  hoe,  small  quantities  at  a  time,  and  mixed  with 
water  until  enough  has  been  added  to  make  a  stiif  paste.  The 
mortar  should  be  vigorously  worked  to  insure  a  uniform  product. 
When  the  mortar  is  of  the  proper  plasticity  the  hoe  should  be  clean 
when  drawn  out  of  it,  or  at  most  but  very  little  mortar  should  stick 
to  the  hoe. 

Cements  vary  greatly  in  their  capacity  for  water  (see  §  104),  the 
naturals  requiring  more  than  the  Portlands,  and  the  fresh-ground 
more  than  the  stale.  An  excess  of  water  is  better  than  a  deficiency, 
particularly  with  a  quick-setting  cement,  as  its  capacity  for  com- 
bining with  water  is  very  great;  and  farther  an  excess  is  better  than 
a  deficiency,  owing  to  the  possibility  of  the  water  evaporating 
before  it  has  combined  with  the  cement.  On  the  other  hand,  an 
excess  of  water  makes  a  porous  and  weak  mortar.  If  the  mortar  is 
stiff,  the  brick  or  stone  should  be  dampened  before  laying;  else  the 
brick  will  absorb  the  water  from  the  mortar  before  it  can  set,  and 
thus  destroy  the  adherence  of  the  mortar.  In  hot  dry  weather,  the 
mortar  in  the  box  and  also  in  the  wall  should  be  shielded  from  the 
direct  rays  of  the  sun. 

When  mortar  is  required  in  considerable  quantities,  as  in  making 
concrete,  it  is  usually  mixed  by  machinery  (see  §  156w). 

125.  Gkout.  This  is  merely  a  thin  or  liquid  mortar  of  lime  or 
cement.  The  interior  of  a  wall  is  sometimes  laid  up  dry,  and  the 
grout,  which  is  poured  on  top  of  the  wall,  is  expected  to  find  its 
way  downwards  and  fill  all  voids,  thus  making  a  solid  mass  of  the 
wall.  G-rout  should  never  be  used  when  it  can  be  avoided.  If 
made  thin,  it  is  porous  and  weak;  and  if  made  thick,  it  fills  only 
the  upper  portions  of  the  wall.  To  get  the  greatest  strength,  the 
mortar  should  have  only  enough  water  to  make  a  stiff  paste — the 
less  water  the  better. 

126.  Data  for  Estimates.  The  following  will  be  found  use- 
fiil  ill  estimating  the  amounts  of  the  different  ingredients  necessary 
to  i^roduce  any  required  quantity  of  mortar: 

Lime  weighs  about  230  pounds  per  barrel.     One  barrel  of  lime 


ART.   1.]  DATA    FOR    ESTIMATES.  87 

will  make  about  2^  barrels  (0.3  ca.  yd.)  of  stiff  lime  paste.  One 
barrel  of  lime  paste  and  three  barrels  of  sand  will  make  about 
three  barrels  (0.4  cu.  yd.)  of  good  lime  mortar.  One  barrel  of 
unslaked  lime  will  make  about  6.75  barrels  (0.95  cu.  yd.)  of  1 
to  3  mortar. 

Portland  cement  weighs  370  to  380  pounds  per  barrel  net  (see 
§  77,  page  54).  The  capacity  of  a  Portland  cement  barrel  varies 
from  3.20  to  3.75  cu.  ft.,  the  average  being  3.49*  or  practically 
3.50  cu.  ft.  A  barrel  of  Portland  will  make  from  1.1  to  1.4  bar- 
rels if  measured  loose.  A  cubic  foot  of  packed  Portland  cement 
(105  pounds)  and  about  0.33  cu.  ft.  of  water  will  make  1  cu.  ft.  of 
stiff  paste;  and  a  cubic  foot  of  loose  cement  (gently  shaken  down 
but  not  compressed)  will  make  about  0.8  cu.  ft.  of  stiff  paste. 

Natural  cement  weighs  from  2G5  to  300  pounds  per  barrel  net 
(see  §  77,  page  54).  The  capacity  of  a  natural  cement  barrel  varies 
from  3.37  to  3.80  cu.  ft.,  the  average  being  3.52,*  or  practically 
3.50  cu.  ft.  A  barrel  of  natural  cement  will  make  from  1.33  to 
1.50  barrels  if  measured  loose.  Volume  for  volume,  natural 
cement  will  make  about  the  same  amount  of  paste  as  Portland;  or 
a  cubic  foot  of  packed  natural  cement  (75  pounds)  and  about  0.45 
cu.  ft.  of  water  will  make  1  cu.  ft.  of  stiff  paste,  and  a  cubic  foot 
of  loose  cement  (gently  shaken  down,  but  not  compressed)  will 
maKe  about  0.8  ou.  ft.  of  stiff  j^aste. 

128.  Quantities  for  a  Yard  of  Mortar.  Table  11,  page  88, 
shows  the  approximate  quantities  of  cement  and  sand  required  for 
a  cubic  yard  of  mortar  by  the  three  methods  of  proportioning 
described  in  §  122.  The  table  is  based  upon  actual  tests  made  by 
mixing  "d^  cubic  feet  of  the  several  mortars;  f  but  at  best  such  data 
can  be  only  approximate,  since  so  much  depends  upon  the  specific 
gravity,  fineness,  compactness,  etc.,  of  the  cement;  upon  the  fine- 
ness, humidity,  sharpness,  compactness,  etc.,  of  the  sand;  and 
npon  the  amount  of  water  used  in  mixing.  The  sand  employed  in 
deducing  Table  11  contained  37  per  cent,  of  voids  when  measured 
loose;  and  the  plasticity  of  the  mortar  was  such  that  moisture 
flushed  to  the  surface  when  the  mortar  was  struck  with  the  back 
of  the  shovel  used  in  mixing. 

The  volume  of  the  resulting  mortar  is  always  les&  than  the  sum 

*  The  Technogeaph,  University  of  Illinois,  No,  11,  p.  104. 
+  By  L.  C.  Sabin,  Assistant  U,  S.  Engineer — see  Report  of  Chief  of  Engineers, 
U,  S.  A.,  1894,  p.  2326. 


MORTAR. 


[chap.  IV. 


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AKT.   i.J 


DATA    FOE   ESTIMATES. 


«9 


TABLE   12. 
Amount  of  Mortar  required  for  a  Cubic  Yard  of  Masonry. 


Ref. 

No 

Dbsckiption  op  Masonry. 

MORTAB, 

cu.  yd. 

Min. 

Max. 

1 

Ashlar, — 18"  courses  and  ^"  joints 

0.03 
0.06 

0.10 
0.25 
0.35 

0.33 
0.20 

0.12 
0.20 

0  04 

3 

12"       "         "     "       "     

0.08 

3 
4 
5 

6 

7 

Brickwork,— standard  size  (§  256)  and  \"       joints 

"    f"  toi"  "     .... 

"     "    "  r tor "  •.-. 

Concrete— see  Tables  \Zd  and  13e,  pages  112^',  112/i. 
Rubble, — small,  rough  stones 

0.15 
0.35 
0.40 

0.40 

8 

9 

10 

1 

large  stones,  rough  hammer- dressed 

Squared-stone  masonry, — 18"  courses  and  f"  joints 

12"       "        "    "       "     .... 

0.30 

0.15 
0.25 

of  the  volumes  of  the  cement  and  sand,  or  of  the  paste  and  sand, 
because  part  of  the  paste  enters  the  Toids  of  the  sand;  but  the 
volume  of  the  mortar  is  always  greater  than  the  sum  of  the  volumes 
of  the  paste  and  the  solids  in  the  sand,  because  of  imperfect  mixing 
and  also  because  the  paste  coats  the  grains  of  sand  and  thereby 
increases  their  size  and  consequently  the  volume  of  the  interstices 
between  them.  This  increase  in  volume  varies  with  the  dampness 
and  compactness  of  the  mortar.  For  example,  the  volume  of  a 
rather  dry  mortar  with  cement  paste  equal  to  the  voids,  when 
compacted  enough  to  exclude  great  voids,  was  126  per  cent,  of  the 
sum  of  the  volumes  of  the  paste  and  solids  of  the  sand;  and  the 
same  mortar  when  rammed  had  a  volume  of  102  to  104  per  cent. 
If  the  paste  is  more  than  equal  to  the  voids,  the  per  cent,  of  in- 
crease is  less ;  and  if  the  paste  is  not  equal  to  the  voids,  the  per  cent, 
of  increase  is  more.  The  excess  of  the  volume  of  the  mortar  over 
that  of  the  sand  increases  with  the  fineness  of  the  sand  and  with 
the  amount  of  mortar  used  in  mixing. 

129.  Mortar  for  a  Yard  of  Masonry.     Table  12,  page  89,  gives 


90  MORTAR.  [CHAP,  IV. 

data  concerning  the  amount  of  mortar  required  per  cubic  yard  for 
the  different  classes  of  masonry,  extracted  from  succeeding  pages 
of  this  volume;  and  are  collected  here  for  greater  convenience  in 
making  estimates. 

130.  Strength  of  Mortae.  The  strength  of  mortar  is 
dependent  upon  the  strength  of  the  cementing  material,  upon  the 
strength  of  the  sand,  and  upon  the  adhesion  of  the  former  to  the 
latter.  The  kind  and  amount  of  strength  required  of  mortar 
depends  upon  the  kind  and  purpose  of  the  masonry.  If  the  blocks 
are  large  and  well  dressed,  and  if  the  masonry  is  subject  to  com- 
pression only,  the  mortar  needs  only  hardness  or  the  property  of 
resisting  compression;  hard  sharp  grains  of  sand  with  comparatively 
little  cementing  material  would  satisfy  this  requirement  fairly  well. 
If  the  blocks  are  small  and  irregular,  the  mortar  should  have  the 
capacity  of  adhering  to  the  surfaces  of  the  stones  or  bricks,  so  as  to 
prevent  their  displacement;  in  this  case  a  mortar  rich  in  a  good 
cementing  material  should  be  used.  If  the  masonry  is  liable  to  be 
subject  to  lateral  or  oblique  forces,  the  mortar  should  possess  both 
adhesion  and  cohesion. 

131.  Tensile  Strength.  E'ig.  5  shows  the  effect  of  time  upon 
the  strength  of  various  mortars.  The  diagram  represents  the 
average  results  of  a  great  number  of  experiments  made  in  connec- 
tion with  actual  practice.  Eesults  which  were  uniformly  extremely 
high  or  low  as  compared  with  other  experiments  were  excluded  on 
the  assumption  that  the  difference  was  due  to  the  method  of  mould- 
ing and  testing.  Since  the  individual  values  jilotted  were  them- 
selves means,  there  were  no  very  erratic  results,  and  consequently 
the  lines  are  quite  reliable.  There  were  fewer  experiments  for  the 
larger  proportions  of  sand  to  cement,  and  hence  the  curves  are  less 
accurate  the  larger  the  proportion  of  sand. 

The  line  for  the  strength  of  lime  mortar  probably  represents  the 
maximum  value  that  can  be  obtained  by  exposing  the  mortar  freely 
to  the  air  in  small  briquettes.     This  line  is  not  well  determined. 

Unusually  hard-burned  Portland  cements  when  tested  neat 
will  show  a  greater  strength  than  that  given  in  the  diagrams. 
Very  fine  cement  when  mixed  with  sand  will  show  greater  strength 
■than  that  given  by  Fig.  5.  Again,  the  diagram  shows  neat  cement, 
both  Portland  and  natural,  stronger  than  any  proportion  of  sand, 
while  frequently  neat  cement  mortar  is  not  as  strong  as  a  mortar 
composed  of  one  part  sand  and  one  part  cement — particularly  at 


ART.  1.] 


TENSILE    STRENGTH. 


91 


the  greater  ages.  However,  notwithstanding  these  exceptions,  it 
is  believed  that  the  results  represent  fair  average  practice.  The 
proportions  of  sand  to  cement  were  determined  by  weight. 

132.  The  results  in  Fig.   5  are  tabulated  in  another  form  in 
Fig.  6,  to  show  the  effect  of  varying  the  proportions  of  the  sand 


100 


Fig.  5. — Diagram  showing  Effect  of  Time  ox  Strength  of  Mortars. 

and  cement,  and  also  to  show  the  relative  strength  of  natural  and 
Portland  cement  mortars  at  difiereut  ages.  The  curves  of  Fig.  6 
are  especially  useful  in  discussing  the  question  of  the  relative 
economy  of  Portland  and  natural  cement  (§  13G).  For  example, 
assume  that  we  desire  to  know  the  strength  of  a  1  to  2  natural 
cement  mortar  a  year  old,  and  also  the  proportions  of  a  Portland 
cement  mortar  of  equal  strength.     iVt  the  bottom  of  the  lower 


92 


MORTAR. 


[chap.  IV. 


right-hand  diagram  of  Fig.  6  find  the  proportion  of  sand  in  the 
mortar,  which  in  this  case  is  2;  follow  the  corresponding  line  np 
nntil  it  intersects  the  "  natural  "  line.  The  elevation  of  this  in- 
tersection above  the  base,  as  read  from  the  figure  at  the  side  of 
the  diagram,  is  the  strength  of  the  specified  mixture,  which  in  this 

60a 1 1 r- — r— 1 1 1 1       I : 1 . 1 1 1 1 ,600 


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600 
500 
400 
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0       12      3      4      5     6      7 

Parts   ^and  to  I  Part  Cement    by   Weight 

Fig.  6. — Diagram  showing  Relative  Strength  of  Cement  Mortars. 

case  is  about  250  pounds  per  square  inch.  The  second  part  of  the 
problem  then  is  to  determine  the  proportions  of  a  Portland  cement 
mortar  which  will  have  a  strength  of  250  pounds  per  square  inch. 
Find  the  250  point  on  the  scale  at  the  side  of  the  diagram,  and 
imagine  a  horizontal  line  passing  through  this  point  and  intersect- 


AtCT.  1.]  COMPRESSIVE    STRENGTH.  93 


ing  the  "  Portland  "  line;  from  this  point  of  intersection  draw  a 
■vertical  line  to  the  base  of  the  diagram,  and  this  point  of  intersec- 
tion gives  the  required  number  of  volumes  of  sand  to  one  volume  of 
cement,  which  in  this  case  is  5.5.  Therefore  a  1  to  2  natural 
mortar  a  year  old  has  a  strength  of  250  pounds  per  square  inch,  and 
is  then  equivalent  to  a  1  to  5.5  Portland  mortar. 

133.  Compressive  Strength.  But  few  experiments  have  been 
made  upon  the  compressive  strength  of  mortar.  An  examination 
of  the  results  of  about  sixty  experiments  made  with  the  Watertown 
testing-machine  seems  to  show  that  the  compressive  strength  of 
mortar,  as  determined  by  testing  cubes,  is  from  8  to  10  times  the 
tensile  strength  of  the  same  mortar  at  the  same  age.  This  ratio 
increases  with  the  age  of  the  mortar  and  with  the  proportion  of 
sand.  The  standard  German  specifications  require  that  the  com- 
pressive strength  of  cement  mortar  shall  be  at  least  10  times  the 
tensile  strength. 

Data  determined  by  submitting  cu^es  of  mortar  to  a  compressive 
stress  are  of  little  or  no  value  as  showing  the  strength  of  mortar 
when  employed  in  thin  layers,  as  in  the  joints  of  masonry.  The 
strength  per  unit  of  bed  area  increases  rapidly  as  the  thickness  of 
the  test  specimen  decreases,  but  no  experiments  have  ever  been 
made  to  determine  the  law  of  this  increase  for  mortar. 

134.  Adhesive  Strength.  Unfortunately  very  few  experiments 
have  been  made  on  the  adhesive  strength  of  mortars,  i.e.,  the 
power  with  which  mortars  stick  to  brick,  stone,  etc.  It  is  com- 
monly assumed  that,  after  the  lapse  of  a  moderate  time,  the 
adhesive  and  cohesive  strengths  of  cement  mortars  are  about  equal, 
and  tbat  in  old  work  the  former  exceeds  the  latter.  Modern 
experiments,  however,  fail  to  establish  the  truth  of  this  assumption, 
and  indicate  rather  that  the  adhesion  of  mortar  to  brick  or  stone  is 
much  less,  during  the  first  few  months,  than  its  tensile  strength ; 
and  also  that  the  relation  between  the  adhesive  strength  and 
cohesive  strength  (the  resistance  of  the  mortar  to  pulling  asunder) 
is  very  obscure.  The  adhesion  of  mortars  to  brick  or  stone  varies 
greatly  with  the  different  varieties  of  these  materials,  and  particu- 
larly with  their  porosity.  The  adhesion  also  varies  with  the  quality 
of  the  cement,  the  character,  grain,  and  quantity  of  the  sand,  the 
amount  of  water  used  in  tempering,  the  amount  of  moisture  in  the 
stone  or  brick,  and  the  age  of  the  mortar.  Some  cements  which 
exhibit  high  tensile  strength  give  low  values  for  adhesion;  and  con- 


94 


MORTAR. 


[chap.  IV. 


TABLE  13. — Adhesive  Stkength  of  Mortals. 


o 

a 
a 

< 

Kind  of  Cement  used. 

Materials 
Cemented  to- 
gether. 

Average  adhesive  strength  In 
pounds  per  square  inch. 

, 

i 

15 

0 

h 

0" 

S  3 

0)  to 

0 

zi. 
ii 

Authority. 

1 

2 
3 
4 
5 
6 

- 

Hard  brick 

*23 
*15 

Robertson 1858 

16 

28 

30 

Sawed  limest'ne 

Cut  granite 

Folislied  marble 
Bridgewater 
brick 

57 
41 
38 

19 
24.1 

168 

I.J.Mann 1883 



II       11              II 

Hydraulic  lime 

Portland 

11       11              11 

7 
8 
9 
10 
11 
12 
13 
14 
15 
16 
17 

+Brick 

21.0 
102 
117 

18.7 

38 

53 

9-15 

5 

25.5 

45 

73 
*59 
*30 

12.3 

12.0 

15.3 

20 

26 

13.2 
9 
16 

Prof.  Warren.. '87 

§      "      

Quicklime 

Lime  and  cement 

Hydraulic  lime 

tBrick 

■■35J 
213 

' '  30.4 
105 
146 

20.9 
24 

48 

17.5 

14 

45 

Dr.  Bohme. . .  .1883 

t    "      

Prof.  Warren.. '87 

§    "     

Quick-setting  cement 
Slow-setting  cement. 

Robertson 1858 

" 

Croton  brick 

Fine-cut  granite 

30.S 
27.5 
78 

1197 
IITl 

1166 
49 

15.7 
20.8 

6.8 
9.2 

■5.2 
7.9 

Gen.  GIllmore.'63 

18 

42 
48 
56 
9U 
95 
110 
180 

19 
20 
21 
22 

I.J.  Mann 1882 

Cut  granite 

Polished  marble 
Bridgewater 
brick 

11 

II       II              II 

II 

,, 

[1       II              11 

23 

Sandstone 

Staffordshire 
brick 

11       II              11 

24 

*40 
*36 
*18 

*5 

46.9 

Building  News,'8(y 

25 

Gray  stock  brick 
Common  soft  '• 
Hard  brick 

26 

11    11       it 

11              11      11 

27 

Lime  and  pozzuolaua 

J.  White 1832 

28 

TTBrick 

68.8 

Bauschinger..l873 

29 

T       "      

24.2 

'28.1 
14.2 
12.8 

30 

11 

t       "    

54.0 
41.9 

56.9 
38.9 

.1                11 

31 

Hydraulic  lime 

+        "     

39.3 

22.6  Dr.  Bohme.... 1883 

32 

IT        "     

Bauschinger..l873 

33 

Hydraulic  lime 

IT       "    

34 

Brick 

♦83 

*15 
*40 
•18 

Rondelet 1831 

35 

36 

11 

Hard  brick 

Robertson 1858 

37 

27U 

II 

Soft  brick 

38 
39 

Portland Sawed  slate 

1162 
1155 
1175 

I.J.  Mann....l88S 

40 

Polished  marble 

f(       11              11 

41 

*8 
24 

J.  White 1832 

42 

320  Rnsfindalft 

Croton  brick 

68 

40 

Gen.Gillmore  1863 

4S 

lyr 

Quicklime 

Good  quicklime 

Ordinary      hydraulic 

lime 

Good  hydraulic  lime 

♦21 
♦51 

Vicat 1818 

44 

45 

11        11 

*85 
*140 

<i                       11 

46 

11        II 

11                       11 

47 

Materiala  in  air 

45 
78 
96 
48 
40 
126 

70 
99 

44 

63 

70 

47 

29 

•  83 

62 

Mallet 182» 

48 

49 

Portland 

Gault-clay  brick 

J.  Grant .1871 

50 

Stock    brick    in 

51 

Stock   brick    in 

11                  ii 

52 

Staf .  blue  brick 

„ 

II                  II 

S3 

Staf.  blue  brick 

in  wntpr 

II                  II 

S4 

"          Fareham      red 

hriok  in  air_ 

1.                  « 

55 

Fareham      red 

11                  II 

•Proportions  of  sand  not  given,  but  presumably  about  those  indicated  in  headings  of  table. 

+  Standard  sand  used  in  mixture.  t  Clean  river-sand  used  in  mixture. 

i  Crushed  sandstone  used  in  mixture. 

I  Coarse  particles  in  cement  sifted  out  before  testing.  T  Fine  river-sand  used  in  mixture. 


ART.  1.]  COST    OF    MORTAR.  95 

versely,  cements  which  are  apparently  poor  when  tested  for  cohesion, 
show  excellent  adhesive  qualities. 

The  table*  on  the  preceding  page  gives  all  the  reliable  data 
known.  A  comparison  of  the  table  with  the  diagram  on  page  92 
shows  that  the  adhesion  of  a  mortar  is  far  less  than  its  tensile 
strength  at  the  same  age,  but  fails  to  show  any  definite  relation 
between  the  two.  In  the  experiments  by  Dr.  Bohme  at  the  Eoyal 
Testing  Laboratory  at  Berlin  the  mortar  was  made  with  standard 
quartz  sand,  and  the  tensile  strength  of  the  mortar  and  its  adhesion 
to  common  brick  were  determined  separately.  By  comparing  the 
tensile  and  adhesive  strengths  at  the  same  ages,  it  was  found  that 
the  former  was  always  about  ten  times  greater  than  the  latter  when 
the  mortar  consisted  of  one  part  of  cement  to  three  or  four  parts 
of  sand,  and  from  six  to  eight  times  greater  when  the  cement  was 
used  neat  or  with  one  or  two  parts  of  sand.  In  the  experiments 
made  by  Prof.  "Warren,  of  Sydney  University,  New  South  Wales, 
the  tensile  strength  of  neat  Portland  cement  mortar  was  three  and 
a  half  times  its  adhesion  to  brick.  The  result  of  twelve  thousand 
special  tests  by  Mr.  Mann  was  that  pure  Portland  cement  of  -425 
pounds  tensile  and  5,640  pounds  compressive  strength  per  square 
inch  has  but  60  to  80  pounds  adhesion  to  limestone,  and  that  the 
ratio  of  tensile  to  adhesive  strength  varies  from  5  to  1  to  9  to  1. 

135.  Cost  of  Mortar.  Knowing  the  ]}Tice  of  the  materials  it 
is  very  easy,  by  the  use  of  Table  11,  page  88,  to  compute  the  cost 
of  the  ingredients  required  for  a  cubic  yard  of  mortar.  The 
expense  for  labor  is  quite  variable,  depending  upon  the  distance  the 
materials  must  be  moved,  the  quantity  mixed  at  a  time,  etc.  As  a 
rough  approximation  it  may  be  assumed  that  a  common  laborer  can 
mix  3  yards  per  day,  at  a  cost  of,  say,  50  cents  per  cubic  yard„  If 
the  mixing  is  done  by  machinery,  the  cost  may  be  as  low  as  25 
cents  per  cubic  yard.  The  cost  of  a  cubic  yard  of  mortar  composed 
of  1  part  Portland  cement  and  2  parts  sand,  both  by  weight  is 
then  about  as  follows : 

Cement 2.80  bbls.  (see  page  88)  @  $3.00  =  $8.40 

Sand 0. 78 cu.  yd.  (seepage  88)  @      .50=       .39 

Labor,  handling  materials  and  mixing , .  J  day  @  $1.50  =       .50 

Total  cost  of\  cubic  yard  of  mortar  =  $9.39 

♦Compiled  by  Emil  Kuichling,  for  the  report  for  1888  of  the  Executive  Board 
of  the  City  of  Rochester,  N.  Y. 


96 


MORTAR. 


[chap.  IV. 


136.  Natural  vs.  Portland  Cement  Mortar.  It  is  sometimes  a 
question  whether  Portland  or  natural  cement  should  be  used.  If 
a  quick-setting  cement  is  required,  then  natural  cement  is  to  be 
preferred,  since  as  a  rule  the  natural  cements  are  quicker  setting, 
although  there  are  many  and  marked  exceptions  to  this  rule.  Other 
things  being  the  same,  a  slow-setting  cement  is  preferable,  since 

13 

h 

.y 
■|/0 

Ug 
id 


\ 

\ 

\ 

\ 
\ 

\ 

\ 

V 

X 

\ 

•\ 

^ 

■-.J 

^^o. 

^ 

■ 

•■H'^/ 

— — .» 



u 

"  c  _..,__ 

P'^rts  Sand  toj  Part  Cement  by  We/ght 

Fig.  7a.— Cost  of  Cement  Mortar. 

it  is  not  so  liable  to  set  before  reacliing  its  place  in  the  wall.  This 
is  an  important  item,  since  with  a  quick-setting  cement  any  slight 
delay  may  necessitate  the  throwing  away  of  a  boxful  of  mortar 
or  the  removal  of  a  stone  to  scrape  out  the  partially-set  mortar. 

Generally,  however,  this  question  should  be  decided  upon 
economical  grounds,  which  makes  it  a  question  of  relative  strength 
and  relative  prices.  Tlie  tensile  strength  of  natural  and  Portland 
cement  mortars  is  shown  in  Fig.  G,  jmge  92.  The  cost  of  mortar 
of  various  proportions  of  sand  may  be  computed  as  in  the  preceding 
section;  but  as  the  cost  of  labor  is  uncertain  and  is  substantially 
the  same  for  both  kinds  of  mortar,  it  is  sufficient  to  deal  with  the 
cost  of  the  materials  only.  Assuming  Portland  cement  to  cost  $3 
per  barrel,  natural  81  per  barrel,  and  sand  50  cents  per  cubic  yard, 
and  using  Table  11,  j^age  88,  the  cost  of  the  materials,  in  a  cnbie 
yard  of  mortar  is  as  in  Fig.  7a. 


ART.   1.]  NATURAL    TS.    PORTLAND    CEMENT   MORTAR. 


97 


500 


to 
c 

Ssoo 


«0  100 

I 


2        3        4        5       6        7       d       9        10       II       12       15 
Cost  of  Forfar  in  Dollars  per  Cubic   Yard 

Fig.  7b. — Relative  Economy  of  Natural  and  Portland  Cements. 


^ 

^ 

^ 

^ 

y 

(<5V 

y 

y 

> 

4 

^'A^ 

r 

/ 

v 

t 

/' 

Age  of  Mortar 
6  Honths 

100 


^90 


tdo 


>70 


5  60 


50 


40 


30 


/ 

\ 

i 

/ 

\ 

/ 

\ 

!Ji2V< 

>/ 

/ 

\ 

' 

^"^ 

"^ 

^ 

^t^. 

>^ 

N 

'\ 

OIZ545670 
fhrti  Sanct  to  I  fhrt   Cement    by    V\^iqht 

Fig.  7c. — Economic  Proportion  of  Sand. 


98  MORTAR.  [chap.  IV. 

By  plotting  the  strength  of  Portland  and  natural  cement  mortar 
6  months  old  and  the  cost  of  a  yard  of  mortar  as  given  in  Fig.  7a, 
Fig.  It  is  obtained,  which  shows  the  relation  between  the  strength 
at  6  months  and  the  cost  of  the  mortar  made  of  the  two  kinds  of 
cement.  Notice  that  for  any  tensile  strength  under  about  370 
pounds  per  square  inch,  either  natural  or  Portland  cement  may  be 
used,  but  that  the  former  is  the  cheaper.  In  other  words,  Fig.  1h 
shows  that  if  a  strength  of  about  370  pounds  per  square  inch  at 
6  months  is  sufficient,  natural  cement  is  the  cheaper.  Nearly  all 
carefully  conducted  tests  of  the  strength  of  cement  mortar  6 
months  old  or  over  give  a  similar  result,  except  that  the  above 
limit  is  usually  between  300  and  350  pounds.  A  considerable 
change  in  prices  does  not  materially  alter  the  result,  and  hence  the 
conclusion  may  be  drawn  that  if  a  strength  of  300  to  350  pounds 
per  square  inch  at  6  months  is  sufficient,  natural  cement  is  more 
economical  than  Portland.  Incidentally  Fig.  7c,  page  97,  shows 
the  same  relation.  However,  in  this  connection  it  should  not  be 
forgotten  that  other  considerations  than  strength  and  cost  may 
govern  the  choice  of  a  cement ;  for  example,  uniformity  of  product, 
rapidity  of  set,  and  soundness  are  of  equal  or  greater  importance 
than  strength  and  cost. 

Mortar  made  of  two  brands  of  Portland  or  natural  cement  will 
differ  considerably  in  economic  values,  and  hence  to  be  of  the 
highest  value  the  above  comparison  should  be  made  between  the 
most  economical  Portland  and  the  most  economical  natural  cement 
as  determined  by  the  method  described  in  §  lllh. 

Short-time  tests  do  not  warrant  any  general  conclusion  as  to  the 
relative  economy  of  natural  and  Portland  cements,  since  the 
strength  at  short  times  varies  greatly  with  the  activity  of  the 
cement.  For  example,  the  two  upper  diagrams  of  Fig.  6,  page 
92,  when  plotted  as  in  Fig.  1h  show  Portland  to  be  the  more 
economical,  while  other  similar  experiments  show  natural  cement  to 
be  the  more  economical. 

137.  Economic  Proportion  of  Sand.  Fig.  7f,  page  97,  shows 
the  ratio  of  strength  to  cost  for  different  proportions  of  sand,  for 
both  Portland  and  natural  cement ;  in  other  words,  Fig.  7c  shows 
the  tensile  strength  in  pounds  per  square  inch  for  each  dollar  of  the 
cost  of  a  cubic  yard  of  mortar.  For  example,  if  a  natural  cement 
mortar  at  6  months  has  a  tensile  strength  of  280  pounds  per  square 
inch,  and  costs  82.95  per  yard,  the  strength  per  dollar  is:  280  -j- 


ART.  1.]  EFFECT    OF    RE-TEMPERING.  99 

2.95  =  94.9  pounds  per  square  inch.  In  this  way  Fig.  7c  was 
constructed,  using  the  cost  of  mortar  as  given  in  Fig.  7a  and  the 
strength  as  determined  by  L.  C.  Sabin  in  connection  with  the  con- 
struction of  the  Poe  lock  on  the  St.  Mary's  Falls  Canal.*  Accord- 
ing to  this  diagram  the  most  economic  mortar,  either  natural  or 
Portland,  consists  of  3  parts  sand  to  1  part  cement,  by  weight. 

A  study  of  the  results  of  other  experiments  shows  that  the  above 
conclusions  are  not  general.  The  maximum  ratio  as  above  is 
different  for  different  ages  for  the  same  cement,  and  at  the  same 
age  is  different  for  different  cements.  The  above  ratio  varies  (1) 
with  the  activity  of  the  cement,  which  determines  the  strength 
neat  at  different  ages;  (2)  with  the  fineness,  which  determines  the 
sand-carrying  power  of  the  cement;  (3)  with  the  fineness  of  the 
sand,  which  determines  the  surface  to  be  covered  by  the  cement; 
and  (-4)  with  the  cost  of  the  cement  and  the  sand.  If  the  strength 
of  any  particular  cement  with  the  various  proportions  of  sand  is 
known  for  a  particular  age,  and  the  price  of  the  cement  and  sand 
also  is  known,  the  most  economic  proportion  of  sand  can  be  com- 
puted as  above.  To  determine  the  most  economic  mortar,  the 
most  economic  cement  should  be  selected  as  described  in  §  111^, 
and  then  be  mixed  with  the  most  economical  proportion  of  sand  as 
above. 

Strictly,  the  maximum  ratio  of  strength  to  cost  determined  as 
above  is  not  necessarily  the  most  economical  mortar.  The  work  in 
hand  may  not  require  a  mortar  as  strong  as  that  giving  the  maxi- 
mum ratio  of  strength  to  cost,  in  which  case  a  mortar  having  a 
smaller  proiDortion  of  cement  may  be  used;  and  similarly,  if  the 
work  requires  a  mortar  stronger  than  that  giving  the  maximum 
ratio  of  strength  and  cost,  then  a  mortar  must  be  used  which  con- 
tains a  greater  proportion  of  cement. 

138.  Effect  of  Re-tempeking.  Frequently,  in  practice, 
cement  mortar  which  has  taken  an  initial  set,  is  re-mixed  and  used. 
Masons  generally  claim  that  re-tempering,  i.e.,  adding  water  and 
re-mixing,  is  beneficial;  while  engineers  and  architects  usually 
specify  that  mortar  which  has  taken  an  initial  set  shall  not  be  used. 

Ee-tempering  makes  the  mortar  slightly  less  "short"  or 
*' brash,"  that  is,  a  little  more  plastic  and  easy  to  handle.  Ke- 
tempering  also  increases  the  time  of  set,  the  increase  being  very 

«  Report  of  Chief  of  Engineers,  U.  S.  A.,  1893,  page  3019,  Table  4. 


100  MORTAR.  [chap.  IV. 

different  for  different  cements.  Bat  on  the  other  hand,  re-temper- 
ing nsually  weakens  a  cement  mortar.  A  quick-setting  natural 
cement  sometimes  loses  30  or  40  per  cent,  of  its  strength  by  being 
re-tempered  after  standing  20  minutes,  and  70  or  80  per  cent,  by 
being  re-tempered  after  standing  1  hour.  With  slow-setting 
cements,  particularly  Portlands,  tbe  loss  by  re-tempering  immedi- 
ately after  initial  set  (§  84)  is  not  material.  A  mortar  which  has 
been  insufficiently  worked  is  sometimes  made  appreciably  stronger 
by  re-tempering,  the  additional  labor  in  re-mixing  more  than  com- 
pensating for  the  loss  caused  by  breaking  the  set. 

The  loss  of  strength  by  re-tempering  is  greater  for  quick-setting 
tban  for  slow-setting  cement,  and  greater  for  neat  than  for  sand 
mortar,  and  greater  with  fine  sand  than  with  coarse.  The  loss 
increases  with  the  amount  of  set.  If  mortar  is  to  stand  a  consider- 
able time,  the  injury  will  be  less  if  it  is  re-tempered  several  times 
during  the  interval  than  if  it  is  allowed  to  stand  undisturbed  to  the 
end  of  the  time  and  is  then  re-mixed.  Ee- tempered  mortar  shrinks 
mo'o  in  setting  than  ordinary  mortar.  This  fact  sometimes 
accounts  for  the  cracks  which  frequently  appear  upon  a  troweled 
surface. 

The  only  safe  rule  for  practical  work  is  to  require  the  mortar 
to  be  thoroughly  mixed,  and  then  not  permit  any  to  be  used  which 
has  taken  an  initial  set  (§  84).  This  rule  should  be  more 
strenuously  insisted  upon  with  natural  than  with  Portland  cements, 
and  more  with  quick-setting  than  with  slow-setting  varieties. 

139.  Lime  with  Cement.  Cement  mortar  before  it  begins  to 
set  has  no  cohesive  or  adhesive  properties,  and  is  what  the  mason 
calls  "  poor,"  "  short,"  "  brash  ";  and  consequently  is  difficult  to 
use.  It  will  not  stick  to  the  edge  of  the  brick  or  stone  already  laid 
sufficiently  to  give  mortar  with  which  to  strike  the  joint.  The 
addition  of  a  small  per  cent,  of  lime  paste  makes  the  mortar  "  fat  " 
or  "  rich,"  and  more  pleasant  to  work.  The  substitution  of  10  to 
20  per  cent,  of  lime  paste  for  an  equal  volume  of  the  cement  paste 
does  not  materially  decrease  the  strength  of  the  mortar,  and 
frequently  the  addition  of  this  amount  of  lime  slightly  increases  its 
strength.  In  all  cases  the  substitution  of  10  to  20  per  cent,  of  lime 
decreases  the  cost  more  rapidly  than  the  strength,  and  hence  is 
economical;  but  the  substitution  of  more  than  about  20  per  cent, 
decreases  the  strength  more  rapidly  than  the  cost,  and  hence  is  not 
economical.     The  economy  of  using  lime  with  cement  is,  of  course, 


ART.  1.]  MORTAR   IMPERVIOUS   TO   WATER.  101 

greater  with  Portland  than  with  natural  cement   owing  to   the 
greater  cost  of  the  former. 

If  the  mortar  is  porous,  i.e.,  if  the  voids  of  the  sand  are  not 
filled  with  cement,  the  addition  of  lime  will  make  the  mortar  more 
dense  and  plastic,  and  will  also  increase  its  strength  and  cost.  The 
increase  in  strength  is  not  proportional  to  the  increase  in  cost,  but 
the  increased  plasticity  and  density  justify  the  increased  cost — the 
former  adds  to  the  ease  of  using  the  mortar,  and  the  latter  to  its 
durability. 

The  addition  of  lime  does  not  materially  affect  the  time  of  set, 
and  usually  slightly  increases  it. 

It  has  lonor  been  an  American  practice  to  reinforce  lime  mortar 
by  the  addition  of  hydraulic  cement.  The  mortar  for  the 
*' ordinary  brickwork"  of  the  United  States  public  buildings  is 
composed  of  "one  fourth  cement,  one  half  sand,  and  one  fourth 
lime."  The  cement  adds  somewhat  to  the  strength  of  the  mortar, 
but  not  proportionally  to  the  increase  in  the  cost  of  the  mortar. 

140.  MoETAR  Impervious  to  Water.  IS^early  every  failure 
of  masonry  is  due  to  the  disintegration  of  the  mortar  in  the  outside 
of  the  joints.  Ordinary  mortar — either  lime  or  cement — absorbs 
water  freely,  common  lime  mortar  absorbing  from  50  to  60  per  cent, 
of  its  own  weight,  and  the  best  Portland  cement  mortar  from  10 
to  20  per  cent. ;  and  consequently  they  disintegrate  under  the 
action  of  frost.  Mortar  may  be  made  practically  non-absorbent 
by  the  addition  of  alum  and  potash  soap.  One  per  cent.,  by 
weight,  of  powdered  alum  is  added  to  the  dry  cement  and  sand,  and 
thoroughly  mixed;  and  about  one  per  cent,  of  any  potash  soap 
(ordinary  soft-soap  made  from  wood  ashes  is  very  good)  is  dissolved 
in  the  water  used  in  making  the  mortar.  The  alum  and  soap  com- 
bine, and  form  compounds  of  alumina  and  the  fatty  acids,  wliich 
are  insoluble  in  water.  These  compounds  are  not  acted  upon  by 
the  carbonic  acid  of  the  air,  and  add  considerably  to  the  early 
strength  of  the  mortar,  and  somewhat  to  its  ultimate  strength. 

With  lime  mortar,  the  alum  and  soaj)  has  a  slight  disadvantage 
in  that  the  compounds  which  render  the  mortar  impervious  to  water 
also  prevent  the  air  from  coming  in  contact  with  the  lime,  and 
consequently  prevent  the  setting  of  the  mortar.  On  the  other 
hand,  the  alum  and  soap  compounds  add  considerably  to  both  the 
early  and  the  ultimate  strength  of  the  mortar. 

This  method  of  rendering  mortar  impervious  is  an  application 


102  MORTAE.  [chap.  IV. 

of  the  principle  of  Sylvester's  method  of  repelling  moistnre  from 
external  walls  by  applying  alam  and  soap  washes  alternately  on  the 
outside  of  the  wall  (see  §  2G3),  The  same  principle  is  applied  in 
McMurtrie's  artificial  stone  (see  §  162).  The  alum  and  soap  are 
easily  used,  and  do  not  add  greatly  to  the  cost  of  the  mortar.  The 
mixture  could  be  advantageously  used  in  plastering,  and  in  both 
cement  and  lime  mortars  of  outside  walls  or  masonry  in  damp 
places.  It  has  been  very  successfully  used  in  the  plastering  of 
cellar  and  basement  walls.  It  should  be  employed  in  all  mortar 
used  for  pointing  (§  204). 

The  addition  of  a  small  amount  of  very  finely  powdered  clay 
(§  114c)  decreases  the  permeability  of  mortar;  but  since  clay  absorbs 
and  parts  with  water  with  the  changing  seasons,  the  use  of  clay  is 
not  efficient  in  preventing  disintegration  by  freezing  and  thawing. 

141.  Feeezing  of  Mortar.  The  freezing  of  mortar  before  it 
has  set  has  two  effects:  (1)  the  low  temperature  retards  the  setting 
and  hardening  action;  and  (2)  the  expansive  force  of  the  freezing 
water  tends  to  destroy  the  cohesive  strength  of  the  mortar. 

142.  Effect  on  Lime  Mortar.  The  freezing  of  lime  mortar 
retards  the  evaporation  of  the  water,  and  consequently  delays  the 
combination  of  the  lime  with  the  carbonic  gas  of  the  atmosphere. 
The  expansive  action  of  the  freezing  water  is  not  very  serious  upon 
lime  mortar,  since  it  hardens  so  slowly.  Consequently  lime  mortar 
is  not  seriously  injured  by  freezing,  provided  it  remains  frozen 
until  fully  set.  Alternate  freezing  and  thawing  somewhat  damages 
its  adhesive  and  cohesive  strength.  However,  even  if  the  strength 
of  the  mortar  were  not  materially  affected  by  freezing  and  tliawing, 
it  is  not  permissible  to  lay  masonry  during  freezing  weather;  for 
example,  if  the  mortar  in  a  thin  wall  freezes  before  setting  and 
afterwards  thaws  on  one  side  only,  the  wall  may  settle  injuriously. 

When  masonry  is  to  be  laid  in  lime  mortar  during  freezing 
weather,  frequently  the  mortar  is  mixed  with  a  minimum  of  water 
and  then  thinned  to  the  proper  consistency  by  adding  hot  water 
just  before  using.  This  is  undesirable  practice  (see  §  118).  When 
the  very  best  results  are  sought,  the  brick  or  stone  should  be 
warmed — enough  to  thaw  off  any  ice  upon  the  surface  is  sufficient 
— before  being  laid.  They  may  be  warmed  either  by  laying  tliem 
on  a  furnace,  or  by  suspending  them  over  a  slow  fire,  or  by  wetting 
with  hot  water,  or  by  blowing  steam  through  a  hose  against  them. 

143.  Effect  on  Cement  Mortar.    Owing  to  the  retardation  of  the 


AET.  1.]  EFFECT    OF   FREEZING,  103 

low  temperature,  the  setting  and  hardening  may  be  so  delayed  that 
the  water  may  be  dried  out  of  the  mortar  and  not  leave  enough  for 
the  chemical  action  of  hardening;  and  consequently  the  mortar  will 
be  Aveak  and  crumbly.  This  would  be  substantially  the  same  as 
using  mortar  with  a  dry  porous  brick.  Whether  the  water  evapo- 
rates to  an  injurious  extent  or  not  depends  upon  the  humidity  of 
the  air,  the  temperature  of  the  mortar,  the  activity  of  the  cement, 
and  the  extent  of  the  exposed  surface  of  the  mortar.  The  mortar 
in  the  interior  of  the  Avail  is  not  likely  to  be  injured  by  the  loss  of 
water  while  frozen;  but  the  edges  of  the  joints  are  often  thus  seri- 
ously injured.  In  the  latter  case  the  damage  may  be  fully  repaired 
by  pointing  the  masonry  (§  204)  after  the  mortar  has  fully  set. 

On  the  other  hand  when  the  cement  has  partially  set,  if  the 
expansive  force  of  the  freezing  water  is  greater  than  the  cohesive 
strength  of  the  mortar,  then  the  bond  of  the  mortar  is  broken,  and 
on  thawing  out  the  mortar  will  crumble.  Whether  this  action  will 
take  place  or  not  will  depend  chiefly  upon  the  strength  and  activity 
of  the  cement,  upon  the  amount  of  free  water  present,  and  ui:)on 
the  hardness  at  the  time  of  freezing.  The  relative  effects  of  these 
several  elements  is  not  known  certainly;  but  it  has  been  proven 
conclusively  that  for  the  best  results  the  following  precautions 
should  be  observed:  1.  Use  a  quick-setting  cement.  2.  Make  the 
mortar  richer  than  for  ordinary  temperatures.  3.  Use  the  mini- 
mum quantity  of  water  in  mixing  the  mortar.  4.  Prevent  freezing 
as  long  as  possible. 

There  are  various  ways  of  preventing  freezing:  1.  Cover  the 
masonry  with  tarpaulin,  straw,  manure,  etc.  2.  Warm  the  stone 
and  the  ingredients  of  the  mortar.  Heating  the  ingredients  is  not 
of  much  advantage,  particularly  with  Portland  cement.  3.  Instead 
of  trying  to  maintain  a  temperature  above  the  freezing  point  of 
fresh  water,  add  salt  to  the  water  to  prevent  its  freezing.  The 
usual  rule  for  addmg  salt  is:  "Dissolve  1  pound  of  salt  in  18 
gallons  of  water  when  the  temperature  is  at  32°  Fahr.,  and  add 
3  ounces  of  salt  for  every  3°  of  lower  temperature."  The  above 
rule  gives  a  slight  excess  of  salt.  The  following  rule  is  scientifically 
correct  and  easier  remembered:  "  Add  one  per  cent,  of  salt  for  each 
Fahrenheit  degree  below  freezing."  Apparently  salt  slightly 
decreases  the  strength  of  cement  mortar  setting  in  air,  and  slightly 
increases  the  strength  when  setting  in  water.* 

*  Report  of  Chief  of  Engineers,  U.  S.  A.,  1895,  pp.  2963-74,  3015. 


104  MORTAR,  [chap.  IV, 

Alternate  freezing  and  thawing  is  more  damaging  than  contin- 
uous freezing,  since  with  the  former  the  bond  may  be  repeatedly 
broken;  and  the  damage  due  to  successive  disturbance  increases 
with  the  number. 

144.  Practice  has  shown  that  Portland  cement  mortar  of  the 
usual  proj^ortions  laid  in  the  ordinary  way  is  not  materially  injured 
by  alternate  freezing  or  thawing,  or  by  a  temperature  of  10°  to 
15°  F.  below  freezing,  except  perhaps  at  the  exposed  edges  of  the 
joints.  Under  the  same  conditions  natural  cement  mortar  is  liable 
to  be  materially  damaged. 

By  the  use  of  salt,  even  in  less  proportions  than  specified  above, 
or  by  warming  the  materials,  masonry  may  be  safely  laid  with 
Portland  at  a  temperature  of  0°  F. ;  and  the  same  may  usually  be 
done  with  natural  cement,  although  it  will  ordinarily  be  necessary 
to  re-point  the  masonry  in  the  spring.  Warming  the  materials  is 
not  as  effective  as  using  salt. 

145.  Change  of  Volume  in  Setting.  The  Committee  of  the 
American  Society  of  Civil  Engineers  draw  the  following  conclu- 
sions:* 1.  Cement  mortars  hardening  in  air  diminish  in  linear 
dimensions,  at  least  to  the  end  of  twelve  weeks,  and  in  most  cases 
progressively.  2.  Cement  mortars  hardening  in  water  increase  in 
like  manner,  but  to  a  less  degree.  3.  The  contractions  and  expan- 
sions are  greatest  in  neat  cement  mortars.  4.  The  quick-setting 
cements  show  greater  expansions  and  contractions  than  the  slow- 
setting  cements.  5.  The  changes  are  less  in  mortars  containing 
sand.  6.  The  changes  are  less  in  water  than  in  air.  7.  The  con- 
traction at  the  end  of  twelve  weeks  is  as  follows:  for  neat  cement 
mortar,  0.14  to  0.32  per  cent.;  for  a  mortar  composed  of  1  part 
cement  and  1  part  sand,  0.08  to  0.17  per  cent.  8.  The  expansion 
at  the  end  of  twelve  weeks  is  as  follows:  for  neat  cement,  0.04  to 
0.25  per  cent. ;  for  1  part  cement  and  1  part  sand,  0.0  to  0.08  per 
cent.  9.  The  contraction  or  expansion  is  essentially  the  same  in 
all  directions. 

146.  Elasticity,  Compression,  and  Set  of  Mortar.  For 
data  on  elasticity  see  page  14.  The  evidence  is  so  conflicting  that 
it  is  impossible  to  determine  the  coefficient  of  compression  and  of 

*  See  the  "  Report  of  Progress  of  the  Committee  on  the  Compressive  Strength  of 
Cements  and  the  Compression  of  Mortars  and  Settlement  of  Masonry,"  in  the 
Transactions  of  that  Society,  vol.  xvii.  pp.  213-37 ;  also  a  similar  report  in  vol.  xvi. 
pp.  717-32. 


ARl-.   L.j    ELASTICITY,  COMPRESSION,  AND    SET    OF   MOETAR.  105 

set  of  mortar,  even  approximately.  For  much  valuable  data  on 
this  and  related  subjects,  see  the  "  Report  of  Progress  of  the  Com- 
mittee on  the  Compressive  Strength  of  Cements  and  the  Compres- 
sion of  Mortars  and  Settlement  of  Masonry,"  in  the  Transactions  of 
the  American  Society  of  Civil  Engineers,  vol.  xvi.  pp.  717-32, 
vol.  xvii.  pp.  213-17,  and  also  vol.  xviii.  pp.  2G-4-80.  The  several 
annual  reports  of  tests  made  with  the  United  States  Government 
testing-machine  at  Watertown  contain  valuable  data — particularly 
the  report  for  188-4,  pp.  09-247 — bearing  indirectly  upon  this  and 
related  subjects;  but  since  some  of  the  details  of  the  experiments 
are  wanting,  and  since  the  fundamental  principles  are  not  well 
enough  understood  to  carry  out  intelligently  a  series  of  experiments. 
it  is  impossible  to  draw  any  valuable  conclusions  from  the  data. 


106  CONCRETE.  [chap.  IV. 


Art.  2.  Concrete. 

147.  Concrete  consists  of  mortar  in  which  is  embedded  small 
pieces  of  some  hard  material.  The  mortar  is  often  referred  to  as 
the  matrix;  and  the  embedded  fragments,  as  the  aggregate.  Con- 
crete is  a  species  of  artificial  stone.  It  is  sometimes  called  beton, 
the  French  equivalent  of  concrete. 

"  Concrete  is  admirably  adapted  to  a  variety  of  most  important 
uses.  For  foundations  in  damp  and  yielding  soils  and  for  subter- 
ranean and  submarine  masonry,  under  almost  every  combination  of 
circumstances  likely  to  be  met  with  in  practice,  it  is  superior  to 
brick  masonry  in  strength,  hardness,  and  durability;  is  more 
economical;  and  in  some  cases  is  a  safe  substitute  for  the  best 
natural  stone,  while  it  is  almost  always  preferable  to  the  poorer 
varieties.  For  submarine  masonry,  concrete  possesses  the  advan- 
tage that  it  can  be  laid,  under  certain  precautions,  without  exhaust- 
ing the  water  and  without  the  use  of  a  diving-bell  or  submarine 
armor.  On  account  of  its  continuity  and  its  impermeability  to 
water,  it  is  an  excellent  material  to  form  a  substratum  in  soils 
infested  with  springs;  for  sewers  and  conduits;  for  basement  and 
sustaining  walls;  for  columns,  piers,  and  abutments;  for  the 
hearting  and  backing  of  walls  faced  with  bricks,  rubble,  or  ashlar 
work;  for  pavements  in  areas,  basements,  sidewalks,  and  cellars; 
for  the  walls  and  floors  of  cisterns,  vaults,  etc.  Groined  and 
vaulted  arches,  and  even  entire  bridges,  dwelling-houses,  and  fac- 
tories, in  single  monolithic  masses,  with  suitable  ornamentation, 
have  been  constructed  of  this  material  alone." 

The  great  value  of  concrete  in  all  kinds  of  foundations  is  slowly 
coming  to  be  appreciated.  It  enables  the  engineer  to  build  his 
superstructure  on  a  monolith  as  long,  as  wide,  and  as  deep  as  he 
may  think  best,  which  cannot  fail  in  parts,  but,  if  rightly  propor- 
.tioned,  must  go  all  together — if  it  fails  at  all. 

148.  The  Mortar.  The  matrix  may  be  either  lime  or  cement 
-mortar,  but  is  usually  the  latter.     The   term   concrete  is  almost 

universally  understood  to  be  cement 'mortar  with  pebbles  or  broken 
stone  embedded  in  it.  Lime  mortar  is  wholly  unfit  for  use  in  large 
masses  of  concrete  since  it  does  not  set  when  excluded  from  the  air 
(see  §  119). 


ART.  3.]  THE    AGGREGATE.  107 

The  cement  mortar  may  be  made  as  already  described  in  Art.  1 
preceding. 

149.  The  Aggkegate.  The  aggregate  may  consist  of  small 
pieces  of  any  hard  material,  as. pebbles,  broken  stone,  broken  brick, 
shells,  slag,  coke,  etc.  It  is  added  to  the  mortar  to  reduce  the 
cost,  and  within  limits  also  adds  to  the  strength  of  the  concrete. 
Ordinarily  either  broken  stone  or  gravel  is  used.  Coke  or  blast- 
furnace slag  is  used  when  a  light  and  not  strong  concrete  is  desired, 
as  for  the  foundation  of  a  pavement  on  a  bridge  or  for  the  floors 
of  a  tall  building.  Of  course  a  soft  porous  aggregate  makes  a  weak 
concrete. 

Whatever  the  aggregate  it  should  be  free  from  dust,  loam,  or 
any  weal^  material.  The  pieces  should  be  of  graduated  sizes,  so 
that  the  smaller  shall  fit  into  the  Toids  between  the  larger.  "When 
this  condition  is  satisfied  less  cement  will  be  required  and  conse- 
quently the  cost  of  the  concrete  will  be  less,  and  at  the  same  time 
its  strength  will  be  greater.  Other  things  being  equal,  the  rougher 
the  surfaces  of  the  fragments  the  better  the  cement  adheres,  and 
consequently  the  stronger  the  concrete. 

150.  It  is  sometimes  specified  that  the  broken  stone  to  be  used 
in  making  concrete  shall  be  screened  to  practically  an  uniform  size; 
bat  this  is  unwise  for  three  reasons;  viz. :  1.  With  graded  sizes  the 
smaller  pieces  fit  into  the  spaces  between  the  larger,  and  conse- 
quently less  mortar  is  required  to  fill  the  spaces  between  the 
fragments  of  the  stone.  Therefore  the  unscreened  broken  stone  is 
more  economical  than  screened  broken  stone.  2.  A  concrete  con- 
taining the  smaller  fragments  of  broken  stone  is  stronger  than 
though  they  were  replaced  with  cement  and  sand.  Experiments 
show  that  sandstone  screenings  give  a  considerably  stronger  mortar 
than  natural  sand  of  equal  fineness,  and  that  limestone  screenings 
make  stronger  mortar  than  sandstone  screenings,  the  latter  giving 
from  10  to  50  per  cent,  stronger  mortar  than  natural  sand.* 
Hence,  reasoning  by  analogy,  we  may  conclude  that  including  the 
finer  particles  of  broken  stone  will  make  a  stronger  concrete  than 
replacing  them  with  mortar  made*  of  natural  sand.  Farther, 
experiments  show  that  a  concrete  containing  a  considerable  propor- 

*  Annual  Report  of  Chief  of  Engineers,  U.  S.  A.,  1893,  Part  3,  p.  3015 ;  do.  1894, 
Part  4,  p.  2321 ;  do.  1895,  Part  4,  p.  2953 ;  Jour.  West.  Soc.  of  Eng'rs,  vol.  ii.  pp.  394 
and  400. 


108  coxcRExr.  [chap.  iv. 

tioa  of  broken  stone  is  stronger  tlian  the  mortar  alone  (see  the  second 
and  third  paragraphs  of  §  153).  Since  the  mortar  alone  is  -weaker 
than  the  concrete,  the  less  the  proportion  of  mortar  the  stronger  the 
concrete,  provided  the  voids  of  the  aggregate  are  filled;  and  there- 
fore concrete  made  of  broken  stone  of  graded  sizes  is  stronger  than 
that  made  of  practically  one  size  of  broken  stone.  3.  A  single  size 
of  broken  stone  has  a  greater  tendency  to  form  arches  while  being 
rammed  into  place,  than  stone  of  graded  sizes. 

Therefore  concrete  made  with  screened  stone  is  more  expensive 
and  more  liable  to  arch  in  being  tamped  into  place,  and  is  less  dense 
and  weaker  than  concrete  made  with  unscreened  stone. 

In  short,  screening  the  stone  to  nearly  one  size  is  not  only  a 
needless  expense,  bnt  is  also  a  positive  detriment. 

The  dust  should  be  removed,  since  it  has  no  strength  of  itself 
and  adds  greatly  to  the  surface  to  be  coated,  and  also  prevents  the 
contact  of  the  cement  and  the  body  of  the  broken  stone.  Particles 
of  the  size  of  sand  grains  may  be  allowed  to  remain  if  not  too  fine 
nor  in  excess.  The  small  particles  of  broken  stone  should  be 
removed  if  to  do  so  reduces  the  proportion  of  voids  (§§  115^/,  lloe). 

151.  Gravel  vs.  Broken  Stone.  Often  there  is  debate  as  to  the 
relative  merits  of  gravel  and  broken  stone  as  the  aggregate  for  con- 
crete ;  but  when  compared  upon  the  same  basis  there  is  no  room  for 
doubt. 

In  the  preceding  section  it  was  shown  that  finely  crushed  stone 
gave  greater  tensile  and  compressive  strengths  than  equal  propor- 
tions of  sand;  and  hence  reasoning  by  analogy,  the  conclusion  is 
that  concrete  composed  of  broken  stone  is  stronger  than  that  con- 
taining an  equal  proportion  of  gravel.  This  element  of  strength  is 
due  to  the  fact  that  the  cement  adheres  more  closely  to  the  rough 
surfaces  of  the  angular  fragments  of  broken  stone  than  to  the 
smooth  surface  of  the  rounded  pebbles. 

Again,  part  of  the  resistance  of  concrete  to  crushing  is  due  to 
the  frictional  resistance  of  one  piece  of  aggregate  to  moving  on 
another;  and  consequently  for  this  reason  broken  stone  is  better 
than  gravel.  It  is  well  known  that  broken  stone  makes  better 
macadam  than  gravel,  since  the  rounded  pebbles  are  more  easily 
displaced  than  the  angular  fragments  of  broken  stone.  Concrete 
differs  from  macadam  only  in  the  use  of  a  better  binding  material; 
and  the  greater  the  frictional  resistance  between  the  particles  the 
stronger  the  mass  or  the  less  the  cement  required. 


ART.  2.]  THEORY    OF   THE    PROPORTIONS.  109 

A  series  of  experiments*  made  by  the  Citj'  of  Washington, 
D,  C,  to  determine  the  relative  value  of  broken  stone  and  gravel 
for  concrete,  which  are  summarized  in  §  loTJ,  page  li2y,  i;i\cs  the 
following  resalts: 

Strength  of  Gravbl  Concrete  in  terms  of  Broken  Stone  Concrete. 

Concrete  made  with 
Natural  Cement.  Portland  Cement. 

38  per  cent.  76  per  cent. 

91    "      •• 

119    "      " 

73    "      " 

108    "      " 

93   "      " 


Age  of 

Concrete 

WHEN  tested. 

10 

days. 

45 

" 

3  months. 

6 

" 

1 

year. 

96 
43 
83 

Mean  68 


Each  result  is  the  mean  for  two  1-foot  cubes,  excejot  that  the  values 
for  a  year  are  the  means  for  five  cubes.  Xotice  that  the  gravel 
concrete  is  relatively  weaker  for  the  earlier  ages,  owing  probably  to 
the  greater  internal  friction  of  the  broken-stone  concrete. 

In  a  series  of  forty-eight  French  experiments,!  the  crushing 
strength  of  gravel  concrete  with  Portland  cement  is  only  79  per 
■cent,  as  great  as  that  of  broken-stone  concrete.  The  gravel  had  40 
per  cent,  of  voids,  while  the  broken  stone  had  47  per  cent.,  which 
favored  the  gravel  concrete  (see  §  154).  The  results  of  these  tests 
are  shown  graphically  in  Fig.  8,  page  ll'2a. 

152.  Since  frequently  gravel  is  cheajDer  than  broken  stone,  a 
mixture  of  broken  stone  and  gravel  may  make  a  more  efficient  con- 
crete than  either  alone,  i.  e.,  may  give  greater  strength  for  the  same 
cost,  or  give  less  cost  for  the  same  strength. 

153.  Theory  of  the  Propoetions.  The  voids  in  the  aggre- 
gate should  always  be  filled  witli  mortar.  If  there  is  not  enough 
mortar  to  fill  the  voids,  the  concrete  will  be  weak  and  porous.  On 
the  other  hand,  more  mortar  than  enough  to  fill  the  voids  of  the 
aggregate  increases  the  cost  of  the  concrete  and  also  decreases  its 
strength.  The  decrease  in  strength  due  to  an  excess  of  mortar  is 
usually  greater  than  would  be  produced  by  substituting  the  same 
amount  of  aggregate,  since  ordinarily  the  sand  and  the  aggregate 
have  approximately  the  same  per  cent,  of  voids,  while  the  sand  has 
the  greater, -and  also  the  smoother,  surface. 

*  Report  of  Engineer  Commissioner  of  the  District  of  Columbia  for  1897,  p.  165. 
t  Cements  at  Chaux  Hydrauliques,  E.  Candlot,  Paris,  1891,  pp.  215,  and  340-41. 


]10 


CONCRETE. 


[chap.  IV. 


A  correctly  proportioned  concrete  is  always  stronger  than  the 
mortar  alone.  For  example,  Table  13a*  shows  that  a  concrete 
containing  a  considerable  proportion  of  pebbles  is  stronger  than  the 
mortar  alone — compare  lines  2,  5,  and  8  with  the  preceding  line  of 
each  gronp,  respectively.  The  results  are  for  gravel  concrete,  and 
they  would  be  more  striking  for  broken-stone  concrete,  since  the 
cement  adheres  better  to  broken  stone  than  to  either  sand  or 
gravel. 

TABLE   13a. 
Relative  Strength  op  Mortar  and  Gravel  Conckete. 


Portland  Cement. 

Tested  when  CS  Days  old. 

Rep.  No. 

Proportions. 

Crusliing  Ptienp;th. 
lUs.  per  sq.  in. 

Strength  of  the 

Cement. 

Sand. 

Pebbles. 

Concrete  in  terms  of 
that,  of  the  Mortar. 

I 

1 

2 

0 

2  158 

100  per  ceut. 

0 

o 

2  783 

129    "      " 

3 

5 

2  414 

12G    "      " 

4 

1 

3 

0 

1  40G 

100  per  cent. 

5 

5 

1  661 

114    "      " 

6 

6.5 

1  534 

109    "      '• 

7 

1 

4 

0 

1068 

100  per  cent. 

8 

5 

1  291 

121    "      " 

9 

8.5 

1  221 

114   "      " 

The  average  strength  of  twenty-four  cubes  ranging  from  3  to 
IG  inches  on  a  side,  made  under  the  direction  of  Gen.  Q.  A.  Gill- 
more,!  and  composed  of  1  volume  of  cement,  3  volumes  of  sand, 
and  G  volumes  of  broken  stone,  was  15  per  cent,  more  than  that  of 
corresponding  cubes  made  of  the  mortar  alone.  In  another  series 
of  the  same  experiments, J  the  average  strength  of  eight  cubes  of 

*Dr.  II.  Dycberhoff,  a  Germau  authority,  as  quoted  iu  "  Der  Portland  Cement 
und  seine  Anwendungen  im  Bauwesen,"  p.  90. 

f  Notes  on  the  Compressive  Resistance  of  Freestones,  Brick  Piers,  Hydraulic 
Cement,  Mortar,  and  Concretes,  Q.  A.  Gillmore.  John  Wiley  &  Sons,  New  York, 
1888,  pp.  137-40  and  143-^6. 

J  Ihid.,  pp.  141^2. 


ART.  2.] 


THEORY    OF   THE    TROPORTIONS. 


Ill 


concrete  composed  of  1  part  cement,  1^  parts  sand,  and  6  parts 
broken  stone  was  95  per  cent,  of  that  of  corresponding  cubes  of  the 
mortar  alone,  which  is  interesting  as  showing  that  a  lean  concrete 
is  nearly  as  strong  as  a  very  rich  mortar, 

A  correctly  proportioned  concrete  is  also  stronger  than  either  a 
richer  or  a  leaner  mixture — see  Table  13/,  page  112^. 

154.  For  the  strongest  and  densest  concrete,  the  voids  of  the 
aggregate  should  be  filled  with  a  rich  strong  mortar;  but  if  a 
cheaper  concrete  is  desired,  fill  the  voids  of  the  aggregate  with  sand 
and  add  as  much  cement  as  the  cost  will  justify.  In  other  words, 
to  make  a  cheap  concrete,  use  as  lean  a  mortar  as  the  circumstances 
warrant,  bat  use  enough  of  it  to  fill  the  voids  of  the  aggregate. 
Sand  is  so  cheap  that  there  is  no  appreciable  saving  by  omitting  it; 
and  the  use  of  it  makes  the  concrete  more  dense. 

The  strength  of  a  concrete  varies  nearly  with  the  amount  and 
strength  of  the  cement  used,  provided  the  mortar  is  not  more  than 
enough  to  fill  the  voids.  Table  13b  shows  the  strength  of  con- 
crete in  terms  of  the   cement  employed.     The   data  from  which 

TABLE   135. 
Relation  between  the  Crushing  Strength  of  Concrete  and  the 
Proportion  of  Cement. 
Mortar  Equal  to  the  Voids  io  the  Aggregate. 


Ref. 
No. 

Composition  of 

Mortar. 
Volumes  Loose. 

Proportion  of 
Cement. 

Crushing  Strength, 
Pounds  per  Square  Inch. 

Cement. 

Sand. 

Actual. 

Relative. 
1.00 

Actual. 

Theoretical. 

Relative. 

1 

1 

0.50 

4,467 

5,000 

1.00 

2 

2 

0.33 

0.67 

3,781 

3,300 

0.66 

3 

3 

0.25 

0.50 

2,553 

2,500 

0.50 

4 

4 

0.20 

0.40 

2,015 

2,000 

0.40 

5 

5 

0.17 

0.3S 

1.796 

1.600 

0.32 

6 

G 

0.14 

0.28     ' 

1,:;;65      i       1.400 

0.28 

this  table  was  made  are  the  same  as  those  suuimarized  in  Table 
13/,  page  1125-,  The  actual  crushing  strengths  v^ere  plotted,  and  it 
was  found  that  they  could  be  reasonably  well  represented  by  a  right 
line  passing  through  the  origin  of  co-ordinates.  The  values  for  this 
average  line  are  shown  in  next  to  the  last  column  of  Table  13h. 


112a 


CONCRETE. 


[chap.  IV. 


These  experiments  seem  to  jirove  that  the  strength  of  concrete 
varies  as  the  quantity  of  cement,  provided  the  voids  are  filled  with 
mortar.     The  same  conclusion  is  jiroved  by  the  data  summarized 


Portland  Cemeni -Barrels  per  Cubic  Yard 

Fig.  8. — Relation  between  the  Strength  of  the  Concrete  and  the  Amount  of  Cement. 

in  Fig.  8.  The  diagram  presents  the  results  of  forty-eiglit  experi- 
ments on  4-inch  cubes.*  Each  point  represents  two  experiments, 
the  age  of  the  mortar  in  one  being  7  days  and  in  the  other  28  days. 
The  points  with  one  circle  around  them  represent  the  strength  of 
broken-stone  concrete,  and  the  points  with  two  circles  gravel  con- 
crete. Both  the  sand  and  the  gravel  employed  in  these  experiments 
were  very  coarse,  and  consequently  the  amount  of  cement  per  cubic 
yard  is  unusually  great. 

155.  When  mortar  is  mixed  with  broken  stone,  the  film  of 
mortar  surrounding  each  fragment  increases  the  volume  of  the 
resulting  concrete.  Table  13c,  page  112J,  gives  the  result  of  fifteen 
experiments  to  determine  this  increase  in  volume.     The  mortar  was 

*  Candlot's  Cements  et  Chaux  Hydrauliques,  jjp.  3i0-il. 


ART.  2.] 


THEORY    OF   THE    PROPORTIONS. 


1126 


moderately  dry,  and  the  concrete  was  fjiiite  dry,  moisture  flushino- 
to  the  surface  only  after  vigorous  tamping.  Tiie  broken  stone  was 
Ko.  10  of  Table  lOh,  page  80,  and  contained  28  per  cent,  of  voids 
when  rammed. 

Line  4  of  Table  13c  shows  that  if  the  mortar  is  equal  to  the 
voids,  the  volume  of  the  rammed  concrete  is  7^  P^r  cent,  more  than 
the  volume  of  the  rammed  broken  stone  alone.     Possibly  part  of 

TABLE   13c. 
Increase  of  Volume  by  Mixing  Moktar  with  Broken  Stone. 


Ref.  No. 

Volume  of  Mortar  in 

terms  of  the  Voids  in 

the  Broken  Stone.* 

Volume  of  Rammed 

Concrete  in  terms  of  the 

Volume  of  Rammed 

Stone. 

Voids  in  the  Rammed 
Concrete  (while  wet). 

1 

70  per  cent. 

105.0  per  cent. 

15.3  per  cent. 

2 

80   "       " 

105.5   •'       " 

12.2    "       " 

3 

90  "       " 

106.5   "       " 

9.5    "       " 

4 

100    "       " 

107.5    "       " 

7.0   "       " 

5 

110   "       " 

109.0    "       " 

4.9    "       " 

6 

120   "       " 

110.5  "       " 

2.8    "       " 

7 

130   "       " 

112.5   "       " 

1.2    "       " 

8 

140    "       " 

114.0    "       " 

0.0   "       " 

the  increase  of  volume  was  due  to  imperfect  mixing,  although  it 
was  believed  that  the  mass  was  perfectly  mixed.  The  table  also 
shows  that  the  voids  in  this  concrete  are  equal  to  7  per  cent,  of  its 
volume;  in  other  words,  even  though  the  volume  of  the  mortar  is 
equal  to  the  volume  of  the  voids,  the  voids  are  not  filled.  Appar- 
ently the  voids  can  be  entirely  filled  with  this  grade  of  mortar  only 
when  the  mortar  is  about  40  per  cent,  in  excess  of  the  voids. 

The  increase  in  volume  in  Table  13c  may  be  regarded  as  the 
maximum,  since  the  mortar  was  quite  dry  and  the  stone  unscreened. 
With  moderately  wet  mortar  and  the  same  stone,  the  increase  in 
volume  Avas  only  about  half  that  in  the  table;  and  with  moist 
mortar  and  stone  ranging  between  2  inches  and  1  inch,  there  was 
no  appreciable  increase  of  volume.  With  pebbles  the  increase  is 
onlv  about  two  thirds  that  with  broken  stone  of  tlie  same  size. 
With  fine  gravel  (Xo.  18,  page  80)  the  per  cent,  of  increase  was 
considerably  greater  than  in  Table  13c;  with  mortar  equal  to  150 
per  cent,  of  the  voids,  it  was  possible  to  fill  only  about  5  to  7  per 


112c  CONCRETE.  [CHAP.  IV. 

cent,  of  the  yoids.  The  mortar  used  in  Table  13c  was  1  volume  of 
cement  to  3  volumes  of  sand,  both  measured  loose;  but  with  richer 
mortars  the  increase  in  volume  was  a  little  less,  and  with  leaner 
mortars  a  little  more.  These  differences  are  so  small  that  they  may 
be  disregraded. 

Notice  that  the  voids  in  Table  13c  are  for  the  wet  concrete. 
When  the  concrete  has  dried  out  the  voids  will  be  more;  since 
ordinarily  all  the  water  employed  in  making  the  concrete  does  not 
enter  into  chemical  combination  with  the  cement,  and  consequently 
when  the  concrete  dries  out  the  space  occupied  by  the  free  water  is 
empty. 

156.  Methods  of  Determining  the  Proportions.  There  are  two 
methods  of  fixing  the  proportions  for  a  concrete;  viz.:  1.  adjust 
the  proportions  so  that  the  voids  of  the  aggregate  shall  be  filled 
with  mortar,  and  the  voids  of  the  sand  with  cement  paste;  or,  2, 
fix  the  proportions  without  reference  to  the  voids  in  the  materials. 
These  two  methods  will  be  considered  in  order. 

156a.  With  Reference  to  the  Voids.  To  find  the  correct  pro- 
portions for  a  concrete,  first  determine  the  per  cent,  of  voids  in  the 
rammed  aggregate  (§  115fZ).  Next  determine  the  voids  in  the 
sand.  Then  use  that  proportion  of  cement  which  will  fill  the  voids 
of  the  sand  with  cement  paste  (see  §  120).  The  amount  of  mortar 
to  be  used  depends  upon  the  per  cent,  of  voids  in  the  aggregate  and 
the  density  desired  in  the  concrete  (see  Table  13c,  page  112J). 

The  details  of  the  method  of  determining  the  amount  of  mortar 
and  of  cement  will  be  illustrated  by  the  following  example.  Assume 
the  aggregate  to  be  broken  stone,  unscreened  except  to  remove  the 
dust,  containing  28  per  cent,  of  voids  when  rammed  (see  No.  10, 
Table  lOA,  page  80).  Also  assume  that  a  concrete  of  maximum 
density  is  desired;  and  that  therefore  the  mortar  should  be  equal 
to  about  140  per  cent,  of  the  voids  (see  Table  13c,  page  112Z*).  The 
aggregate  compacts  5  per  cent,  in  ramming  (No.  10,  Table  IQh), 
and  therefore  a  yard  of  loose  material  will  equal  0.95  of  a  yard 
rammed.  Adding  mortar  equal  to  140  per  cent,  of  the  voids 
increases  the  volume  to  about  114  percent.  (Table  13c);  and  tliere- 
fore  adding  the  mortar  will  increase  the  volume  of  the  rammed 
aggregate  to  0.95  X  1.14  =  1.08  cu.  yd.,  which  is  the  volume  of 
concrete  produced  by  a  yard  of  loose  aggregate.  To  produce  a  yard 
of  concrete  will  therefore  require  1  -4-  1.08  =  0.93  en.  yd.  of  loose 
broken  stone.     Since  the  mortar  is  to  be  equal  to  140  per  cent,  of 


ART.  2.]      METHODS    OF    DETERMINING   THE    PROPORTIONS.  112d 

the  voids,  a  yard  of  concrete  will  require  1.40  X  0.28  =  0.39  cu. 
yd.  of  mortar.  Assume  the  rammed  sand  to  contain  37  per  cent, 
of  voids  (see  Table  10^,  page  79 i).  Therefore  to  fill  the  voids  of 
the  sand  with  cement  paste  will  require  37  per  cent,  as  much 
packed  cement  as  loose  sand ;  or  in  other  words,  the  proportions  of 
the  mortar  should  be  about  1  volume  packed  cement  to  2^  vohnnes 
loose  sand.  Interpolating  from  Table  11,  page  88,  we  see  that  to 
produce  a  yard  of  this  mortar  will  require  about  2.40  bbl.  of  Port- 
land cement  and  0.79  cu.  yd.  of  sand.  Consequently  a  yard  of  the 
concrete  will  require  0.39  X  2.40  =  0.94  bbl.  of  Portland  cement, 
and  0.39  X  0.79  =  0.31  cu.  yd.  of  sand.  The  quantities  for  a 
cubic  yard  of  the  rammed  concrete  are:  0.94  bbl.  of  packed  Port- 
land cement,  0.31  cu.  yd.  of  loose  sand,  and  0.93  cu.  yd.  of  loose 
broken  stone;  and  since  1  bbl.  =  0.13  cu.  yd.,  the  proportions 
are:  1  volume  of  packed  Portland  cement,  2^  volumes  of  loose 
sand,  and  7^  volumes  of  loose  broken  stone. 

156b.  }yitliout  Reference  to  ihe  Voids.  Usually  the  proportions 
of  a  concrete  are  fixed  without  any  reference  to  the  method  to  be 
employed  in  measuring  the  cement,  and  also  without  reference  to 
the  voids  in  the  sand  and  in  the  aggregate.  The  proportions  are 
usually  stated  in  volumes,  that  of  the  cement  being  the  unit.  For 
example,  a  concrete  is  described  as  being  1  part  cement,  2  parts 
sand,  and  4  parts  broken  stone. 

This  method  is  inexact,  in  the  first  place,  since  it  does  not  state 
the  degree  of  compactness  of  the  cement.  If  the  unit  of  cement  is 
a  commercial  barrel  of  packed  cement,  the  resulting  concrete  will 
be  much  richer  than  if  the  cement  were  measured  loose  (see  §  126). 
In  the  second  place,  this  method,  in  name  and  usually  in  fact, 
takes  no  account  of  the  proportion  of  voids  in  either  tht  sand  or 
the  aggregate.  If  the  stone  is  screened  to  practically  one  size,  it 
may  have  45  to  50  per  cent,  of  voids  when  rammed;  but  if  it  is 
unscreened  except  to  remove  the  dust,  it  may  have  only  30  per 
cent,  of  voids  (see  Table  lOA,  page  80). 

156c.  To  exj^lain  the  method  of  testing  whether  or  not  the  voids 
are  filled  in  a  concrete  described  in  the  above  form,  take  the  com- 
mon proportions:  1  volume  cement,  2  volumes  sand,  and  4  volumes 
broken  stone.  If  the  cement  is  measured  by  volumes  loose,  as  is 
usually  the  case,  1  volume  of  dry  cement  will  make  about  0.8  of  a 
volume  of  paste.  If  tlie  sand  is  the  best,  it  will  probably  have 
about  30  percent,  of  voids  when  rammed  (see  Table  10^,  page  79t); 


Il2e  coiircRETE.  [chap.  IV. 

and  hence  the  2  vohimes  of  sand  will  contain  about  0.6  of  a  Yolume 
of  voids.  The  cement  is  then  25  per  cent,  more  than  enough  to 
fill  the  voids  of  the  sand.  The  cement  and  sand  when  rammed 
will  make  2  +  (0.8  —  0.6)  =  2.2  +  volumes  of  mortar.*  If  the 
broken  stone  is  unscreened,  it  will  probably  have  about  30  per  cent, 
voids  when  rammed  (see  Table  lOh,  page  80) ;  and  hence  the  4 
volames  of  stone  will  contain  1.2  volumes  of  voids.  The  excess  of 
mortar  is  then  2.2  —  1.2  =  1.0  units,  or  83  per  cent,  more  than 
enough  to  fill  the  voids  of  the  broken  stone.  The  mortar  and  the 
broken  stone  will  make  4  +  (2.2  —  1.2)  =  5.0  +  volumes  of 
rammed  concrete. f 

For  the  materials  assumed,  the  preceding  proportions  are  very 
uneconomical,  since  there  is  25  per  cent,  more  cement  than  the 
voids  in  the  sand  and  83  per  cent,  more  mortar  than  the  voids  in 
the  broken  stone.  The  possible  saving  in  cement  may  be  computed 
as  follows:  25  per  cent,  of  the  cement  could  be  omitted  iu  making 
the  mortar.  The  mortar  would  then  be  2  volumes,  of  which  0.8 
of  a  volume,  or  40  per  cent.,  is  in  excess  of  the  voids  in  the  aggre- 
gate. The  omission  of  this  surplus  mortar  is  equivalent  to  omitting 
0.40  X  0.75  =  30  per  cent,  of  the  original  cement.  The  total 
surplus  of  cement  is  then  25  +  30  ==  55  j^er  cent.  If  the  above 
proportions  were  intended  to  give  a  concrete  of  maximum  density, 
then  the  mortar  employed  should  be  about  40  per  cent,  in  excess 
of  the  void  (§  155).  In  this  case,  the  surplus  mortar  would  be 
(2.0  —  1.40  X  1.2)  =  0.32  volumes,  or  16  per  cent,  of  the  total 
mortar;  and  the  surplus  cement  in  this  mortar  would  be  (0.75x0.16) 
=  12  per  cent.  Therefore  the  total  surplus  cement  is  25  +  12  = 
37  per  cent.     Even  in  this  case  the  proportions  are  uneconomical. 

15Qd.  The  above  example  shows  how  extravagant  the  above 
proportions  are  with  the  best  grades  of  sand  and  broken  stone.  If 
the  sand  lias  37|  per  cent,  of  voids  and  the  broken  stone  40  per 
cent.,  then  with  the  preceding  proportions  there  will  be  practically 
no  surplus  cement,  and  there  will  be  an  excess  of  mortar  of  about 
25  per  cent.  In  other  words,  with  coarse  sand  and  screened  stone, 
the  voids  of  the  sand  will  be  filled  with  cement  paste,  and  the  voids 

*  The  mortar  when  rammed  will  make  from  2  to  4  per  cent,  more  volume  than 
the  sum  of  the  sand  and  the  excess  of  the  paste  (see  the  last  piaragraph  of  §  128, 
page  87). 

f  The  volume  of  the  concrete  will  he  slightly  more  than  5.0  units,  since  some 
sand  will  remain  between  the  fragments  of  stone,  and  thereby  increase  the  volume 
(see  Table  13c,  page  1126.) 


ART.  2.]  DATA    FOU   ESTIMATES.  113/ 

of  the  broken  stone  will  be  filled  with  mortar.  However,  it  ia 
exceedingly  nneconomical  to  use  a  very  porous  aggregate  and 
attempt  to  make  a  very  dense  concrete. 

The  above  comparisons  show  how  unscientific  it  is  to  proportion 
concrete  regardless  of  the  condition  of  the  materials  to  be  used. 

156e.  Occasionally  specifications  state  the  quality  of  the  mortar 
to  be  used,  and  require  the  mortar  and  the  aggregate  to  be  so  pro- 
portioned that  the  mortar  shall  at  least  be  equal  to  the  voids  in  the 
aggregate.  Under  this  method  of  procedure,  to  guard  against  lack 
of  uniformity  in  the  aggregate,  imperfect  mixing,  and  insufficient 
tamping,  it  is  customary  to  require  more  than  enough  mortar  to 
fill  the  voids,  this  excess  varying  from  0  to  50  per  cent.,  but  usually 
being  from  15  to  25.  Apparently  15  per  cent,  is  frequently  used 
in  Germany.* 

Xotice  that  this  method  is  an  approximation  to  that  discussed 
in  §  15G«  preceding. 

156/'.  Data  for  Estimates.  Table  13cl  and  Table  13e,  pages 
112^  and  112/i,  give  the  quantities  of  cement,  sand,  and  broken  stone 
required  to  make  a  cubic  yard  of  concrete,  for  the  two  methods  of 
proportioning  described  in  §  156a  and  §  1563,  respectively.  Each 
table  gives  the  quantities  for  unscreened  and  also  for  screened 
broken  stone;  and  Table  13f7  gives  also  the  quantities  of  cement 
and  gravel  required  for  a  cubic  yard  of  concrete. 

The  barrel  of  cement  in  both  tables  is  the  commercial  barrel  of 
packed  cement. 

156^.  Taile  13d  is  recommended  for  general  use.  The  first  line 
gives  a  concrete  'of  the  maximum  density  and  maximum  strength, 
i.e.,  tlie  quantity  of  mortar  is  sufficient  to  fill  the  voids  (see  §  155); 
and  the  successive  lines  give  concretes  of  decreasing  density  and 
strength.  The  third  and  subsequent  lines  give  concretes  containing 
mortar  equal  to  the  voids,  the  mortar  in  the  third  line  being  1  to  3, 
in  the  fourth  1  to  4,  etc. 

The  quantities  were  computed  as  described  in  §  156a,  and  were 
afterwards  checked  by  making  8-inch  cubes  of  concrete.  While 
the  results  are  only  approximate  for  any  particular  case,  it  is 
believed  that  they  represent  average  conditions  with  reasonable 
accuracy. 

The  quantities  in  the  table  are  for  stone  uniform  in  quality,  and 

*  Der  Portland  Cement  und  seine  Anwendungen  im  Bauwesen,  pp.  124  and  128. 


n2g 


CONCEETE. 


[chap.  IV. 


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il'Zl  CONCRETE.  [CHAP.  IV, 

for  concrete  thoroughly  and  vigoronsly  rammed;  and  if  it  is  desired 
certainly  to  secure  the  densest  concrete,  it  might  be  wise  to  increase 
somewhat  the  cement  and  sand  given  in  the  first  line  of  Table  13d. 
The  per  cent,  of  increase  should  vary  with  the  circumstances  of  the 
case  in  hand  (see  §  lo6e). 

The  proportions  of  the  concretes  can  be  determined  by  remem- 
bering that  a  barrel  of  cement  is  equal  to  0.13  cu.  yd.  For  example, 
for  unscreened  broken  stone  and  Portland  cement,  the  0.0-4  bbl.  of 
cement  is  equal  to  0.12  cu.  yd. ;  and  the  2:>roportions  are:  1  volume 
of  packed  cement,  2.5  volumes  of  loose  sand,  and  7.5  volumes  of 
loose  unscreened  broken  stone.  If  it  be  assumed  that  a  barrel  of 
packed  cement  will  make  1.25  barrels  when  measured  loose  (see 
§  126),  the  above  proportions  become:  1  volume  loose  cement, 
2.0  volumes  loose  sand,  and  6.0  volumes  loose  unscreened  broken 
stone. 

1567i.  Table  13e  is  given  for  use  in  determining  the  ingredients 
required  for  a  concrete  designed  in  the  ordinary  way — see  §  loQb. 
The  quantities  were  computed  substantially  as  illustrated  in  156c. 
This  table  is  not  as  accurate  as  Table  13f/,  and  besides  many  of  the 
proportions  are  uneconomical  (see  the  second  paragraph  of  §  156c). 

156/.  Proportions  from  Practice.  While  a  statement  of  the 
proportions  used  in  practice  may  be  of  interest,  it  can  not  be  of  any 
great  value  since  it  is  impracticable,  if  not  impossible,  to  describe 
fully  the  circumstances  and  limitations  under  which  the  work  was 
done.  Farther  the  specifications  and  records  from  which  such  data 
must  be  drawn  are  frequently  very  indefinite.  It  is  believed  that 
the  following  examples  are  as  accurate  as  it  is  possible  or  practicable 
to  make  them,  and  also  that  they  are  representative  of  the  best 
American  practice. 

For  foundations  for  pavements:  1  volume  oi  natural  cement, 
2  volumes  of  sand,  and  4  or  5,  and  occasionally  6,  volumes  of  broken 
stone;  or  1  volume  of  Portland  cement,  3  volumes  of  sand,  and  6 
or  7  volumes  of  broken  stone.  Occasionally  gravel  is  specified,  and 
more  rarely  gravel  and  broken  stone  mixed. 

For  foundations  and  minor  railroad  work:  1  volume  of  natural 
cement,  2  volumes  of  sand,  and  2  to  6,  usually  4  or  5,  parts  of 
broken  stone.     See  also  pages  532  and  535. 

For  important  bridge  and  tunnel  Avork :  1  part  of  Portland 
cement,  3  parts  of  sand,  and  4  or  5  parts  of  broken  stone. 


ART.  2.]  PROPORTIONS   FROM    PRACTICE.  112/ 

For  steel-grillage  foundations :  1  part  Portland  cement,  1  part 
sand,  and  2  parts  broken  stone. 

For  the  ]\Ielan  steel  and  concrete  construction  the  usual  pro- 
portions are:  1  volume  of  Portland  cement,  2^  volumes  of  sand, 
5  volumes  of  broken  stone. 

For  the  retaining  walls  on  the  Chicago  Sanitary  Canal :  1  part 
natural  cement,  14^  parts  sand,  and  4  parts  unscreened  limestone. 

For  the  dams,  locks,  etc.,  on  the  Illinois  and  Mississippi  Canal: 
1  volume  of  loose  Portland  cement,  8  volumes  of  gravel  and  broken 
stone;  or  1  volume  loose  natural  cement  and  5  volumes  gravel  and 
broken  stone. 

For  the  Poe  Lock  of  the  St.  Mary's  Fall  Canal:  1  part  natural 
cement,  \\  parts  of  sand,  and  -4  parts  of  sandstone  broken  to  pass 
a  2^-inch  ring  and  not  a  f-inch  screen.  The  broken  stone  had  46 
jier  cent,  voids  loose  and  38  when  rammed. 

In  harbor  improvements  the  proj^ortions  of  concrete  range  from 
the  richest  (used  to  resist  the  violent  action  of  waves  and  ice)  to 
the  very  lowest  (used  for  filling  in  cribwork).  At  Buffalo,  N.  Y., 
an  extensive  breakwater  built  in  1800  by  the  U.  S.  A.  engineers, 
consisted  of  concrete  blocks  on  the  faces  and  a  backing  of  concrete 
deposited  in  place.  Portland  was  used  for  the  blocks  and  natural 
for  the  backing,  the  proportions  being:  1  volume  cement,  3  sand, 
and  8^  of  broken  stone  and  pebbles  mixed  in  equal  parts. 

For  the  concrete  blocks  used  in  constructing  the  Mississippi 
Jetties  the  proportions  were:  1  part  Portland  cement,  1  part  sand, 
1  part  gravel,  and  5  parts  broken  stone. 

For  incidental  information  concerning  proportions  used  in  prac- 
tice, see  Cost  of  Concrete,  §  158rt,  page  Wlv. 

156/.  Water  Required.  There  is  a  considerable  diversity  of 
oijinion  among  engineers  as  to  the  amount  of  water  to  be  used  in 
making  concrete.  According  to  one  extreme,  the  amount  of  water 
should  be  snch  that  the  concrete  will  quake  when  tamped;  or  in 
other  words,  it  should  have  the  consistency  of  liver  or  jelly. 
According  to  the  other  extreme,  the  concrete  should  be  mixed  so 
dry  that  when  thoroughly  tamped  moisture  just  flushes  to  the  sur- 
face. The  advocates  of  wet  mixture  claim  that  it  makes  the 
stronger  and  more  dense  concrete;  while  the  advocates  of  dry  mix- 
ture claim  the  opposite.  The  difference  in  practice  is  not  as  great 
as  in  theory;  the  apparent  difference  is  chiefly  due  to  differences 
in  condition. 


112/t  CONCRETE.  [chap.  IV. 

It  is  unquestionably  true  that  dry  mixtures  of  neat  cement,  and 
also  of  cement  and  sand,  are  stronger  than  wet  mixtures,  provided 
the  amount  of  water  is  sufficient  for  the  crystallization  of  the 
cement.  It  is  also  certainly  true  then  in  even  a  dry  mortar  or  con- 
crete, the  water  is  considerably  in  excess  of  that  necessary  for  the 
crystallization  of  the  cement,  this  excess  increasing  with  the  amount 
of  sand  and  aggregate.  Of  course  the  excess  water  is  an  element 
of  weakness.  Bnt  the  amount  of  water  to  be  used  in  making  con- 
crete is  usiially  a  question  of  expediency  and  cost,  and  not  a  ques- 
tion of  the  greatest  attainable  strength  regardless  of  expense. 

1.  Dry  mixtures  set  more  quickly  and  gain  strength  more 
rapidly  than  wet  ones;  and  therefore  if  quickset  and  early  strength 
are  desired,  dry  concrete  should  be  preferred.  2.  Wet  concrete 
contains  a  great  number  of  invisible  pores,  while  dry  concrete  is 
liable  to  contain  a  considerable  number  of  visible  voids;  and  for 
this  reason  there  is  liability  that  wet  concrete  will  be  pronounced 
the  more  dense,  even  though  both  have  the  same  density.  3.  Wet 
concrete  is  more  easily  mixed;  and  therefore  if  the  concrete  is  mixed 
by  hand  and  the  supervision  is  insufficient  or  the  labor  is  careless, 
or  if  the  machine  by  which  it  is  mixed  is  inefficient,  wet  mixtures 
are  to  be  preferred.  4.  Wet  mixtures  can  be  compacted  into  place 
with  less  effort  than  dry;  but  on  the  other  hand  the  excess  of  water 
makes  the  mass  more  porous  than  though  the  concrete  had  been 
mixed  dry  and  thoroughly  compacted  by  ramming.  Dry  con- 
crete must  be  compacted  by  ramming,  or  it  will  be  weak  and 
porous;  therefore  if  the  concrete  can  not  be  rammed  into  place,  it 
should  be  mixed  wet  and  then  the  weight  of  the  stones  will  bury 
themselves  in  the  mortar,  and  the  mortar  will  flow  into  the  voids. 
5.  A  rich  concrete  can  be  compacted  much  easier  than  a  lean  one, 
owing  to  the  lubricating  effect  of  the  mortar;  and  hence  rich  con- 
cretes can  be  mixed  dryer  than  lean  ones.  The  quaking  of  concrete 
frequently  is  due  more  to  an  excess  of  mortar  than  to  an  excess  of 
water.  G.  Lean  concretes  sliould  be  mixed  dry,  since  if  wet  the 
cement  will  find  its  way  to  the  bottom  of  the  layer  and  destroy  the 
uniformity  of  the  mixture.  7.  Machine-made  concrete  may  be 
mixed  dryer  than  hand-made,  owing  to  the  more  thorough  incor- 
;poration  of  the  ingredients.  8.  Gravel  concrete  can  be  more  easily 
■  compacted  than  broken  stone,  and  hence  may  be  mixed  dryer. 
>Cement  and  sand  alone  is  more  easily  compacted  than  when  mixed 
rwith  coarser   material,  particularly  broken   stone;   and   therefore 


ART,  2.]         ■  WATER   REQUIRED.  1121 

mortar  to  be  deposited  in  mass  should  be  mixed  dryer  than  concrete. 
9.  In  mixing  dry  by  hand  there  is  a  tendency  for  the  cement  to  ball 
up,  or  form  nodules  of  neat  cement,  while  in  mixing  wet  this  does 
not  occur.  10.  If  wet  concrete  is  deposited  in  a  wood  form,  there 
is  liability  of  the  water  exuding  between  the  planking  and  carrying 
away  part  of  the  cement  and  thus  weakening  the  face — which  should 
be  the  strongest  part  of  the  mass. 

The  conclusion  is  that  sometimes  wet  concrete  must  be  used 
regardless  of  any  question  of  strength  and  cost;  while  with  thorough 
mixing  and  vigorous  ramming,  dry  concrete  is  strongest  but  also 
most  expensive  to  mix  and  lay. 

156Z,-.  The  following  experiments  are  the  only  ones  of  any  im- 
portance made  to  determine  the  relative  strength  of  wet  and  dry 
concretes.  The  mean  crushing  strength  of  four  hundred  and 
ninety-six  1-foot  cubes  *  made  with  mortar  as  "  dry  as  damp  eartli  " 
was  11  per  cent,  stronger  than  cubes  made  Avith  mortar  of  the 
"ordinary  consistency  used  by  the  average  mason,"  and  13  j)er 
cent,  stronger  than  cubes  that  "  quaked  like  liver  under  moderate 
ramming."  The  cubes  were  made  of  five  brands  of  Portland 
cement,  with  broken  stone  and  five  proportions  of  sand  varying 
from  1  to  1  to  1  to  5 ;  half  the  cubes  had  a  little  more  mortar  than 
enough  to  fill  the  voids,  while  the  other  half  had  only  about  80  per 
cent,  as  much  mortar  as  voids.  One  quarter  of  the  cubes  were 
stored  in  water,  one  quarter  in  a  cellar,  one  quarter  under  a  wet 
cloth,  and  one  quarter  in  the  open  air;  and  all  were  broken  when 
approximately  2  years  old.  The  difference  in  the  amount  of  mortar 
made  no  appreciable  difference  in  the  strength. 

The  mean  of  twelve  cubes  of  dry  concrete  was  51  per  cent. 
stronger  than  corresponding  cubes  of  quaking  concrete,  f 

A  few  minor  experiments  have  been  found  confirming  the  above, 
and  none  have  been  found  that  contradict  them. 

156/.  The  amount  of  water  required  to  produce  any  particular 
plasticity  varies  so  greatly  with  the  proportions  of  the  ingredients, 
the  kind  and  fineness  of  the  cem_ent,  the  dampness  of  the  sand,  the 
kind  of  aggregate,  etc.,  that  it  is  scarcely  possible  to  give  any  valu- 
able general  data.  The  water  varies  from  10  to  40  pounds  per  cubic 
foot  of  concrete.     The  only  general  rule  that  can  be  given  is  that  for 

*  Geo.  W.  Rafter,  in  Report  of  the  New  York  State  Engineer  for  1897,  pp.  375-460, 
particularly  Table  i,  page  398. 

t  Feret,  Engineering  Neics,  vol.  xxvii,  p.  311. 


112m  CONCEETE.  [chap.  IV^ 

dry  concrete  the  aggregate  should  be  wet  but  have  no  free  water  ia 
the  lieap;  and  that  the  mortar  should  be  damp  enough  to  show 
water  only  when  it  is  thoroughly  rammed,  or  so  that  water  will 
flush  to  the  surface  when  it  is  tightly  squeezed  for  a  considerable 
time  in  the  hand. 

In  the  experiments  referred  to  in  the  first  paragraph  of  the 
■  preceding  section,  the  average  quantity  of  water  for  the  different 
'  grades  of  dry  mortar  was  19.8  lbs.  per  cu.  ft.,  and  for  the  plastic 
'  21.4,  and  for  the  wet  22.5,  the  sand  being  reasonably  dr}-. 

158/;?.  Mixing. — The  value  of  the  concrete  dejjends  greatly 
upon  the  thoroughness  of  the  mixing.  Every  grain  of  sand  and 
every  fragment  of  agg.'egate  should  have  cement  adhering  to  every 
point  of  its  surface.  Thorough  mixing  should  cause  the  cement 
not  only  to  adhere  to  all  the  surfaces,  but  should  force  it  into 
intimate  contact  at  every  point.  It  is  possible  to  increase  the 
strength  of  really  good  concrete  100  per  cent,  by  prolonged  tritura- 
tion and  rubbing  together  of  its  constituents.  The  longer  and  more 
thorough  the  mixing  the  better,  provided  the  time  does  not  trench 
upon  the  time  of  set  or  the  working  does  not  break  and  pulverize 
the  angles  of  the  stone.  Uniformity  of  the  mixture  is  as  important 
as  intimacy  of  contact  between  the  ingredients.  Of  course  thorough- 
ness of  mixing  adds  to  the  cost,  and  it  may  be  wiser  to  use  more 
cement,  or  more  concrete,  and  less  labor. 

Concrete  may  be  mixed  by  hand  or  by  machinery.  The  latter 
is  the  better;  since  the  work  is  more  quickly  and  more  thoroughly 
done,  and  since  ordinarily  the  ingredients  are  brought  into  more 
intimate  contact.  jMachine  mi.-cing  is  frequently  specified.  If  any 
considerable  quantity  is  required,  machine  mixing  is  the  cheaper, 
ordinarily  costing  only  about  half  as  much  as  hand  mixing. 

156w.  Hand  Mixing.  The  sand  and  aggregate  are  usually 
measured  in  wheelbarrows,  the  quantit}''  being  adjusted  for  a  Ijag 
or  barrel  of  cement.  The  dry  cement  and  sand  are  mixed  as 
described  in  the  first  paragraph  of  §  124  (page  85),  which  see. 
The  proper  quantity  of  water  is  then  added,  preferably  with  a 
spray  to  secure  greater  uniformity, and  prevent  the  washing  away 
of  the  cement.  The  mass  should  be  again  turned  until  it  is  of 
nniform  consistency.  The  broken  stone,  having  previously  been 
sprinkled  but  having  no  free  water  in  the  heap,  is  then  added.  The 
whole  is  then  turned  until  every  fragment  is  covered  with  cement. 
Specifications  usually  require  concrete  to  be  turned  at  least  four 


ART.  2.]  MIXING.  112 n 

times,  and  frequently  six.  The  concrete  appears  wetter  each  time 
it  is  turned,  and  should  appear  too  dry  until  the  very  last. 

If  gravel  is  used  instead  of  broken  stone,  the  mixing  is  done  as 
■described  for  cement  and  sand. 

156o.  Machine  Mixing.  A  variety  of  concrete-mixing  machines 
are  in  use.  Some  forms  are  intermittent  and  some  continuous  in 
their  action.  Some  of  the  latter  automatically  measure  the  in- 
gredients. A  simple  variety  of  the  former  consists  of  a  cubical  box 
revolved  slowly  about  a  diagonal  axis.  The  dry  materials  are 
inserted  through  a  door,  and  the  water  is  admitted  through  the 
axis  daring  the  process  of  mixing.  Eight  or  ten  revolutions  are 
sometimes  specified;  but  eighteen  or  twenty  are  more  frequently 
specified  and  give  a  much  better  concrete.  Sometimes  an  inclined 
cylinder  or  long  box  revolving  about  the  long  axis  is  employed. 
Another  form  consists  of  a  vertical  box  having  a  series  of  inclined 
shelves  projecting  alternately  from  opposite  sides,  the  materials 
being  thrown  in  at  the  top  and  becoming  mixed  by  falling  sncces- 
sively  from  the  inclined  shelves.  A  modification  of  this  form  sub- 
stitutes rods  for  the  shelves,  the  mixing  being  accomplished  by  the 
ingredients  in  their  descent  striking  the  rods.  Still  another  type 
form  consists  of  a  spiral  conveyor  or  a  bladed  screw-sliaft  revolving 
in  a  trough  in  which  the  materials  are  thrown.  All  of  these  forms, 
and  also  modifications  of  them,  are  to  be  had  on  the  market. 

1567).  Laying.  After  mixing,  the  concrete  is  conveyed  in  wheel- 
barrows or  in  buckets  swung  from  a  crane,  deposited  in  layers  G  to 
8  inches  tliick,  and  compacted  by  ramming.  In  dumping,  the  mass 
should  not  be  allowed  to  fall  from  any  considerable  height,  as  doing 
so  separates  the  ingredients.  If  in  handling,  the  larger  fragments 
become  separated,  they  should  be  returned  and  be  worked  into  the 
muss  with  the  edge  of  a  shovel. 

The  rammer  usually  employed  consists  of  a  block  of  iron  having 
a  face  G  to  8  inches  square  and  weighing  anything  up  to  30  or  40 
pounds.  The  face  of  the  rammer  is  sometimes  corrugated,  to  keep 
the  surface  of  the  layer  rough  and  thus  afford  a  better  bond  with 
the  next,  and  also  to  transfer  the  compacting  effect  of  the  blow  to 
the  bottom  of  the  layer.  The  tamping  should  be  vigorous  enougli 
to  thoroughly  compact  the  mass;  but  too  severe  or  too  long-con- 
tinued pounding  injures  the  strength  of  the  concrete  by  forcing  the 
broken  stone  to  the  bottom  of  the  layer,  or  by  disturbing  the 
incipient  set  of  the  cement. 


112o  CONCRETE.  [CHAP.  IV. 

When  one  layer  is  laid  on  another  already  partially  set,  the 
entire  surface  of  the  latter  should  be  thoroughly  wet;  but  water 
should  not  stand  in  puddles.  In  case  the  first  layer  is  fully  set,  it 
is  wise  to  sweep  the  surface  with  neat  cement  paste  to  make  sure 
that  the  two  layers  adhere  firmly.  If  the  sand  or  gravel  contains 
any  appreciable  clay  and  the  concrete  is  mixed  wet,  clay  is  liable  to 
be  flushed  to  the  surface  and  prevent  the  adherence  of  the  next 
layer;  therefore  under  these  conditions  particular  care  should  be 
given  to  secure  a  good  union  between  the  layers.  After  the  con- 
crete is  in  2)lace  it  should  be  protected  from  the  sun,  and  not  be 
disturbed  by  walking  ujion  it  until  fully  set:  this  limit  should  be 
at  least  12  hours  and  is  frequently  specified  as  4  or  5  days. 

l5Qq.  Depositing  Concrete  under  Water.  In  laying  concrete 
under  water,  an  essential  requisite  is  tiiat  tlie  materials  shall  not 
fall  from  any  height,  but  be  deposited  in  the  allotted  jilace  in  a 
compact  mass;  otherwise  the  cement  will  be  separated  from  the 
other  ingredients  and  the  strength  of  the  work  be  seriously  im- 
paired. If  the  concrete  is  allowed  to  fall  through  the  water,  its 
ingredients  will  be  deposited  in  a  series,  the  heaviest — the  stone- — ■ 
at  the  bottom  and  the  lightest — the  cement — at  the  to]:),  a  fall  of 
even  a  few  feet  causing  an  appreciable  separation.  Of  course  con- 
crete should  not  be  used  in  running  water,  as  the  cement  would  be 
washed  out. 

A  common  method  of  depositing  concrete  under  water  is  to 
place  it  in  a  A^-shaped  box  of  wood  or  plate-iron,  which  is  lowered 
to  the  bgttom  by  a  crane.  The  box  is  so  constructed  that,  on 
reaching  the  bottom,  a  pin  may  be  drawn  out  by  a  string  reaciiing 
to  the  surface,  thus  j^ermitting  one  of  the  sloping  sides  to  swing 
open  and  allowing  the  concrete  to  fall  out.  The  box  is  then  raised 
to  be  refilled.  It  usually  has  a  lid.  Concrete  under  water  should 
not  be  rammed;  but,  if  necessary,  may  be  leveled  by  a  rake  or 
other  suitable  tool  immediately  after  being  deposited. 

A  long  box  or  tube,  called  a  freinie,  is  also  sometimes  used.  It 
consists  of  a  tube  open  at  top  and  bottom,  built  in  detachable  sec- 
tions so  that  the  length  may  be  adjusted  to  the  depth  of  water. 
The  tube  is  susjjended  from  a  crane,  or  movable  frame  running  on  a 
track,  by  which  it  is  moved  about  as  the  work  progresses.  The 
upper  end  is  hopper-shaped,  and  is  kept  above  the  water;  the  lower 
end  rests  against  the  bottom.  The  tremie  is  filled  in  the  beginning 
by  placing  the  lower  end  in  a  box  with  a  movable  bottom,  filling 


ART.  2  ]  STRENGTH.  Il2p 

the  tube,  lowering  all  to  the  bottom,  and  then  detaching  tlie 
bottom  of  the  box.  The  tnbe  is  kept  full  of  concrete,  as  the  mass 
issues  from  the  bottom  more  is  thrown  in  at  the  top. 

Concrete  has  also  b^en  successfully  deposited  under  water  by 
enclosing  it  in  paper  bags,  and  lowering  or  sliding  them  down  a 
chute  into  place.  The  bags  get  wet  and  the  pressure  of  the  con- 
crete soon  bursts  them,  thus  allowing  the  concrete  to  unite  into  a. 
solid  mass.  Concrete  is  also  sometimes  dejiosited  under  Avater  by 
enclosing  it  in  open-cloth  bags,  the  cement  oozing  through  the 
meshes  sufficiently  to  unite  the  whole  into  a  single  mass. 

When  concrete  is  deposited  in  water,  a  pulpy  gelatinous  fluid  is 
washed  from  the  cement  and  rises  to  the  surface.  This  causes  the 
water  to  assume  a  milky  hue;  hence  the  term  laitance,  which 
French  engineers  apply  to  this  substance.  It  is  more  abundant  in 
salt  water  than  in  fresh  water.  It  sets  very  slowly,  and  sometimes 
scarcely  at  all,  and  its  interposition  between  the  layers  of  concrete 
forms  strata  of  separation.  The  proportion  of  laitance  is  greatly 
diminished  by  using  large  immersing  boxes,  or  a  tremie,  or  paper 
or  cloth  bags. 

157.  Strength.  The  strength  of  concrete  depends  upon  the 
kind  and  amount  of  cement,  and  upon  the  kind,  size,  and  strength 
of  the  ballast.  Mortar  adheres  to  broken  stone  better  than  to 
pebbles,  and  therefore  concrete  containing  the  former  is  stronger 
than  that  containing  the  latter  (see  §  151).  If  the  sizes  of  the  indi- 
vidual pieces  of  the  ballast  are  so  adjusted  that  the  smaller  fit  into 
the  interstices  of  the  larger,  successively,  then  the  cementing 
material  will  act  to  the  best  advantage  and  consequently  the  con- 
crete will  be  stronger.  Hamming  the  concrete  after  it  is  in  place 
brings  the  pieces  of  aggregate  into  closer  contact,  and  consequently 
makes  it  stronger.  The  strength  of  concrete  also  depends  somewhat 
u.pon  the  strength  of  the  ballast,  but  chiefly  upon  the  adhesion  of 
the  cement  to  the  ballast. 

There  are  comparatively  few  experiments  ujion  the  strength 
of  concrete  in  which  the  data  was  complete  enough  to  make  the 
results  of  any  considerable  value. 

157a.  Compressive  Strength.  In  a  series  of  experiments  made 
by  Geo.  W,  Eafter*  to  determine  the  crushing  strength  of  concrete, 
three  varieties  of  Portland  cement  were  used,  all  of  which  were 

*  Report  of  the  New  York  State  Engineer,  1897,  pp.  375-460. 


112^ 


CONCRETE. 


[chap.  IV, 


equal  to  the  maximum  both  neat  and  with  sand  in  Table  10,  page 
78a.  The  sand  was  pure,  clean,  sharp  silica,  containing  o'l  per 
cent,  of  voids.  The  aggregate  was  sandstone  broken  to  pass  a 
2-inch  ring,  having  37  per  cent,  voids  when  tamped.  In  half  the 
blocks  the  mortar  was  a  little  more  than  enough  to  fill  the  voids; 
and  in  the  other  half  the  mortar  was  equal  to  about  80  per  cent,  of 
the  voids.     The  mortar  was  mixed  as  "  dry  as  damp  earth." 

The  test  specimens  were  1-foot  cubes,  and  were  stored  under 
water  for  four  months  and  then  buried  in  sand.  The  age  when 
tested  ranged  from  550  to  G50  days,  the  average  being  about  600. 
The  cubes  were  crushed  on  the  U.  S.  Watertown  Arsenal  testing- 
machine.  The  means  are  shown  in  Table  13/.  The  individual 
results  agreed  well  among  themselves. 

TABLE   13/. 
Crushing  Strength  op  Portland  Concrete. 
Voids  of  broken  stone  practically  filled  with  mortar — see  the  text. 
Age  when  tested  600  daj-s. 


Ref.  No. 

Composition 

op  Mortar. 

No. 
OF  Cubes 
Tested. 

Crushing 

Strength. 

Cement. 

Sand. 

lbs.  per  sq.  iu. 

tons  per  sq.  ft. 

1 

1 

3 

4,467 

322 

2 

2 

6 

3,731 

268 

3 

3 

6 

2,553 

184 

4 

4 

6 

2,015 

145 

5 

5 

2 

1,796 

129 

6 

6 

1 

1,365 

98 

The  cubes  summarized  in  Table  13/"  were  stored  under  water. 
Companion  blocks  stored  in  a  cool  cellar  gave  82  per  cent,  as  much 
strength;  those  fully  exposed  to  the  Aveather,  81  per  cent.;  and 
those  covered  with  burlap  and  wetted  several  times  a  day  for  about 
three  months  and  afterwards  exposed  to  the  weather,  80  per  cent. 

The  cubes  of  Table  13/ were  mixed  as  "  dry  as  damp  earth." 
Companion  blocks  of  which  the  mortar  was  mixed  to  the  "  ordinary 
consistency  nsed  by  the  average  mason,"  gave  90  per  cent,  as  much 
strength;  and  those  mixed  to  "quake  like  liver  nnder  moderate 
ramming,"  88  per  cent. 


ART.  2.] 


COMPRESSIVE    STRENGTH. 


1127 


157^.  Table  log  shows  the  results  of  a  series  of  experiments 
made  by  A.  W.  Dow,  Inspector  of  Asphalt  and  Cement,  "Washing- 
ton, D.'C* 

TABLE    13g. 
Crushing  Strength  op  Concrete  in  Pounds  per  Square  Inch. 


Composition  of  Concrete 
BY  Volumes  Loose. 

Voids 
IN  Aggregate. 

Age  of  Cubes  when  Broken. 

Ref. 

Ho. 

Mortar. 

Apgregafe  in 
Sizes  from 
214"  to  jV'- 

Per 

Cents. 

of 
volume 

Per 

Cent. 

of 
Voids 
filled 
with 
Mortar 

10 
Days. 

45 
Days. 

3 
Mos. 

6 
Mos. 

1 
Year. 

Cement 

Sand. 

Broken 
Stone. 

Gravel. 

1 
9, 

2 

2 

2 

2, 

2 

2 

2 
2 
2 
2 
2 
2 

6 

6* 
6t 

3 

4 

6 

6* 

6t 

3 

4 

Nan 

6 

3 

2 

Portl 

6 
3 
2 

iral  Ce 
45.3 
45.7 
39.5 
29.3 
35.5 
37.8 

and  Ce 
45.3 
45.7 
39.5 
29.3 
35.5 
37.8 

ment. 

83.9 

83.9 

96.2 

129.1 

107.0 

100.6 

ment. 

83.9 

83.9 

96.2 

129.1 

107.0 

100.6 

228 

539 

375 
596 

795 

915 
829 

R 

800 

4 
5 
6 

87 
108 

421 
364 

361 
593 

344 
632 

763 
841 
915 

7 
8 

908 

1,790 

2,260 
1,630 

2,510 
1,530 

3,060 
1.850 

9 

2  700 

10 
11 
19, 

694 
950 

1,630 
1,850 

2,680 

1.840 
2,070 

2,820 
2,750 
2,840 

*  Coarse.  t  Three  fourths  ordinary  stone,  one  fourth  granolithic. 

The  strength  of  the  cement  is  shown  in  Table  13h.  Notice  that 
the  Portland  cement  did  not  gain  strength  proportionally  as  fast  as 
the  natural  cement;  for  example,  the  Portland  mortar  in  line  7  is 
two  and  two-thirds  times  as  strong  as  the  preceding  natural-cement 
mortar,  wliile  that  in  line  10  is  not  quite  as  strong  as  the  natural- 
cement  mortar  immediately  preceding. 


*  Report  of  the  Operations  of  the  Engineering  Department  of  the  District  ol 
Columbia  for  the  year  ending  June  30,  1897,  pp.  160-66. 


112s 


CONCRETE. 


[chap.  IV., 


TABLE  137*. 
Tensile  Stkength  of  Cement  used  in  Table  l'6g. 


Rep  No. 

AOE  WHEN  Tested. 

Parts  Standard 

Quartz  to  1  hart 

Cement. 

Tensile  Steength 

IN.  LBS.  PER  SQ.  IN. 

Natural. 

Portland. 

1 

0 

3 

1  day. 
7  days. 
7     •' 
7     " 
1  nio. 
1      " 
3     " 
3     " 
6     '• 
6     " 
1  vear. 

0 
0 
o 

3 
3 
3 

2 

3 

2 
3 
2 
3 

9(5 

180 

91 

188 

441 
889 

4 
5 

248 

6 

429 

7 

327 

8 

398 

9 

414 

10 

428 

11 

485 

12 

474 

The  fineness  of  the  sand  was  as  follows:*  "3"  6 '-^  8"  10'"  20 '^ 
40^  60-  80"-^  100  "-5  and  contained  44.1  per  cent,  of  voids.  With 
the  natural  cement  the  water  used  was  0.317  cu.  ft.  (20  lbs.)  per 
ca,  ft.  of  rammed  concrete,  and  with  Portland  cement  0.24  cu.  ft. 
(12  lbs.) — in  both  cases  including  the  moisture  in  the  sand.f 

The  broken  stone  was  gneiss  broken  to  pass  a  2^-inch  ring,  none 
passing  a  No.  10  sieve,  the  voids  for  each  particular  concrete  being 
as  stated  in  Table  13_^.  The  gravel  was  clean  quartz  passing  a 
l|-inch  ring  and  only  3  per  cent,  passing  a  No.  10  ring,  and  had 
29  j)er  cent,  of  voids.  The  per  cent,  of  voids  in  the  aggregate  filled 
with  mortar  is  stated  in  Table  13^.  Each  result  in  the  table  is  the 
mean  of  two  cubes,  except  those  for  one  year,  which  are  the  mean 
of  five.  Owing  to  the  friction  of  the  press  with  which  the  tests 
were  made,  the  results  are  3  to  8  per  cent,  too  high. 

157c.   Table  13i  shows  the  relative  strength  of  rich  and  lean 

*For  explanation  of  the  nomenclature,  see  the  second  paragraph  of  §  114e. 
f  The  sand  contained  4.4  per  cent,  of  water,  which  increased  the  volume  of  the 
aand  and  made  the  mortar  slightly  richer  than  as  stated. 


ART.  2.] 


COMPRESSIVE   STRENGTH. 


112^ 


TABLE    13t. 
Relative  Strength  of  Rich  and  Lean  Concretes. 


I 

'ropohtions. 

Crushing  Strength. 

Rbf.  No. 

One  Week. 

Four  Weeks. 

Cement. 

Sand. 

Bi-oken 
Stone. 

i 

Lbs.  per 
f-q.  in. 

Relative. 

Lbs.  per 
sq.  in. 

Relative. 

Portia 

id  sand-cement 

1 

1 

n 

3 

412 

0.77 

490 

0.66 

2 

4 

446 

0.83 

679 

0.92 

3 

^ 

5:;6 

1.00 

741 

1.00 

4 

1 

2 

4 

316 

0.61 

441 

0.60 

5 

5 

275 

0.53 

477 

0.64 

6 

6 

521 

1.00 

639 

1.00' 

7 

1 

3 

5 

,  144 

0.69 

374 

0.85. 

8 

6 

110 

0.52 

182 

0.57 

9 

7 

210 

1.00 

332 

i.oa 

English  Porthind  cement 

10 

1 

2 

2 

494 

0.60 

565 

0.81 

11 

3 

611 

0.75 

555 

0.8a 

12 

4 

819 

1.00 

613 

0.8& 

13 

5 

581 

0.71 

680 

0.97 

14 

6 

500 

0.61 

698 

1.00 

15 

1 

3 

3 

333 

205 

0.53 

16 

4 

366 

0.95 

17 

5 
6 

386 
357 

1  00 

18 



0.92 

German  Portland  cement 

19 

1 

2 

4 
5 
6 

626 
703 

728 

0.86 

20 

0.97 

31 

1.00 

11'2h  COSrCRETE.  [CIIAP.  IV. 

concretes.*  The  water  was  equal  to  20  per  cent,  of  the  weight  of 
the  cement  and  the  sand.  The  test  specimens  for  the  Portland 
sand-cement  were  9  inches  square  and  12  inches  high,  and  for  the 
remainder  12-inch  cubes.  All  were  crashed  between  sheets  of  rubber 
(see  §  12,  page  0).  Each  value  in  the  table  is  the  result  for  a  single 
cube.  Table  13i  is  valuable  chiefly  as  showing  the  relative  strength 
of  rich  and  lean  concretes.  The  table  shows  that  a  moderately  lean 
concrete  is  stronger  than  a  very  rich  one,  which  is  in  accordance 
with  the  conclusion  from  Table  13a,  page  110,  that  a  concrete  is 
stronger  than  the  mortar  alone.  Table  13i  also  shows  tliat  the 
strength  of  the  concrete  increases  with  the  richness  of  the  mortar, 
which  agrees  with  Table  loZ*,  page  111,  and  Fig.  8,  page  112a. 

157d.  For  data  on  the  crushing  strength  of  gravrl  concrete,  see 
Table  13a,  page  110. 

For  data  on  the  crushing  strength  of  gravel  and  broken-stone 
corncretes  approximately  17  days  old,  see  Fig.  8,  page  112rr. 

157e.  Tlie  strength  of  concrete  made  of  coke  does  not  increase 
with  age  owing  to  the  soft  and  friable  nature  of  the  aggregate. 
Apparently  the  maximum  strength  of  1  volume  loose  cement, 
3  volumes  sand,  and  5  volumes  crushed  coke  is  about  600  to  700  lbs. 
per  sq.  in.  with  Portland,  and  about  300  to  350  with  natural  cement. 

157/.  Transverse  Strength.  Table  13/,  page  112y,  is  a  summary 
of  191  tests  on  concrete  bars  30  inches  long  and  4  inches  square. f 
The  cement  stood  497  lbs.  per  sq.  in.  neat  at  7  days,  and  209  lbs. 
with  3  parts  sand  at  4  weeks.  In  most  of  the  bars  the  mortar  was 
made  of  pulverized  sandstone^  although  in  some  cases  river  and  pit 
sands  were  used.  The  aggregate  was  generally  broken  sandstone, 
but  gravel  and  broken  whinstone  were  also  used.  "  In  each  case 
the  voids  in  the  '  sand  '  were  filled  with  cement,  and  those  in  the 
aggregate  with  mortar." 

The  results  are  tabulated  in  the  order  of  the  ratio  of  the  cement 
to  the  total  sand  and  aggregate.  Xotice  that  the  results  in  the  last 
line  are  proportionally  higher  than  those  in  the  remainder  of  the 
table.  This  difference  is  probably  due  to  tlie  fact  that  the  speci- 
mens for  tlie  first  four  lines  were  mado  with  natural  sand  and 
stone,  while  in  those  for  the  last  line  only  crushed  sandstone  was 
used  for  both  the  sand  and  the  aggregate. 

*  W.  B.  Anderson,  Student  Can.  Soc.  C.  E.,  in  Trans.  Can.  Soc.  C.  E.,  vol.  xiii., 
Part  1. 

t  A.  F.  Bruce,  in  Proc.  of  lust,  of  C.  E.  (London),  vol.  cxiii,  pp.  217-28. 


ART.  3. J 


TRANSVERSE    STRENGTH. 


112y 


TABLE    13  j. 

]\IoDULus  OF  Rupture  op  Portland  Co>crete  Bahs,  Pounds  per 

Square  Inch. 


Composition. 

Age  in  AVeeks  when  Tested. 

Ref. 
No. 

Cement 

Sand. 

Aggre- 
gate. 

1 

4 

8 

13 

19 

S6 

39 

1 
2 
3 
4 
5 

2 

2i 

3 

2i 

3 

3 
5 
5 
6 

7 

95 
37 

37 

145 

144 
88 
81 

113 

215 
165 
129 
130 
154 

266 
194 

176 
156 

187 

301 
268 
191 
193 
216 

303 
236 
214 
199 
243 

320 
259 
214 
212 
263 

157^.  In  connection  with  the  construction  of  the  Poe  Lock  of 
the  St.  Mary's  Falls  Canal*  a  series  of  one  hundred  and  forty-seven 
concrete  beams  10  inches  square  were  tested.  The  experiments 
were  very  carefully  conducted,  but  there  were  so  many  variables 
that  it  is  impossible  to  draw  any  general  conclusions  therefrom. 
The  beams  made  with  Portland  cement  were  tested  when  about  19 
months  old  and  those  with  natural  cement  when  about  12  months. 

157//.  Weight  of  Concrete.  The  weight  of  concrete  varies 
with  the  materials  and  the  proportions,  and  with  the  amount  of 
ramming.  The  weight  varies  from  130  to  160  lbs.  per  cu.  ft.,  but 
is  usually  from  140  to  150.  The  difference  in  weight  of  the  con- 
crete due  to  the  aggregate  and  to  the  ramming  is  greater  than  that 
due  to  the  difference  in  weight  between  Portland  and  natural 
cement.  The  maximum  difference  between  Portland  and  natural 
concrete,  due  to  the  greater  weight  of  Portland  cement,  is  4  or 
5  lbs.  per  cubic  foot.  Concrete  made  of  blast-furnace  slag  weighs 
from  110  to  120  lbs,  per  cubic  foot;  and  that  made  of  coke  from 
80  to  90  lbs.  per  cu.  ft. 

l5Sa.  Cost  of  Conckete.  The  cost  of  concrete  varies  greatly 
with  the  materials,  the  proportions,  the  cost  of  material  and  labor, 
etc. 

The  following  is  the  analysis  of  the  composition  and  cost  of  the 
concrete  employed  for  the  foundations  of  the  sea-wall  at  Lovell's 
Island,  Boston  Harbor:  f 

*  Report  of  Chief  of  Engineers,  U.  S.  A.,  1895.    Part  4,  pp.  2922-31. 

t  Compiled  from  Qillmore's  Limes,  Hydraulic  Cements  and  Mortars,  p.  247. 


112w  •  CONCRETE.  [CHAP.  IV. 

Cement,  0.83  bbl 0.12  cu.  yd.  @  $1  54  =  $1  26 

Sand 0.25  cu.  yd.  @       70  17 

Gravel 0.90  cu.  yd.  @       27  24 

Total  materials 1 .27  cu.  yd.  $1  67 


Labor,  making  mortar 0.06  days  @  1  20  =  08 

Labor,  making  concrete 0.11  days  @  1  20  13 

Labor,  t  aiisportiug  concrete 0.06  days  @  1  20  08 

Labor,  packing  concrete 0.03  days  @  1  20  04 

Total  labor 0. 26  days  33 


Tools,  implements,  etc 11 

Total  cost  1  cu.  yd.  of  concrete,  in  j)lace  $2  11 

The  proportions  for  this  concrete  were  1  cement,  3  sand,  and 
4  gravel.  It  was  tinnsually  cheap,  owing  partly  to  the  use  of 
pebbles  instead  of  broken  stone.  If  the  latter  had  been  used,  it 
would  have  cost  j^robably  4  to  6  times  as  much  as  the  gravel.  The 
amount  of  labor  required  was  also  unusually  small,  this  item  alone 
being  frequently  6  to  8  times  as  much  as  in  this  case. 

The  following  is  the  analysis  *  of  the  cost  of  nearly  10,000  yards 
of  concrete  as  laid  for  the  foundations  of  a  blast-furnace  plant  near 
Troy,  N.  Y.,  in  1886.  The  conditions  were  unusually  favorable 
for  cheap  work.  The  concrete  consisted  of  1  volame  of  packed 
cement  to  7  of  sand,  gravel,  and  broken  stone. 

Cement,  1.23  bbl 0.18  cu.  yd.  @  61  00  =  $1  23 

Sand 0.10      "  @    0  30=        03 

Gravel 0.36      "  @    0  30  =        11 

Broken  stone 0.74      "  @    1  41  =     1  04 


Total  materials 1.38      "  =$12  41 


Labor,  handling  cement 0.02  day  @  1  00  =  02 

unloading  stone 0.14     "  @  1  00  =  14 

mixing ....0.85     "  @  100=  85 

"       superintendence 0.01     "  @  9  61  =  10 


Total  labor 1.02."  =    109 


Total  cost  of  a  cubic  yard  of  concrete,  in  place =  |3  52 


*  Trans.  Am.  Soc.  of  C.  E.,  voL  xv.  p.  875. 


ART.  2.]  COST   OF    CONCRETE.  112z 

The  following  is  the  cost  of  the  concrete  used  in  the  construc- 
tion of  Hiland  Avenue  reservoir,  Pittsburg,  Penn.*  The  stone  was 
broken  so  as  to  pass  through  a  2^-inch  ring.  The  mortar  was 
1  part  liosendale  natural  cement  to  2  jiarts  sand.  The  concrete 
was  1  part  mortar  to  2^  of  stone.  The  concrete  was  mixed  by  hand. 
Common  laborers  received  $1.25  per  day,  and  foremen  ^2.50.  The 
contract  price  was  $G.OO  per  yard. 

Quarrying  stoue $0  45 

Transpor,  iug  stone 50 

Breaking  sloue 35 

Cement  @  $1.35  per  bbl 180 

Sand,  cost  of  digging 10 

Water 05 

Labor,  mixing  and  laying 75 

Incidentals 05 

Total  cost  per  cubic  yard,  in  place $4  05 

The  following  is  the  cost  of  concrete  in  the  foundations  of  an 
electric  power-house  at  Pittsburg,  Pa.,  in  1890. f  The  proportions 
were  1  volume  of  packed  cement,  3  volumes  of  sand,  and  5  volumes 
of  broken  stone.  The  cost  of  labor  was  abnormally  high.  The  day 
was  ten  working  hours. 

Portland  cement  1.28  bbl 0.17  cu.  yd.  @|2.60  $3.33 

Sand 0.50      "  @    1.30  0.65 

Broken  stone 0.90      "  @    1.35  1.13 

Labor 0.91  day  @    1.75  1.59 

Superintendence 0.07    "  @    3.00  0.21 

7^otal  cost  per  cu.  yd.,  in  p/ac(! $6  90 

The  following  is  the  cost  of  constructing  the  concrete  retain- 
ing wall  on  the  Chicago  Sanitary  Canal.  J  The  average  height  of 
the  wall  was  10  ft.  in  Sec.  1-4,  and  22  ft.  in  Sec.  15.  The  thickness 
on  top  was  6  ft.,  and  at  the  bottom  it  was  equal  to  half  the  height. 
The  stone  was -taken  from  the  adjacent  canal  excavation.  The  body 
of  the  wall  was  made  with  natural  cement,  but  the  coping  and 
facing,  each  3    inches  thick,   were   made   with   Portland   cement, 

*  Einilo  Low  in  Engineering  TWfo.s,  vol.  xiii.  p.  51,  52. 

t  E.  T.  Chibas  in  The  Polytechnic,  Rensselaer  Polytechnic  Institute,  vol.,  vii. 
p.  145. 

X  Jour.  West.  Soc.  of  Eng'rs,  vol.  iii.  pp.  1310-32. 


lV2y  CONCRETE.  [chap.  iv» 

The  proportions  were  1  volume  of  cement,  1^  volumes  of  sand,  and 
4  volumes  of  unscreened  limestone.  The  cost  of  plant  employed  in 
Sec.  14  was  80,600,  and  in  Sec.  15  was  $25,4-20.  The  contract 
price  for  the  concrete  in  Sec.  14  was  12.74,  and  in  Sec.  15  13.40 
per  on.  yd. 

Items  of  Expense. 

Labor,  general f 0.078 

on  the  wall ....   

mixing  concrete 

placing  and  removing  forms.  ... 

transporting  materials 

quarrying  stone 

crushing  stone 

Total  for  labor $0,975         $1,074 


Cost  per  1 
Sec.  14 

Cubic  yard. 
Sec.  15 

$0,078 

$0,082 

.108 

AH: 

.121 

.250 

.150 

.142 

.143 

.081 

.303 

.275 

.073 

.128 

Material,  cement,  natural    @  $0.65  per  bbl.       0.863  .930 

"       Portland®  $2.25  "      "             .805  .180 

sand @|1.35percu.  yd.     .465  .476 

Total  for  materials $  1 .  633  $1 .  586 


Machinery,  cost  of  operating 407  .567 

Total  cost  per  cu.  yd $3,015  $3 .  227 

For  additional  data  concerning  the  cost  of  concrete,  see 
§§  233-34,  page  157. 

1586.  The  following  items  relate  only  to  the  labor  of  making 
concrete. 

Table  13^'  gives  the  details  of  the  cost  per  cubic  yard  of  the 
labor  required  in  mixing  and  laying  concrete  for  the  Buffalo,  ]^.  Y., 
breakwater,  constructed  in  1887-89.  The  data  were  communicated 
by  Capt.  F.  A.  ]\Iahan,  Corps  of  Engineers,  U.  S.  A.,  who  had 
charge  of  the  work.  The  total  amount  of  concrete  laid  was  14,587 
cu,  yds.  The  conditions  under  which  the  work  was  done  varied 
considerably  from  year  to  year.* 

Table  13m  gives  the  details  of  the  labor  required  in  mixing  and 
laying  concrete  in  the  construction  of  the  Boyd's  Corner  dam.f 

*  The  work  is  fuUj'  described  in  Report  of  Chief  of  Eugiueors,  U.  S.  A.,  for  1890, 
pp.  2808-35. 

t  Fi-nm  an  account  of  the  construction  of  the  Boyd's  Corner  dam  on  the  Croton 
Kiver  near  New  York.  City,  by  J.  James  R.  Croes,  in  Trans.  Am.  Soc.  of  C.  E.,  vol. 
ill.  p.  360. 


ART.  2.] 


COST    OF    CONCRETE. 


U2z 


TABLE  13k. 
Cost  of  Mixing  and  Laying  Concrete. 


Ref. 

No. 


10 


Items. 


Transporting  cement  from  store-house. 

Measuring  cement 

Mixing  cement  paste 

Measuring  sand  and  pebbles 

Measuring  broken  stone 

Mixing  concrete 

Transporting  concrete 

Spreading  and  ramming  concrete 

Placing  forms 

Building  temporary  railway 

Total  labor  per  cu.  yd , 


Concrete  mixed  by 


hand. 


.078 

.212 

.172 

.070 
.557 
.185 
.270 
.240 


$1,790 


iiiachiiiery. 


1887         1889 


$0. 


128 

26 

186 

285 

198 

152 

445 

502 

176 


$0,098 
.024 
.084 
.116 
.101 
.103 
.160 
.892 
.263 
.181 


$2. 098:  $1,528 


TABLE  13to. 
Labor  Required  i::  .Mixing  and  Laying  Concrete. 


Kind  op  Labor. 


Mixers— hand  work,  days  . . 
Derrick  and  car  men,  days. 

Engine,  hours.   

Handling  sand,  days 

Handling  stone,  days 

Carts,  days  

Ramming,  days 


Labor  per  Cubic  Yard. 


New  York  Storage  Reservoir.  St.  Louis  Reservoir 


Mixed  on  level 
and  wheeled  in 


2s= 


i-0.161 

'  0.065 
0.125 


2.2.'%^ 


!•  0.227 


■0.114 

0.076 
0.078 


Hoisted  by 

steam  and  run 

on  cars. 


00   t) 

o*  > 

C  «  = 
O  «  3 


0.145 
0.088 
0.152 
0.065 
0.127 
0.046 
0.071 


»n  0* 


fe 


0.121 
0.070 
0.108 
0.071 
0.098 
0.035 
0.073 


All  work  on  level 
— wheeled  in. 


j- 0.183 

o.nss 

0.125 


-0  1:34 

0  0.17 
0.107 


0  250 

0.068 
0.128 


113a  CONCRETE.  [chap.  IV. 


158c.  The  cost  of  mixing  and  laying  6  inches  of  concrete  for  a 
pavement  foundation  is  about  7  cents  jier  sq.  yd.,  for  1  part  cement, 
2  parts  sand,  and  4  parts  broken  stone,  turned  six  times — exclusive 
of  casting  into  place.  With  gravel  instead  of  broken  stone,  the  cost 
is  about  6  cents  per  sq.  yd. ;  and  with  four  turnings  instead  of  six, 
the  cost  is  about  half  a  cent  less  than  the  prices  above. 

158^.  Economic  Concrete.  The  relative  economy  of  natural  and 
Portland  cement  mortars  can  be  investigated  as  explained  in 
§§  136,  137. 

The  relative  strengths  of  gravel  and  of  broken-stone  concretes 
<are  stated  in  the  last  two  paragraphs  of  §  151.  The  relative 
economy  of  concrete  made  with  broken  stone  and  gravel  will  vary 
with  the  cost  of  each;  but  as  a  rule,  when  gravel  costs  less  than  30 
per  cent,  of  that  of  broken  stone,  gravel  is  more  economical. 

The  strengths  of  both  broken-stone  and  gravel  concretes  are 
given  in  Table  13^5^,  page  112r,  for  both  natural  and  Portland 
cements  at  different  ages.  A  study  of  these  results  shows  that  the 
relative  strength  of  natural  and  Portland  concrete  is  different  at 
different  ages.  For  example,  taking  averages  for  10  days,  the 
Portland  concrete  was  6  times  as  strong  as  the  natural  concrete; 
while  at  a  year  the  Portland  concrete  was  only  3  times  as  strong  as 
the  natural  concrete.  At  45  days  and  also  at  6  months,  tlie  Port- 
land concrete  was  4  times  stronger  than  the  natural  concrete;  and 
at  3  months  5  times  as  strong.  Taking  averages  for  like  dates 
and  compositions,  the  Portland  cement  concrete  was  3.7  times 
as  strong  as  natural  cement  concrete.  "With  the  data  in  Table  13d 
or  13^,  page  112i7  or  112//,  it  is  easy  to  compute  the  cost  of 'each 
kiud  of  concrete,  if  the  cost  of  a  cubic  yard  of  Portland  cement 
'Concrete  is  more  than  3.7  times  that  of  a  cubic  yard  of  natural 
cement  concrete,  then  the  latter  is  on  the  average  the  more  econom- 
ical; but  if  the  Portland  cement  concrete  costs  less  than  3.7  times 
that  of  the  natural  cement  concrete,  then  the  former  is  on  the 
average  the  more  economical.  Of  course  the  relative  cost  will  vary 
with  the  condition  of  the  cement  market  and  with  the  locality. 

IbSe.  The  following  example,  from  actual  practice,  illustrates 
the  possibilities  in  the  way  of  combinations  between  Portland  and 
natural  cements,  and  gravel  and  broken  stone.  The  specifications 
called  for  a  concrete  composed  of  1  volume  of  natural  cement, 
2  volumes  of  sand,  and  4  volumes  of  screened  broken  stone.  The 
contractor  found  that  at  current  prices  a  concrete  composed  of 
1  volume  of  Portland  cement  and  9  volumes  of  gravel  would  cost 


ART.  3.]  ARTIFICIAL    STONE.  Il3b 

about  the  same  as  the  concrete  specified.  A  test  of  the  strength  of 
the  two  concretes  showed  that  at  a  week  the  Portland-gravel  con- 
crete was  1.52  times  as  strong  as  the  natural  cement  and  broken- 
stone  concrete;  and  at  a  month  1.59  times  as  strong.  Therefore  the 
Portland-gravel  concrete  was  the  more  economical,  and  was  used. 

Art.  3.  Artificial  Stone. 

159.  Several  kinds  of  artificial  stone  have  come  into  use  within 
the  hist  twenty-five  years  for  architectural  and  artistic  purposes,  and 
for  the  pavements  of  cellars,  for  footpaths,  areas,  and  other  locali- 
ties not  subjected  to  the  tread  of  heavy  animals.  They  are  all  a 
combination  of  hydraulic  cement  and  sand,  pebbles,  etc.  Some  of 
them  possess  very  considerable  merit,  and  are  of  value  in  districts 
where  durable  and  cheap  building-stone  is  not  supplied  by  nature. 

The  strength  and  hardness  of  all  varieties  of  artificial  stone  vary 
directly  with  the  ultimate  strength  and  hardness  attainable  by  the 
hydraulic  ingredients  employed.  An  obvious  means  of  improving 
the  quality  of  the  stone,  therefore,  is  the  employment  of  the  highest 
grades  of  cement. 

160.  Beton-Coignet.  As  made  by  its  inventor,  Coignet,  of 
Paris,  its  usual  ingredients  are:  Portland  cement,  siliceous  hydraulic 
lime  (like  that  obtained  at  Teil,  France),  and  clean  sand,  mixed 
together  with  a  little  fresh  water.  The  proportions  are  varied  con- 
siderably for  different  kinds  of  work.  The  dry  ingredients  are  first 
thoroughly  mixed  by  hand,  and  again  in  a  mill  after  moistening 
them  very  slightly  with  clean  water.  Moulds  are  then  filled  with 
the  mixture,  which  is  compacted  by  ramming.  The  peculiarities 
of  this  stone  result  from  (1)  the  small  quantity  of  water  used  in  its 
manufacture,  (2)  a  judicious  choice  of  the  qualities  and  proportions 
of  the  ingredients,  and  (3)  the  thoroughness  with  which  the  mixing 
is  done.  It  is  nothing  more  than  hydraulic  concrete,  from  which 
the  coarse  fragments  have  been  omitted,  and  upon  which  have  been 
conferred  all  the  advantages  to  be  derived  from  their  thorough 
manipulation.  It  is  used  in  France  to  a  considerable  extent  in 
constructing  the  walls  of  houses,  and  in  repairing  masonry, — as 
bridge  piers,  culverts,  etc. 

In  this  country  a  mixture  of  either  natural  or  Portland  cement 
and  sand  is  frequently,  but  improperly,  called  Beton-Coignet. 

161.  Portland  Stone.  This  is  a  mixture  of  Portland  cement 
and  sand,  or  sand  and  gravel,  compacted  into  form  by  tamping. 


11-i  ARTIFICIAL    STONE.  [cHAP.  lY^ 

When  properly  made  it  possesses  the  essential  requisites  of  strength 
and  hardness  in  a  degree  proportionate  to  the  value  of  the  cement 
employed.  The  proportions  of  1  measure  of  dry  cement  to  2  or  "21- 
measures  of  sand  will  answer  for  most  purposes.  The  manipulation 
should  be  prolonged  and  thorough  to  insure  the  production  of  a 
homogeneous  stone.  It  is  much  used  for  flagging,  for  which  pur- 
pose the  surface  layer,  to  the  thickness  of  about  half  an  inch,  may 
advantageously  be  composed  of  1  measure  of  cement  to  1^  or  li  of 
sand. 

162.  McMuRTRlE  Stone.  This  stone  consists  essentially  of  the 
Portland  stone  described  above,  in  the  pores  of  which  are  formed 
compounds  of  alumina  with  the  fatty  acids  by  the  double  decom- 
position of  alum  and  a  potash  soap  (see  §  140,  page  101).  These 
compounds  are  insoluble  in  water,  are  not  acted  upon  by  the  car- 
bonic acid  of  the  air,  and  add  considerably  to  the  early  strength  of 
the  stone  and  somewhat  to  its  ultimate  strength. 

The  peculiar  merit  of  this  stone  is  that  its  power  of  absorbing^ 
water  is  decreased  by  the  use  of  the  alum  and  the  soap.  All  mor- 
tars and  most  of  the  artificial  stones  absorb  water  freely,- — porous 
mortar  from  50  to  GO  per  cent,  of  its  own  weight  and  the  best  Port- 
land from  10  to  20  per  cent., — and  consequently  they  disintegrate 
rapidly  under  the  action  of  frost.  The  absorbed  water  also  dissolves 
the  salts  of  magnesia,  lime,  soda,  and  potash  (of  all  of  which  there 
is  always  more  or  less  in  cement),  and  on  evaporating  leaves  a  white 
efflorescence  on  the  surface,  which  injures  the  appearance  of  the 
wall.  For  these  reasons  the  ordinary  artificial  stones  are  in  dis- 
repute for  architectural  purposes.  The  absorptive  power  of  the  Mc- 
Murtrie  stone  is  about  twice  that  of  granite,  about  equal  to  that  of 
the  best  limestones,  and  about  one  tenth  or  less  of  that  of  the  best 
sandstones.  It  has  been  used  in  Washington,  D.  C,  to  a  limited 
extent,  the  window  trimmings  of  the  National  Museum  and  also  the 
fronts  of  a  few  stores  and  dwellings  being  of  this  stone.  It  a^^pears 
to  have  given  entire  satisfaction. 

163.  Frear  Stone.  This  is  composed  of  siliceous  sand  and  good 
Portland  cement,  to  which  gum  shellac  is  added.  The  composition 
used  by  the  inventor  was  1  measure  of  cement  and  2^  measures  of 
sand  moistened  with  an  alkaline  solution  of  shellac  of  sufficient 
strength  to  furnish  an  ounce  of  the  shellac  to  a  cubic  foot  of  stone. 
The  shellac  adds  to  the  early  strength  of  the  stone  ;  but  it  is  not 
certain  that  it  adds  to  the  ultimate  strength,  nor  is  it  certain  that 


AKI.  3]  SOKEL   STOXE.  115 

the  shellac  may  not    decay  and    ultimately  prove   an    element  of 
weakness. 

\>  lieu  mixed,  it  is  rammed  into  wooden  moulds,  and  after  setting 
is  laid  away  to  season, — which  requires  several  months  for  best 
results.  It  was  much  used  in  architectural  work  in  the  West  a  few 
years  ago,  but  did  not  give  satisfaction. 

164.  Ransome  Stone.  This  is  made  by  forming  in  the  in- 
terstices of  sand,  gravel,  or  any  pulverized  stone  a  hard  and 
insoluble  cementing  substance,  by  the  natural  decomposition  of 
two  chemical  compounds  in  solution.  Sand  and  the  silicate  of 
soda  are  mixed  in  the  proportion  of  a  gallon  of  the  latter  to  a 
bushel  of  the  former  and  rammed  into  moulds,  or  it  may  be 
rolled  into  slabs  for  footpaths,  etc.  At  this  stage  of  the  process 
the  blocks  or  slabs  may  be  easily  cut  into  any  desired  form.  They 
are  then  immersed,  under  pressure,  in  a  hot  solution  of  chloride  of 
€alcium,  after  which  they  are  thoroughly  drenched  with  cold  water 
— for  a  longer  or  shorter  period,  according  to  their  size — to  wash 
■out  the  chloride  of  sodium  formed  dui-ing  the  operation.  In 
England  grindstones  are  frequently  made  by  this  process. 

165.  SOREL  Stone.  Some  years  ago,  M.  Sorel,  a  French  chemist, 
discovered  that  the  oxychloride  of  magnesium  possessed  hydraulic 
energy  in  a  remarkable  degree.  This  cement  is  the  basis  of  the 
Sorel  stone.  It  is  formed  by  adding  a  solution  of  chloride  of  mag- 
nesium, of  the  proper  strength  and  in  the  proper  proportions,  to 
the  oxide  of  magnesium.  The  strength  of  this  stone,  as  well  as  its 
hardness,  exceeds  that  of  any  other  artificial  stone  yet  produced, 
and  may,  when  desirable,  be  made  equal  to  that  of  the  natural 
stone  which  furnishes  the  powder  or  sand  used  in  its  fabrication- 
The  process  is  patented,  and  is  used  mainly  in  making  emery-wheels. 
By  incorporating  large  pebbles  and  cobble-stones  in  the  mixture 
the  stone  can  be  made  quite  cheaply,  and  is  therefore  suitable  for 
ioundations  and  plain  massive  walls. 


CHAPTER  V. 
QUARRYING. 

166.  This  is  so  large  a  subject  that  it  cannot  be  more  than  en- 
tered upon  here  ;  for  greater  detail,  see  treatises  on  Quarrying,  Rock- 
blasting,  and  Tunneling. 

167.  Sources  of  Building  Stones.  The  bowlders,  Avhich  are 
scattered  promiscuously  over  the  surface  of  the  ground  and  also 
frequently  buried  in  it,  furnish  an  excellent  building  stone  for  massive 
structures  where  strength  is  essential.     They  are  usually  of  tough 

,  granite  or  of  a  slaty  structure,  and  are  difficult  to  work.  Sometimes 
they  have  a  cleavage  plane  or  rift,  along  which  they  may  be  split. 
They  can  be  broken  into  irregular  pieces  by  building  a  fire  about 
them,  and  drenching  them  while  hot  with  water,  or  they  may  be 
broken  by  explosives. 

Of  course  by  far  the  greater  quantity  of  stone  is  taken  directly 
from  quarries.  All  building-stone  deposits  have  usually  a  certain 
amount  of  covering,  consisting  either  of  a  portion  of  the  same  de- 
posit, which  has  been  disintegrated  by  atmospheric  influences,  or  of  a 
later  deposit.  This  covering  is  called  the  ''cap-rock"  or  "strip- 
ping." In  opening  the  quarry,  the  solid  portions  of  cap-rock  are 
broken  up  by  blasting,  and  the  whole  is  carted  out  of  the  way.  After 
a  sufficient  space  is  stripped,  the  next  step  necessary,  when  the  quarry 
rock  does  not  stand  out  in  cliffs,  is  to  excavate  a  narrow  space  on 
one  side  for  a  quarry  face,  either  by  blasting  or  by  some  of  the 
methods  to  be  described  presently. 

168.  Methods  of  Quarrying.  After  a  considerable  area  has 
tlius  been  laid  bare,  the  stone  is  quarried  in  one  of  three  ways. 

169.  J.  By  Hand  Tools.  When  the  stone  is  thin-bedded,  it  may 
be  quarried  by  hand-tools  alone.  The  principal  tools  are  pick,  crow- 
bar, drill,  hammer,  wedge,  and  plug  and  feathers.  The  layers  are 
forced  apart  by  the  crow-bar  or  wedges.  The  flat  pieces  are  broken 
up  with  the  hammer  or  by  drilling  holes  for  the  plug  and  feathers. 

116 


QUARRYIXG   BY    EXPLOSIVES.  117 

The  plug  is  a  Barrow  wedge  with  plane  faces;  the  feathers  are 
wedges  flat  on  one  side  and  ronnded  on  the  other  (Fig.  25,  page  128). 
When  a  plug  is  placed  between  two  feathers,  the  three  will  slip  into 
a  cylindrical  hole  ;  if  the  plug  is  then  driven,  it  exerts  a  great  force. 
If  these  plugs  and  feathers  are  placed  a  few  inches  apart  in  a  row, 
and  all  driven  at  the  same  time,  the  stone  will  be  cracked  along  the 
line  of  the  holes,  even  though  it  be  comparatively  thick. 

The  drill  used  to  cut  the  holes  for  the  plug  and  feathers  is  a  bar 
of  steel  furnished  with  a  wide  edge  sharpened  to  a  blunt  angle  and 
hardened.  It  is  operated  by  one  man,  who  holds  the  drill  witli  one 
hand  and  drives  it  with  a  hammer  in  the  other,  rotating  the  drill 
between  blows.  The  holes  are  usually  from  f  to  f  of  an  inch  in 
diameter. 

Sandstones  and  limestones  occurring  in  layers  thin  enough  to 
be  quarried  as  above  are  usually  of  inferior  qiiality,  suitable  only 
for  slope  walls,  paving,  riprap,  concrete,  etc. 

170.  JJ.'  By  Explosives.  Generally,  the  cheapest  method  of 
quarrying  small  blocks  is  by  the  use  of  explosives.  However,  ex- 
plosives are  used  mainly  for  detaching  large  blocks,  which  are  after- 
wards worked  up  by  means  of  wedges.  In  this  method  of  quarry- 
ing, drill-holes  are  put  down  to  the  depth  to  which  the  rock  is  to 
be  split,  and  the  requisite  amount  of  powder  or  other  explosive  put 
in,  covered  with  sand,  and  fired  by  a  fuse.  Sometimes  numerous 
charges  in  a  line  of  drill-holes  are  fired  simultaneously  by  means  of 
electricity. 

Quick-acting  explosives,  like  dynamite,  have  a  tendency  to  shatter 
the  stone  and  break  it  in  many  directions,  the  texture  being  affected 
by  the  sudden  explosion  in  the  same  manner  as  by  the  blow  of  a 
hammer.  Coarse  gunpowder  is  generally  preferred  for  quarrying 
stone.  Light  charges  of  powder  lightly  covered  with  sand  are  better 
than  heavy  charges  tightly  tamped  ;  *  and  experience  goes  to  show 
that  better  work  is  done  by  repeated  light  blasts  in  the  same  hole, 
than  by  a  single  heavy  blast.  By  means  of  light  charges  often  re- 
peated, a  mass  of  rock  may  be  detached  without  being  broken  up, 
which  would  be  badly  shattered  by  a  single  charge  strong  enough  to 
detach  it. 

In  each  locality  the  structure   of  the  rock   mi;st   be  carefully 

*  For  an  article  showing  that  an  air-space  should  be  left  between  th«»  exploaive 
and  the  tampins,  see  Eiujineering  Xews,  vol.  xviii.  p.  332. 


118  QUARKYIN-G.  [chap.  Y, 

studied  with  a  view  to  take  advantage  of  the  cleavage  planes  and 
natural  Joints.  For  quarrying  each  -class  of  rocks  there  is  a  charac- 
teristic method  employed,  which  is,  however,  varied  in  detail  in 
different  quarries.  The  minor  details  of  quarry  methods  are  as 
various  as  the  differences  existing  in  the  textures,  structures, 
and  modes  of  occurrence  of  the  rocks  quarried.  Much  depends 
upon  how  the  blast  is  made.  The  direction  in  which  the  rock  is 
most  liable  to  break  depends  upon  the  structure  of  the  rock  and 
the  shape  of  the  drill-hole.  Even  such  an  apparently  unimportant 
matter  as  the  form  of  the  bottom  of  the  drill-hole  into  which  the 
explosive  is  put  has  a  very  marked  effect.  If  bored  with  a  hand- 
drill,  the  hole  is  generally  triangular  at  the  bottom,  and  a  blast  in 
such  a  hole  will  break  the  rock  in  three  directions.  In  some  quar- 
ries the  lines  of  fracture  are  made  to  follow  predetermined  directions 
by  putting  the  charge  of  powder  into  canisters  of  special  forms.* 

171.  Drills.  The  holes  are  bored  by  jumpers,  churn-drills,  or 
machine-drills.  The  first  is  a  drill  similar  to  the  one  used  for  drill- 
ing holes  for  plugs  and  feathers  (§  169),  except  that  it  is  larger  and 
longer.  It  is  usually  held  by  one  man,  who  rotates  it  between  the 
alternating  blows  from  hammers  in  the  hands  of  two  other  men. 
Churn-drills  are  long,  heavy  drills,  usually  6  to  8  feet  in  length. 
They  are  raised  by  the  workmen,  let  fall,  caught  on  the  rebound, 
raised  and  rotated  a  little,  and  then  dropped  again,  thus  cutting 
a  hole  without  being  driven  by  the  hammer.  They  are  more  eco- 
nomical than  jumpers,  especially  for  deep  holes,  as  they  cut  faster 
and  make  larger  holes  than  hand-drills. 

172.  Machine  rock-drills  bore  much  more  rapidly  than  hand- 
drills,  and  also  more  economically,  provided  the  work  is  of  sufficient 
magnitude  to  justify  the  preliminary  outlay.  They  drill  in  any 
direction,  and  can  often  be  used  in  boring  holes  so  located  that  they 
could  not  be  bored  by  hand.  They  are  worked  either  by  steam 
directly,  or  by  air  compressed  by  steam  or  water-power  and  stored 
in  a  tank  called  a  receiver  and  thence  led  to  the  drills  through  iron 
pipes. 

A  variety  of  rock-drilling  machines  has  been  invented,!  but 
they  can  be  grouped  in  two  classes,  viz.,  percussion-drills  and  rotat- 
ing drills.     The  method  of  action  of  the  percussion-drill  is  the  same 

*  See  Report  on  Quarry  Industry  in  Vol.  X.  of  the  10th  Census,  pp.  33,  34. 
+  For  a  full  account  of  the  more  important  ones,  see  Drinker's  "  Tunneling.'' 


QUAKRYIXG    BY    EXPLOSIVES.  119 

as  that  of  the  churn-drill  already  described.  The  usual  form  is 
that  of  a  cylinder,  in  which  a  piston  is  moved  by  steam  or  com- 
pressed air,  and  the  drill  is  attached  to  this  piston  so  as  to  make  a 
stroke  with  every  complete  movement  of  the  piston.  An  automatic 
device  causes  it  to  rotate  slightly  at  each  stroke. 

173.  In  the  rotating  drills,  the  drill-rod  is  a  long  tube,  revolving 
about  its  axis.  The  end  of  the  tube — hardened  so  as  to  form  an 
annular  cutting  edge — is  kept  in  contact  with  the  rock,  and  by  its 
rotation  cuts  in  it  a  cylindrical  hole,  generally  with  a  solid  core  in 
the  center.  The  drill-rod  is  fed  forward,  or  into  the  hole,  as  the 
drilling  proceeds.  The  debris  is  removed  from  the  hole  by  a  con- 
stant stream  of  water  which  is  forced  to  the  bottom  of  the  hole 
through  the  hollow  drill-rod,  and  which  carries  the  debris  up 
througli  the  narrow  space  between  the  outside  of  the  drill-rod  and 
the  sides  of  the  hole. 

The  diamond  drill  is  the  only  form  of  rotary  rock-drill  exten- 
sively used  in  this  country.  The  tube  has  a  head  at  its  lower  end, 
in  which  are  set  a  number  of  carbons  or  black  diamonds.  The 
diamonds  usually  project  slightly  beyond  the  circumference  of  the 
head,  which  is  perforated  to  permit  the  ingress  and  egress  of  the 
water  used  in  removing  the  debris  from  the  hole  and  at  the  same 
time  prevent  the  head  from  binding  in  the  hole.  When  it  is  desir- 
able to  know  the  precise  nature  and  stratification  of  the  rock  pene- 
trated, the  cutting  points  are  so  arranged  as  to  cut  an  annular  groove 
in  the  rock,  leaving  a  solid  core,  which  is  broken  off  and  lifted  out 
whenever  the  head  is  brought  up.  Where  it  is  not  desired  to  pre- 
serve the  core  intact,  a  solid  boring-bit  is  used  instead  of  the  core- 
bit.     They  are  made  of  any  size  up  to  15  inches  in  diameter. 

174.  Bxplosives.*  The  principal  explosives  are  gunpowder, 
nitro-glycerine,  and  dynamite.  Only  a  coarse-grained  and  cheap 
variety  of  the  first  is  used  in  quarrying,  the  others  being  too  sudden 
and  too  strong  in  their  action. 

The  pressure  exerted  by  gunpowder  when  fired  in  a  confined 
space  depends  upon  the  relative  weight  and  quality  of  powder  used, 
and  upon  the  space  occupied  by  the  gases  evolved.  The  absolute 
force  of  gunpowder,  the  force  which  it  exerts  when  it  exactly  fills 
the  space  in  which  it  is  confined,  has  never  been  satisfactorily  ascer- 

*  For  a  full  account  of  all  the  various  explosives,  see  Drinker's  "  Tunneliug,*' 
and  Drinker's  "  Modern  Explosives." 


120  QUARRYING.  [CHAP.  V. 

tuined.  It  has  been  variously  estimated  at  from  15,000  to  1,500,000 
pounds  per  square  inch.  Experiments  by  Gen.  Rodman  sliow  that 
for  the  powder  used  in  gunnery  the  absolute  force  of  explosion  is 
at  least  200,000  pounds  per  square  inch.  "  In  ordinary  quarrying, 
a  cubic  yard  of  solid  rock  in  place  (or  about  1.9  cubic  yards  piled 
up  after  being  quarried)  requires  from  ^  to  f  pound  of  powder. 
In  very  refractory  rock,  lying  badly  for  quarrying,  a  solid  yard  may 
require  from  1  to  2  pounds.  In  some  of  the  most  successful  great 
blasts  for  the  Holyhead  Breakwater,  Wales,  (where  several  thou- 
sands of  pounds  of  powder  were  exploded,  usually  by  galvanism,  at 
a  single  shot,)  from  2  to  4  cubic  yards  (solid)  were  loose ited  per 
pound  of  powder  ;  but  in  many  instances  not  more  than  1  to  1^ 
yards.  Tunnels  and  shafts  require  2  to  6  pounds  per  solid  yard, 
usually  3  to  5  pounds.  Soft,  partially  decomposed  rock  frequently 
requires  more  than  harder  ones."  * 

The  explosion  of  the  powder  splits  and  loosens  a  mass  of  rock 
whose  volume  is  approximately  proportional  to  the  cube  of  the  line 
of  least  resistajice, — that  is,  of  the  shortest  distance  from  the  charge 
to  the  surface  of  the  rock, — and  may  be  roughly  estimated  at  iivice 
that  cube  ;  but  this  proportion  varies. much  in  different  cases.  The 
oiwiinary  rule  for  the  weight  of  powder  in'  small  blasts  is 

Powder,  in poitnds,  =  (Line  of  Resistance,  infect,)'  -^  32. 

Powder  is  sold  in  kegs  of  25  lbs.,  costing  about  §2.00  to  82.25 
per  keg,  exclusive  of  freight, — which  is  very  high,  owing  to  the  risk. 

175.  Most  of  the  explosives  which  of  late  years  have  been  tak- 
ing the  place  of  gunpowder  consist  of  a  powdered  substance,  partly 
saturated  Avith  uitro-glycerine — a  fluid  produced  by  mixing  glycerine 
with  nitric  and  sulphuric  acids.  Xitro-glycerine,  and  the  powders 
containing  it,  are  always  exploded  by  means  of  sharp  j^ercussion, 
which  is  applied  by  means  of  a  cap  and  fuse.  The  cap  is  a  hollow 
copper  cylinder,  about  ^  inch  in  diameter  and  an  incli  or  two  in 
length,  containing  a  cement  composed  of  fulminate  of  mercury  and 
some  inert  substance.  The  cap  is  called  single-force,  double-force, 
etc.,  according  to  the  amount  of  explosive  it  contains. 

The  principal  advantages  of  nitro-glycerine  as  an  explosive  con- 
sist (1)  in  its  instantaneous  development  of  force,  due  to  the  fact 
that,  pound  for  pound,  it  produces  at  least  three  and  a  half  times 
*  Traut'ft'iiie's  Engineer's  Pocket-book. 


QUARRYING   BY   EXPLOSIVES.  121 

as  much  gas,  and  twice  as  much  heat,  as  gunpowder  ;  and  (2)  in  its 
high  specific  gravity,^which  permits  the  use  of  small  drill-holes. 

Nitro-glycerine  is  rarely  used  in  the  liquid  state  in  ordinary 
quarrying  or  blasting,  owing  to  the  liability  of  explosion  through 
accidental  percussion,  and  owing  to  its  liability  to  leakage.  It  ex- 
plodes so  suddenly  that  very  little  tamping  is  required,  the  mere 
weight  of  moist  sand,  earth,  or  water  being  sufficient.  This  fact, 
and  the  additional  one  that  nitro-glycerine  is  unaffected  by  immer- 
sion in  water  and  is  heavier  than  water,  render  it  particularly  suit- 
able for  sub-aqueous  work,  or  for  holes  containing  water.  If  the 
rock  is  seamy,  the  nitro-glycerine  must  be  confined  in  water-tight 
casings.  Such  casings,  however,  necessarily  leave  some  spaces  be- 
tween the  rock  and  the  explosive,  which  diminishes  the  effect  of  the 
latter.  The  liquid  condition  of  nitro-glycerine  is  useful  in  causing 
it  to  fill  the  drill-hole  completely,  so  that  there  are  no  empty  spaces 
in  it  to  waste  the  foi'ce  of  the  explosion.  On  the  other  hand,  the 
liquid  form  is  a  disadvantage,  because  when  thus  used  in  seamy 
rock  without  a  containing  vessel  portions  of  the  nitro-glycerine  leak 
away  and  remain  unexploded  and  unsuspected,  and  may  cause  acci- 
dental explosion  at  a  future  time. 

The  price  of  nitro-glycerine  is  from  50  to  60  cents  per  quart. 

176.  Dynamite  is  the  name  given  to  any  explosive  which  con- 
tains nitro-glycerine  mixed  with  a  granular  absorbent.  If  the 
absorbent  is  inert,  the  mixture  is  called  true  dynamite;  if  the 
absorbent  itself  contains  explosive  substances,  the  mixture  is  called 
false  dynamite.  The  absorbent,  by  its  gi-anular  and  compressible 
condition,  acts  as  a  cushion  to  the  nitro-glycerine,  and  protects  it 
from  percussion  and  from  the  consequent  danger  of  explosion,  but 
does  not  diminish  its  power  when  exploded.  Nitro-glycerine 
undergoes  no  change  in  composition  by  being  absorbed  ;  and  it 
then  freezes,  burns,  explodes,  etc.,  under  the  same  conditions  as 
to  pressure,  temperature,  etc.,  as  when  in  the  liquid  form.  The 
cushioning  effect  of  the  absorbent  merely  renders  it  more  difficult 
to  bring  about  sufficient  percussive  pressure  to  cause  explosion. 
The  absorption  of  the  nitro-glycerine  in  dynamite  renders  the  lat- 
ter available  in  horizontal  holes  or  in  holes  drilled  upward.  True 
dynamite  loses  only  a  very  small  percentage  of  its  explosive  power 
when  saturated  with  water,  but  is  then  much  more  difficult  to  ex- 
plode. 


123  QUAREYIXG.  [CHAP.  V. 

True  dynamites  must  contain  at  least  50  per  cent,  of  nitro- 
glycerine, otherwise  the  latter  will  be  too^ompletely  cushioned 
by  the  absorbent,  and  the  powder  will  be  too  difficult  to  explode. 
False  dynamites,  on  the  contrary,  may  contain  as  small  a  percentage 
of  nitro-glycerine  as  may  be  desired,  some  containing  as  little  as  15 
per  cent.  The  added  explosive  substances  in  the  false  dynamites 
generally  contain  large  quantities  of  oxygen,  which  are  liberated 
upon  explosion,  and  aid  in  effecting  the  complete  combustion  of 
any  noxious  gases  arising  from  the  nitro-glycerine.  The  false  are 
generally  inferior  to  the  true  dynamites,  since  the  bulk  of  the 
former  is  increased  in  a  higher  ratio  than  the  power;  and  as  the 
cost  of  the  work  is  largely  dependent  upon  the  size  of  the  drill- 
holes, there  is  no  economic  gain. 

Dynamites  which  contain  large  percentages  of  nitro-glycerine 
explode  with  great  suddenness,  tending  to  break  the  rock  into 
small  fragments.  They  are  most  useful  in  blasting  very  hard  rock. 
In  such  rock  dynamite  containing  75  per  cent,  of  nitro-glycerine 
is  roughly  estimated  to  have  about  6  times  the  force  of  an  equal 
weight  of  gunpowder  ;  but  in  soft  rock  or  clay  its  power,  at  equal 
cost,  is  inferior  to  that  of  common  gunpowder,  because  its  action 
is  akin  to  a  sudden  blow,  rather  than  to  a  continued  push.  For 
soft  or  decomposed  rocks,  sand,  and  earth,  the  lower  grades  <f 
dynamite,  or  those  containing  a  smaller  percentage  of  nitro-glycei- 
ine,  are  more  suitable.  They  explode  with  less  suddenness,  and 
their  tendency  is  rather  to  upheave  large  masses  of  rock  than  to 
splinter  small  masses. 

*' Judgment  must  be  exercised  as  to  the  grade  and  quantity  of 
explosive  to  be  used  in  any  given  case.  Where  it  is  not  objection- 
able to  break  the  rock  into  small  pieces,  or  where  it  is  desired  to  do 
so  for  convenience  of  removal,  the  higher  shattering  grades  are  use- 
ful. Where  it  is  desired  to  get  the  rock  out  in  large  masses,  as  in 
quarrying,  the  lower  grades  are  preferable.  For  very  difficult  work 
in  hard  rock,  and  for  submarine  blasting,  the  highest  grades,  con- 
taining 70  to  75  per  cent,  of  nitro-glycerine,  are  used.  A  small 
charge  does  the  same  execution  as  a  larger  charge  of  lower  grade, 
and  of  course  does  not  require  the  drilling  of  so  large  a  hole.  In 
submarine  work  their  sharp  explosion  is  not  deadened  by  the 
water.  For  general  railroad  work,  ordinary  tunneling,  mining  of 
ores,  etc.,  the  average  grade,  containing  40  per  cent,  of  nitro-glycer- 


NITRO-GLTCERINE    EXPLOSIVES.  12S 

ine,  is  used  ;  for  quarrying,  35  per  cent. ;  for  blasting  stumps,  trees, 
piles,  etc.,  30  per  cent.;  for  sand  and  earth,  15  per  cent." 

177.  A  great  variety  of  dynamites  is  made.  Each  manufacturer 
usually  makes  a  number  of  grades,  containing  different  percentages 
of  nitro-glycerine,  and  gives  to  his  powder  some  fanciful  name. 
Dynamite  is  sold  in  cylindrical,  paper-co'^ered  cartridges,  from  ^  of 
an  inch  to  2  inches  in  diameter^  and  6  to  8  inches  long,  or  longer, 
which  are  packed  in  boxes  containing  25  or  50  pounds  each.  They 
are  furnished,  to  order,  of  any  required  size.  The  price  per  pound 
ranges  from  15  cents  for  15  per  cent.  nitro-gl3'cerine  to  50  cents  for 
75  per  cent,  nitro-glycerine. 

Table  14  (page  124)  gives  the  names  of  all  the  explosives  con- 
taining nitro-glycerine,  with  the  per  cent,  in  each  case.* 

178.  Ill'  By  Channeling  and  Wedging.  By  channeling  is  meant 
the  process  of  cutting  long  narrow  channels  in  rock  to  free  the  sides 
of  large  blocks  of  stone.  Quite  a  large  number  of  machines  have 
been  invented  for  doing  this  work,  all  of  which  make  the  channels 
by  one  form  or  the  other  of  the  machine  drills  already  described 
(see  the  second  paragraph  of  §  172).  The  machines  are  mounted 
upon  a  track  on  the  bed  of  the  quarry,  and  can  be  moved  forward 
as  the  work  progresses.  If  the  rock  is  in  layers,  it  is  only  necessary 
to  cut  the  channels  part  way  through  the  layer,  when  the  block  can 
be  detached  with  wedges,  the  groove  guiding  the  fracture.  If  the 
rock  is  not  in  layers,  after  the  necessary  channels  have  been  cut 
around  the  block,  it  is  necessary  to  under-cut  the  block  in  order  ta 
release  it.  This  is  accomplished  by  drilling  a  series  of  holes  along 
the  bottom,  which  process  is  called  "gadding"  by  quarry-men.  The 
block  is  then  split  from  its  bed  by  means  of  Avedges.  The  method 
of  channeling  and  wedging  is  much  employed  in  quarrying  marble, 
the  massive  limestones,  and  the  thick-bedded  sandstones.  The 
method  is  very  economical  and  expeditious,  except  in  granite  and 
the  hardest  sandstones.  For  illustrations  of  the  two  principal  clian- 
neling  machines  and  also  quarries  being  worked  by  this  method,  see 
Eeport  on  the  Quarry  Industry,  pp.  44-52,  in  Vol.  X.  ci  the  Tenth 
Census  of  the  United  States. 

*W.  C.  Foster,  in  Etigineering  Xewa^  vol.  xix.  p.  25-t.  For  a  list  of  all  the  explo- 
sives employed  as  blasting  agents,  together  with  a  description  of  their  composition 
and  references  to  the  literature  of  each,  see  Engineering  Xews,  vol.  xix.  pp.  533-34, 
and  vol.  xx.  pp.  8-10. 


124 


QUARRYING. 


[chap.  Y. 


TABLE  14. 
List  of  lixPLosivEs  containing  Nitro-glycerine. 


Name  of  Explosive. 


Per  cent, 
of 
Nitro- 
glycerine. 


^tna  powder,  No.  1 

"    3XX... 

"     3 

''    3X.... 
"    4X... 

"     5 

Auiinouia  powder 

Asbestos  powder 

Atlas  powder,  A 

"       B-f 

"       B 

"       C+ , 

"       c 

'•       D+ 

"       D 

"      E+ 

"       E 

"       F+ 

Brady's  dynamite 

Brain's  powder 

Colon iii  powder 

Dualiu  (Dittmar's) 

Dynamite  (Nobel's,  Kiesel- 
gulir  dynamite), 

Old  No.  1 
Old  No.  2 
Old  No.  3 

Electric  powder 

Explosive  gelatine 

Forcite,  2  grades 

Fulgurite  (solid) 

"         (liquid) 

Gelatine  dynamite,  A. . . 

No.  1 

"<  <<  <(    2 

Gelatine   explosive  de 

guerre. 

Gelignite 

Giant  powder,  No.  1 

"  New  '•     1 

"  "    2  extra 

"    2 

"     2c 

"    XXX.. 

"    M 

Giant  powder  (Nobel's), 
No.  2 


65 
50 
40 
35 
25 
15 
16  to 


20 


varies 
75 
60 
50 
45 
40 
35 
30 
25 
20 
15 
33 
40 
40 
50 


75 

40 

25 

33 

93 

70 

60 

90 

97.5 

58 

38.8 


70, 


89.3 

56.5 

75 

50 

45 

40 

33 

27 

20 

20 


Name  op  Explosive. 


Glyxoline 

Hecla  powder,  No.  IXX 

Gun  Sawdust 

"       No.  IX.... 

"     1 

"    2X.... 

"     2 

"     3X.... 

"3 

Hercules  powder,  No.  IXX 
"  1.... 
"  2SSS 
"  2SS.. 
"   2S... 


3S... 
3.... 

4S... 
4.... 
(some 


Horsley's    powder 

varieties) 

Judson  Giant  Powder, No. 2 
Judsou  powder,  FFF 

FF 

F 

RRP 

Lithof  racteur 

Metalline  Nitroleum 

Mica  powder,  No.  1 


Per  cent. 

of 

Nitro- 

gljcerine. 


3. 


Miners'  Powder  Co.'s  Dy- 
namite  

Neptune  powder 

Nitro  Tolnol 

Norrbin  &  Ohlsson's  pow- 
der  

Pontopolite 

Porifera  Nitroleum 

Rendrock 

Sebastin,  No.  1 

"     2 

Selenitic  powder 

Seranim 

Vigorite  (U.  S.) .. 

Vitrite,  No.  1 

"     3 

Vulcan  powder 


75 
16  to  20 
50 
40 
35 
30 
35 
30 
75 
65 
55 
50 
45 
40 
35 
30 
35 
20 

20 

40 

20 

15 

10 
5  to  6 

53 
varies 

40 

52 

33 
32.7 

70 

25  to  50 

varies 
33.4 

78 

68 

varies 

43.8 


32.6 


CHAPTER  VI. 


BTONE    CUTTING. 


Art.  1.  Tools. 

179.  In  order  to  describe  intelligibly  the  various  methods  of 
preparing  stones  for  use  in  masonry,  it  will  be  necessary  to  begin 
with  a  description  of  the  tools  used  in  stone-cutting,  as  the  names 
of  many  kinds  of  dressed  stones  are  directly  derived  from  those  of 
the  tools  used  in  dressing  them. 

"With  a  view  to  securing  uniformity  in  the  nomenclature  of 
building  stones  and  of  stone  masonry,  a  committee  of  the  American 
Society  of  Civil  Engineers  prepared  a  classification  and  recommended 
that  all  specifications  should  be  made  in  accordance  therewith.  The 
old  nomenclature  Avas  very  unsystematic  and  objectionable  on  many 
grounds.  The  new  system  is  good  in  itself,  is  recommended  by  the 
most  eminent  authority,  has  been  quite  generally  adopted  by  en- 
gineers, and  should  therefore  be  strictly  adhered  to.  The  following 
description  of  the  liand  tools  used  in  stone  cutting  is  from  the 
report  of  the  American  Society's  committee.^" 

180.  Hand  Tools.  "The  Double  Face  Hammer,  Fig.  9,  is  a 
heavy  tool  weighing  from  20 
to  30  pounds,  used  for  rough- 
ly shaping  stones  as  they 
come  from  the  quarry  and 
for  knocking  off  projections. 
This  is  used  only  for  the 
rougliest  work. 

"  The  Face  Hammer,  Fig.  10,  has  one  blunt  and  one  cutting 
end,  and  is  used  for  the  same 
purpose  as  the   double   face 
hammer  where  less  weight  is  C 
required.     The   cutting   end 
is  used  for  roughly  squaring 

stones,  preparatory  to  the  use  Fig.  lo.— Face  hammer. 

of  finer  tools. 


D 


Fig.  9.— Double  Face  Hammer. 


*  Trans.  Am.  Soc.  of  C.  E.,  voL  vi.  pp.  297-304. 


125 


126 


STONE   CUTTING. 


[chap.    V"I. 


"The  Cavil,  Fig.  11,  has  one  blunt  and  one  pyramidal,  or 
pointed,  end,  and  weighs  from  15  to  20  pounds. 
It  is  used  in  quarries  for  roughly  shaping  stone 
for  transportation. 

The  Pick,  Fig.  12,  somewhat  resembles  the 
Fig.  11.— Cavil,  pick  used  in  digging,  and  is  used  for  rough  dress- 
ing, mostly  on  limestone  and  sandstone.  Its  length  varies  from 
15  to  24  inches,  the  thickness 
at  the  eye  being  about  2 
inches. 

"  The  Ax,  or  Peaoi  Ham- 
mer, Fig.  13,  has  two  opposite  [Up 
cutting  edges.  It  is  used  for 
making  drafts  around  the  arris, 
or  edge,  of  stones,  and  in  re- 
ducing faces,  and  sometimes  ^'^-  ^s.-pick. 
joints,  to  a  level.     Its  length  is  about  10  inches,  and  the  cutting 


D 


edge  about  4  inches.  It  is  used  after 
the  point  and  before  the  patent  ham- 
mer. 

''The  Tooth  Ax,  Fig.  14,  is  like 
Fig.  13.— Ax.  the  ax,  except  that  its  cutting  edges 

are  divided  into  teeth,  the  number  of  which  varies  with  the  kind 
of  work  required.     This  tool 
is   not   used   in   granite    and 
gneiss  cutting. 

"  The  Bush  Ha  m  in  e  r , 
Fig.  15,  is  a  square  prism  of 
steel  whose  ends  are  cut  into 
a  number  of  pyramidal  points. 
The  length  of  the  hammer  is  from  4  to  8  inches,  and  the  cutting 

face  from  2  to  4  inches  square. 


J 


Fig.  14.— Tooth  Ax. 


The  points  vary  in  number  and 
in  size  with  the  work  to  be  done. 
One  end  is  sometimes  made 
with  a  cutting  edge  like  that  of 
Fig.  15.— Bush  Hammer.  ^]^g  j^x. 

The  Crandall,  Fig.  16,  is  a  malleable-iron  bar  about  two  feet 


ART.  1.] 


TOOLS. 


127 


In  this  end  is  a  slot  3  inches 


long,  slightly  flattened  at  one  end. 

long  and  f  inch  wide.  Through  this 

slot  are  passed  ten  double-headed 

points    of   5-inch   square   steel,    9 

inches   long,    which   are    held    in 

place  by  a  key. 

''The   Patent    Hammer,   Fig. 

17,  is    a     double-headed    tool    so  Fig.  ig.-Crandali,. 

formed  as  to  hold  at  each  end  a  set  of  wide  thin  chisels.     The  tool 

is  in  two  parts,  which  are  held  to- 
gether by  the  bolts  .which  hold  the 
chisels.  Lateral  motion  is  prevented 
by  four  guards  on  one  of  the  pieces. 

Fig.  17.— Patent  Hammer.  The      tool      without      the     teeth      is 

5-|-  X  2f  X  li  inches.     The  teeth  are  2f  inches  wide.     Their  thickness 

varies  from  -jig  to  ^  of  an    inch.      This  tool  is 

used  for  giving  a  finish  to  the  surface  of  stones. 
"  The  Hand  Hammer,  Fig.    18,   weighing 

from  2  to  5  pounds,  is  used    in  drilling  holes,   fig.  i8.-Hand  hammer. 

and  in  pointing  and  chiseling  the  harder  rocks. 

''The  Mallet,  Fig.  19,  is  used  where  the  softer  limestones  and 

sandstones  are  to  be  cut. 

"The  Pitching  Chisel,  Fig.  20, 
is  usually  of  l|-inch  octagonal  steel, 
spread  on  the  cutting  edge  to  a 
rectangle  of  |  X  2^  inches.  It  is 
used  to  make  a  well-defined  edge  to 

the  face  of  a  stone,  a  line  being  marked  on  the  joint  surface  to 

which  the  chisel  is  applied  and  the  portion  of  the  stone  outside  of 

the  line  broken  off  by  a  blow  with  the  hand-hammer  on  the  head 

of  the  chisel. 

"The  Point,  Fig.  21,  is  made  of  round  or  octagonal  rods  of 

steel,  from  \  inch  to  1  inch  in  diameter.     It  is  made  about  12 

inches  long,  with  one  end  brought  to  a  point. 

It  is  used  until  its  length  is  reduced  to  about  CC 

5  inches.     It  is  employed  for  dressing  off  the  (T 

irregular  surface  of  stones,  either  for  a  perma- 
nent finish  or  preparatory  to  the  use  of  the  ax.  ^^°-  21.— Point. 

According  to  the  hardness  of  the  stone,  either  the  hand-hammer 

or  the  mallet  is  used  with  it. 


Fig.  19.— Mallkt. 


:> 


128 


STOJfE   CUTTING. 


[chap.  VI. 


"The  Chisel,  Fig.  22,  of  round  steel  of  ^  to  |  inch  iu  diameter 

and  about  10  inches  long,  with  one  end  brought 

I  to  a  cutting  edge  from  ^  inch  to  2    iuclies 

wide,  is  used  for  cutting  drafts  or  margins  on 

the  face  of  stones. 

'^The  Tooth  Chisel,  Fig.  23,  is  the  same 


d 


d 


Fig.  22. — Chisel. 

as  the  chisel,  except  that  the  cutting  edge  is  divided  into  teeth. 
It  is  used  only  on  mar 
bles  and  sandstones. 

''The  Splitting   c^ 
Chisel,  Fig.  24,  is  used 
chiefly  on   the   softer, 
stratified  stones,  and  sometimes  on  fine  architectural  carvings  in 
granite. 

''The  Plug,  a  truncated  wedge  of  steel,  and  the  Feathers  of 
half-round  malleable  iron,  Fig.  25,  are  used  for  splitting  uustrati- 
fied  stone.     A  row  of  holes  is  made  with  the  Drill,  Fig.  26,  on  the 


Fig.  23. 
Tooth  Chisel. 


Fig.  24. 
Splitting  Chisel. 


Fitt.  -25. 
Plug  and  Ieathees. 


Fig.  26.— Drills. 


line  on  which  the  fracture  is  to  be  made  ;  in  each  of  these  holes 
two  feathers  are  inserted,  and  the  plugs  lightly  driven  in  between 
them.  The  plugs  are  then  gradually  driven  home  by  light  blows 
of  the  hand  hammer  on  each  in  succession  until  the  stone  splits." 

181.  Machine  Tools.  In  all  large  stone-yards  machines  are 
used  to  prepare  the  stone.  There  is  great  variety  in  their  form, 
but  since  the  surface  never  takes  its  name  from  the  tool  which 
forms  it,  it  will  be  neither  necessary  nor  profitable  to  attempt  a  de- 
scription of  individual  machines.  They  include  stone-saws,  stone- 
cutters, stone-planers,  stone-grinders,  and  stone-polishers. 

The  saws  may  be  either  drag,  circular,  or  band  saws  ;  the  cut- 
ting may  be  done  by  sand  and  water  fed  into  the  kerf,  or  by  carbons 
or  black  diamonds.  Several  saws  are  often  mounted  side  by  side  and 
operated  by  the  same  power. 

The  term  "  stone-cutter"  is  usually  applied  to  the  machine  which 


AET.  2. J  METHOD   OF   EORMIXG    SURFACES.  129 

attacks  the  rough  stone  and  reduces  the  inequalities  somewhat. 
After  this  treatment  the  stone  goes  in  succession  to  the  stone- 
planer,  stone-grinder,  and  stone-polisher. 

Those  stones  which  are  homogeneous,  strong  and  tough,  and 
comparatively  free  from  grit  or  hard  spots,  can  be  worked  by  ma- 
chines which  resemble  those  used  for  iron  ;  but  the  harder,  more 
brittle  stones  require  a  mode  of  attack  more  nearly  resembling  that 
employed  in  dressing  stone  by  hand.  Stone-cutters  and  stone- 
planers  employing  both  forms  of  attack  are  made. 

Stone-grinders  and  stone-polishers  differ  only  in  the  degree  of 
fineness  of  the  surface  produced.  They  are  sometimes  called  "rub- 
bing-machines." Essentially  they  consist  of  a  large  iron  plate  re- 
volving in  a  horizontal  plane,  the  stone  being  laid  upon  it  and  braced 
to  prevent  its  sliding.  The  abradent  is  sand,  which  is  abundantly 
supplied  to  the  surface  of  the  revolving  disk.  A  small  stream  of 
water  works  the  sand  under  the  stone  and  also  carries  away  the 
debris. 

Art.  2.  Method  of  forming  the  Surfaces. 

182.  It  is  important  that  the  engineer  should  understand  the 
methods  employed  by  the  stone-cutter  in  bringing  stones  to  any  re- 
quired form.  The  surfaces  most  frequently  required  in  stone  cutting 
are  plane,  cylindrical,  warped,  helicoidal,  conical,  spherical,  and 
sometimes  irregular  surfaces. 

183.  Plane  Surfaces.  In  squaring  up  a  rough  stone,  the  first 
thing  the  stone-cutter  does  is  to  draw  a  line,  with  iron  ore  or  black 
lead,  on  the  edges  of  the  stone,  to  indicate  as  nearly  as  possible  the 
required  plane  surface.  Then  with  the  hammer  and  the  pitching- 
tool  he  pitches  off  all  debris  or  waste  material  above  the  lines, 
thereby  reducing  the  surface  approximately  to  a  plane.  With  a 
chisel  he  then  cuts  a  draft  around  the 
edges  of  this  surface,  i.  c,  he  forms  nar- 
row plane  surfaces  along  the  edges  of  the 
stone.  To  tell  when  the  drafts  are  in 
the  same  plane,  he  uses  two  straight- 
edges having  parallel  sides  and  equal 
widths.     See  Fig.  27.     The  projections  fig.27. 

on  the  surface  are  then  removed  by  the  pitching  chisel  or  the  point, 
until  the  straight-edge  will  just  touch  the  drafts  and  the  inter- 
mediate surface  when  applied  across  the  stone  in  any  direction. 


130 


STONE   CUTTING. 


[chap.  VI.. 


The  surface  is  usually  left  a  little  "  slack,"  i.  e.,  concave,  \<^  allow 
room  for  the  mortar  ;  however,  the  surface  should  be  but  a  very 
little  concave. 

The  surface  is  then  finished  with  the  ax,  patent  hammer,  bu»a 
hammer,  etc.,  according  to  the  degree  of  smoothness  required. 

184.  To  form  a  second  plane  surface  at  right  angles  to  the  first 
one,  the  workman  draws  a  line  on  the  cut  face  to  form  the  inter» 
section  of  the  two  planes  ;  he  also  draws  a  line  on  the  ends  of  the 
stone  approximately  in  the  required  plane.  With  the  ax  or  the 
chisel  he  then  cuts  a  draft  at  each  end  of  the  stone  until  a  steel 
square  fits  the  angle.  He  then  joins  these  drafts  by  two  others  at 
right  angles  to  them,  and  brings  the  whole  surface  to  the  same 
plane.     The  other  faces  may  be  formed  in  the  same  way. 

If  the  surfaces  are  not  at  right  angles  to  each  other,  a  bevel  is 
used  instead  of  a  sqnare,  the  same  general  method  being  pursued. 

185.  Cylindrical  Surfaces.  These  may  be  either  concave  or 
convex.  The  former  are  frequently  required,  as  in  arches;  and  the 
latter  sometimes,  as  in  the  outer  end  of  the  face-stones  of  an  arch. 
The  stone  is  first  reduced  to  a  paralellopipedon,  after  which  the 
curved  surface  is  produced  in  either  of  two  ways  :  (1)  by  cutting 
a  circular  draft  on  the  two  ends  and  applying  a  straight-edge  along 
the  rectilinear  elements  (Fig.  28);  or  (2)  by  cutting  a  draft  along 
the  line  of  intersection  of  the  plane  and  cylindrical  surface,  and. 
applying  a  curved  templet  to  the  required  surface  (Fig.  29). 


Fig. 


Fig.  29. 


186.  Conical  Surfaces  may  be  formed  by  a  process  very  similai 
to  the  first  one  given  above  for  cylindrical  surfaces.  Such  surfaces 
are  seldom  used. 

187.  Spherical  Surfaces  are  sometimes  employed,  as  in  domes. 
They  are  formed  by  essentially  the  same  general  method  as  cylin- 
drical surfaces. 

188.  Warped  Surfaces.     Under  this  head  are  included  what 


-ART.  3.]  METHODS   OF    FINISHING   SURFACES.  131 

the  stone-cutters  call  "  twisted  surfaces,  "  helicoidal  surfaces,  and 
the  general  warped    surface.      None    of 
these  are  common  in  ordinary  stone-work. 

The  method  of  forming  a  surface 
equally  twisted  right  and  left  will  be  de- 
scribed ;  by  obvious  modifications  the  same 
method  can  be  applied  to  secure  other 
forms.  Two  twist  rules  are  required,  the 
angle  between  the  upper  and  lower  edges  ^^*^-  ^*'- 

being  half  of  the  required  twist.  Drafts  are  then  cut  in  the  ends  of 
the  stone  until  the  tops  of  the  twist  rules,  when  applied  as  in  Fig. 
30,  are  in  a  plane.  The  remainder  of  the  projecting  face  is  removed 
until  a  straight-edge,  when  applied  parallel  to  the  edge  of  the  stone, 
will  just  touch  the  end  drafts  and  the  intermediate  surface. 

If  the  surface  is  to  be  twisted  at  only  one  end,  a  parallel  rule 
and  a  twist  rule  are  used. 

189.  Making  the  Drawings.  The  method  of  making  work- 
ing drawings  for  constructions  in  stone  will  appear  in  subsequent 
chapters,  in  connection  with  the  study  of  the  structures  them- 
selves; but  for  detailed  instructions,  see  the  text-books  on  Stere- 
otomy  or  Stone  Cutting. 

Art.  3.  Methods  of  Finishing  the  Surfaces.* 

190.  "All  stones  used  in  building  are  divided  into  three  classes, 
according  to  the  finish  of  the  surface;  viz.  : 

I.  Rough  stones  that  are  used  as  they  come  from  the  quarry. 

II.  Stones  roughly  squared  and  dressed. 

III.  Stones  accurately  squared  and  finely  dressed. 

"  In  practice,  the  line  of  separation  between  them  is  not  very 
distinctly  marked,  but  one  class  gradually  merges  into  the  next. 

191.  I.  "  Unsquared  Stones.  This  class  covers  all  stones 
which  are  used  as  they  come  from  the  quarry,  without  other 
preparation  than  the  removal  of  very  acute  angles  and  excessive  pro- 
jections from  the  general  figure.  The  term  ^  backing,^  which  is 
frequently  applied  to  this  class  of  stone,  is  inappropriate,  as  it  prop- 
erly designates  material  used  in  a  certain  relative  position  in  a  wall, 
whereas  stones  of  this  kind  may  be  used  in  any  position. 

192.  II.  "  Squared  Stones.     This  class  covers  all  stones  that 

*  This  article  is  taken  from  the  report  of  the  committee  of  the  American  Socieljr 
of  Civil  Engineers  previously  referred  to. 


13a  STONE    CUTTIXG.  [CHAP.  VI. 

are  roughly  squared  and  roughly  dressed  on  beds  and  joints.  The 
dressing  is  usually  done  with  the  face  hammer  or  ax,  or  in  soft 
stones  with  the  tooth  hammer.  In  gneiss  it  may  sometimes  be 
necessary  to  use  the  point.  The  distinction  between  this  class  and 
the  third  lies  in  the  degree  of  closeness  of,  the  joints.  Where  the 
dressing  on  the  joints  is  such  that  the  distance  between  the  general 
planes  of  the  surfaces  of  adjoining  stones  is  one  half  inch  or  more, 
the  stones  properly  belong  to  this  class. 

*'  Three  subdivisions  of  this  class  may  be  made.,  depending  on 
the  character  of  the  face  of  the  stones: 

"  (a)  Q,uarry-faced  stones  are  those  whose  faces  are  left  un- 
touched as  they  come  from  the  quarry. 

'^  (b)  Pitch-faced  stones  are  those  on  wliich  the  arris  is  clearly 
defined  by  a  line  beyond  which  the  rock  is  cut  away  by  the  pitching 
chisel,  so  as  to  give  edges  that  are  approximately  true. 

"  (c)  Drafted  Stones  are  those  on  which  the  face  is  surrounded  by 
a  chisel  draft,  the  space  inside  the  draft  being  left  rough.  Ordi- 
narily, however,  this  is  done  only  on  stones  in  which  the  cutting  of 
the  joints  is  such  as  to  exclude  them  from  this  class. 

"  In  ordering  stones  of  this  ciass  the  specifications  should  always 
state  the  width  of  the  bed  and  end  joints  which  are  expected,  and 
also  how  far  the  surface  of  the  face  may  project  beyond  the  plane 
of  the  edge.  In  practice,  the  projection  varies  between  1  inch  and 
6  inches.  It  should  also  be  specified  whether  or  not  the  faces  are  to 
be  drafted. 

193.  III.  "  Cut  Stones.  This  class  covers  all  squared  stones 
with  smoothly-dressed  beds  and  joints.  As  a  rule,  all  the  edges  of 
cut  stones  are  drafted,  and  batween  the  drafts  the  stone  is  smoothly 
dressed.  The  face,  however,  is  often  left  rough  where  construction 
is  massive. 

"In  architecture  there  are  a  great  many  ways  in  which  the  faces 

of  cut  stone  may  be  dressed. 


.  ^  -  --  \  \  ;■ 


but  the  following  are  those 
that  will  usually  be  met  in 
engineering  work: 

"  Rough-pointed.     When  it 
is  necessary  to  remove  an  inch 
Fig.  31.-ROUGH-POINTED.  or   jnore  from   the  face    of  a 

stone,  it  is  done  bv  th?  pick  or  heavy  point  until  the  projections. 


ART.  3.] 


METHODS   OF   FI^'"ISHI^"G    SURFACES. 


183 


vary  from  ^  inch  to  1  inch.     The  stone  is  then  said  to  be  rough- 
pointed  (Fig.  31).     In  dressing 


Fig.  32. — Fine-pointed. 


limestone    and     granite,    this 
operation  precedes  all  others. 

"Fine-pointed.  (Fig.  32). 
If  a  smoother  finish  is  desired, 
rough  pointing  is  followed  by 
fine  pointing,  which  is  done 
with  a  fine  point.  Fine  point- 
ing is  used  only  where  the  finish  made  by  it  is  to  be  final,  and  never 
as  a  preparation  for  a  final  finish  by  another  tool. 

"  Crandalled.  This  is  only  a  speedy  method  of  pointing,  the 
effect  being  the  same  as  fine  pointing,  except  that  the  dots  on  the 
stone  are  more  regular.  The  variations  of  level  are  about  ^  inch, 
and  the  rows  are  made  parallel.  When  other  rows  at  right  angles 
to  the  first  are  introduced,  the  stone  is  said  to  be  cross-crandalled. 
Fig.  33. 


Fig.  33.— Crandalled. 


Fig.  34. — Axed. 


"  Axed,  or  Pean-hammered,  and  Patent-hammered.     These  two 

vary  only  in  the  degree  of  smoothness  of  the  surface  which  is  pro- 
duced. The  number  of  blades  in  a  patent  hammer  varies  from  6  to 
12  to  the  inch;  and  in  precise  specifications  the  number  of  cuts  to 
the  inch  must  be  stated,  such  as  6-cut,  8-cut,  10-cut,  12-cut.  The 
effect  of  axing  is  to  cover  the  surface  with  chisel  marks,  which  are 
made  parallel  as  far  as  practicable.     Fig.  34.     Axing  is  a  final  finish. 

"  Tooth-axed.  The  tooth-ax  is  practically  a  number  of  points, 
and  it  leaves  the  surface  of  a  stone  in  the  same  condition  as  fine 
pointing.  It  is  usually,  however,  only  a  preparation  for  bush-ham- 
mering, and  the  work  is  then  done  without  regard  to  effect  so  long 
as  the  surface  of  the  stone  is  sufficiently  leveled. 

"  Bush-hammered.     The  roughnesses  of  a  stone  are  pounded  off  by 


134 


STONE   CUTTING. 


[chap.  VI. 


the  bush  hammer,  and  the  stone  is  then    said   to  be   'bushed/ 

This  kind  of  'finish  is  dangerous 


Fig.  35.— Bush-hammered. 


Fig.  36.— Rubbed. 


on  sandstone,  as  experience  has 
shown  that  sandstone  thus  treated 
is  very  apt  to  scale.  In  dressing 
limestone  which  is  to  have  a  bush- 
hammered  finish,  the  usual  se- 
quence of  operation  is  (1)  rough- 
pointing,  (2)  tooth-axing,  and  (3) 
bush-hammering.     Fig.  35. 

"  Rubbed.  In  dressmg  sandstone  and  marble,  it  is  very  common 
to  give  the  stone  a  plane  surface  at  once 
by  the  use  of  tlife  stone-saw  [§  181].  Any 
roughnesses  left  by  the  saw  are  removed 
by  rubbing  with  grit  or  sandstone  [§  181]. 
Such  stones,  therefore,  have  no  margins. 
They  are  frequently  used  in  architecture 
for  string-courses,  lintels,  door-jambs,  etc. ; 
and  they  are  also  well  adapted  for  use  in  facing  the  walls  of  lock- 
chambers  and  in  other  localities  where  a  stone  surface  is  liable  to  be 
rubbed  by  vessels  or  other  moving  bodies.     Fig.  36. 

"  Diamond  Panels.      Sometimes  the  space  between  the  margins 
is  sunk  immediately  adjoining  them  and 
then  rises  gradually  until  the  four  planes 
form  an  apex  at  the  middle  of  tlie  panel. 
In  general,  such  panels  are  called  diamond 
panels,  and  the  one  just  described.  Fig. 
37,    is    called   a    sunk    diamond    panel. 
When  the  surface  of  the  stone  rises  grad- 
ually from  the  inner  lines  of  the  margins 
to  the  middle  of  the  panel,  it  is  called  a 
raised  diamond  panel.     Both  kinds  of  finish  are  common  on  bridge 
quoins  and  similar  work.     The  details  of  this  method  should  be 
given  in  the  specifications." 


FiQ.  37.— Diamond  Panel. 


CHAPTER  VII. 
STONE  MASONRY. 

In  preparing  specifications,  it  is  not  safe  to  depend  alone  upon 
the  terms  in  common  use  to  designate  the  various  classes  of  masonry; 
but  every  specification  should  contain  an  accurate  description  of  the 
character  and  quality  of  the  work  desired.  Whenever  practicable, 
samples  of  each  kind  of  cutting  and  masonry  should  be  prepared 
beforehand,  and  be  exhibited  to  the  persons  who  propose  to  under- 
take the  work. 

194.  Definitions  of  Parts  of  the  Wall.*  Face,  the  front 
surface  of  a  wall;  hack,  the  inside  surface. 

Facing,  the  stone  which  forms  the  face  or  outside  of  the  wall. 
Backing,  the  stone  which  forms  the  back  of  the  wall.  Filling,  the 
interior  of  the  wall. 

Batter.     The  slope  of  the  surface  of  the  wall. 

Course.     A  horizontal  layer  of  stone  in  the  wall. 

Joints.  The  mortar-layer  between  the  stones.  The  horizontal 
joints  are  called  bed-Joints  or  Bim^p]y  beds;  the  vertical  joints  are 
sometimes  called  the  builds.  Usually  the  horizontal  joints  are 
called  beds,  and  the  vertical  ones  Joints. 

Coping.     A  course  of  stone  on  the  top  of  the  wall  to  protect  it. 

Fainting.  A  better  quality  of  mortar  put  in  the  face  of  the 
joints  to  help  them  to  resist  weathering. 

Bond.     The  arrangement  of  stones  m  adjacent  courses  (§  202). 

Stretcher.  A  stone  whose  greatest  dimension  lies  parallel  to  the 
face  of  the  wall. 

Header.  A  stone  whose  greatest  dimension  lies  perpendicular 
to  the  face  of  the  wall. 

Quoin.  A  corner-stone.  A  quoin  is  a  header  for  one  face  and  a 
stretcher  for  the  other. 

DoivelSo  Straight  bars  of  iron  which  enter  a  hole  in  the  upper 
side  of  one  stone  and  also  a  hole  in  the  lower  side  of  the  stone  next 
•above. 

Cramps.     Bars  of  iron  having  the  ends  turned  at  right  angles  to 

*  The  definitions  in  this  chapter  are  in  accordance  with  the  recommendations  of 
the  Committee  of  the  American  Society  of  Civil  Engineers  previously  referred  to, 
and  conform  to  the  best  practice.     Unfortunately  they  are  not  universally  adopted. 

135 


136 


STONE   MASONKY. 


[chap.  VII. 


to  the  body  of  the  bar,  which  enter  holes  in  the  upper  side  of  ad- 
jacent stones. 

195.  Definitions  of  Kinds  of  Masonry.  Stone  masonry  is 
classified  (1)  according  to  the  degree  of  finish  of  the  face  of  the 
stones,  as  quarry-faced,  pitch-faced,  pointed,  bush-hammered,  etc. ; 
(2)  according  to  whether  the  horizontal  joints  are  more  or  less  con- 
tinuous, as  range,  broken  range,  and  random;  and  (3)  according 
to  the  care  employed  in  dressing  the  beds  and  joints,  as  ashlar, 
sqnared-stone,  and  rubble. 

196.  Quarry-faced  Masonry.  That  in 
which  the  face  of  the  stone  is  left  as  it 
comes  from  the  quarry.     Fig.  38. 

Pitch-faced  Masonry.  That  in  which 
the  face  edges  of  the  beds  are  pitched  to 
a  right  line.     Fig.  39. 

Cut-stone  Masonry.  That  in  which 
the  face  of  the  stone  is  finished  by  one  of  the  methods  described  in 
§§  190-193. 

197.  Range.  Masonry  in  which  a  course  is  of  the  same  thick- 
ness throughout.     Fig.  40. 

BroTcen  Range.  Masonry  in  which  a  course  is  not  continuous 
thronghout.     Fig.  41. 

Random.     Masonry  which  is  not  laid  in  courses  at  all. 


Fig.  38. 


Fig.  39. 


Fig.  42. 


~~T~— 


Fig.  40.— Range. 


Fig.  41.— Broken  Range. 


Fig.  42.— Random. 


Any  one  of  these  three  terms  may  be  employed  to  designate  the 
coursing  of  either  ashlar  (§  196)  or  sqnare-stone  masonry  (§  197), 
but  can  not  be  applied  to  rubble  (§  198). 

198.  Ashlar.  Cut-stone  masonry,  or  masonry  composed  of  any 
of  the  various  kinds  of  cut-stone  mentioned  in  §  193.  According 
to  the  Report  of  the  Committee  of  the  American  Society  of  Civil 
Engineers,  "  when  the  dressing  of  the  joints  is  such  that  the  dis- 
tance between  the  general  planes  of  the  surfaces  of  adjoining  stones 
is  one  half  inch  or  less,  the  masonry  belongs  to  this  class. ' '     From 


DEFINITIONS    OF    KINDS    OF    MASONRY. 


137 


its  derivation  ashlar  apparently  means  large,  square  blocks;  but 
practice  seems  to  have  made  it  synonymous  with  "  cut-stone,"  and 
this  secondary  meaning  has  been  retained  for  convenience.  The 
coursing  of  ashlar  is  described  by  prefixing  range,  broken  range, 
or  random;  and  the  finish  of  the  face  is  described  by  prefixing  the 
name  of  the  cut-stone  (see  §§  190-93)  of  which  the  masonry  is 
composed. 

Small  Ashlar.  Cut-stone  masonry  in  which  the  stones  are  less 
than  one  foot  thick.     The  term  is  not  often  used. 

Rough  Ashlar.  A  term  sometimes  given  to  squared-stone 
masonry  (§  197),  either  quarry-faced  or  pitch-faced,  when  laid  as 
range-work ;  but  it  is  more  logical  and  more  expressive  to  call  such 
work  range  squared-stone  masonry. 

Dimension  Sfo?ies.  Cut-stones,  all  of  whose  dimensions  have 
been  fixed  in  advance.  "  If  the  specifications  for  ashlar  masonry 
are  so  written  as  to  prescribe  the  dimensions  to  be  used,  it  will  not 
be  necessary  to  make  a  new  class  for  masonry  composed  of  suck 
stones. ' ' 

Squared-stone  Masonry.  Work  in  which  the  stones  are  roughly 
squared  and  roughly  dressed  on  beds  and  joints  (§  192).  The 
distinction  between  squared-stone  masonry  and  ashlar  (§  196) 
lies  in  the  degree  of  closeness  of  the  joints.  According  to  the 
Eeport  of  the  Committee  of  the  American  Society  of  Civil  Engineers, 
"  when  the  dressing  on  the  joints  is  such  that  the  distance  between 
the  general  planes  of  the  surface  of  adjoining  stones  is  one  half  inch 
or  more,  the  stones  properly  belong  to  this  class ; ' '  nevertheless, 
such  masonry  is  often  classed  as  ashlar  or  cut-stone  masonry. 

Bubble    Masonry. 

(§  191). 

Uncoursed  Bubble. 
laid  without  any  at- 
tempt at  regular 
courses.     Fig.  43. 

Coursed  Bubble. 
Unsquared-stone  ma- 
sonry which  is  leveled 
off  at  specified  heights 
to  an  approximately 
horizontal  surface.     It  may  be  specified  that  the  stone  shall  be  rough 


Masonry   composed    of    unsquared    stone 
Masonry  composed   of   unsquared   stones 


^^^ 


Fig.  43. 


Fig.  44. 


]y  shaped  with  the  hammer,  so  as  to  fit  approximately.     Fig.  44. 


138  STOISTE    MASON"RT.  [CHAP.  VII. 

199.  Genesal  Rules.  Eankine  gives  the  following  rules  to  be 
observed  in  the  building  of  all  classes  of  stone  masonry: 

"  I.  Build  the  masonry,  as  far  as  possible,  in  a  series  of  courses, 
perpendicular,  or  as  nearly  so  as  possible,  to  the  direction  of  the 
pressure  which  they  have  to  bear;  and  by  breaking  joints  avoid  all 
long  continuous  joints  parallel  to  that  pressure. 

"  II.   Use  the  largest  stones  for  the  foundation  course. 

"  III.  Lay  all  stones  which  consist  of  layers  in  such  a  manner  that 
tl>e  principal  pressure  which  they  have  to  bear  shall  act  in  a  direction 
perpendicular,  or  as  nearly  so  as  possible,  to  the  direction  of  the 
layers.  This  is  called  laying  the  stone  on  its  natural  led,  and  is  of 
primary  importance  for  strength  and  durability. 

''IV.  Moisten  the  surface  of  dry  and  porous  stones  before  bed- 
ding them,  in  order  that  the  mortar  may  not  be  dried  too  fast  and 
reduced  to  powder  by  the  stone  absorbing  its  moisture. 

"V.  Fill  every  part  of  every  joint,  and  all  spaces  between  the 
stones,  with  mortar,  taking  care  at  the  same  time  that  such  spaces 
shall  be  as  small  as  possible." 

Another  and  very  important  rule  is:  the  rougher  the  stones,  the 
better  the  mortar  should  be.  The  principal  object  of  the  mortar  is 
to  equalize  tiie  pressure;  and  the  more  nearly  the  stones  are  reduced 
to  closely  fitting  surfaces,  the  less  important  is  the  mortar.  Not 
infrequently  this  rule  is  exactly  reversed ;  i.  e.,  the  finer  the  dressing, 
the  better  the  quality  of  the  mortar  used. 

200.  Ashlar  Masonry.  For  definitions  of  this  class  of  masonry 
and  its  subdivision,  see  §  196. 

The  strength  of  a  mass  of  ashlar  masonry  depends  upon  the 
size  of  the  blocks  in  each  course,  upon  the  accuracy  of  the  dressing, 
and  upon  the  bond. 

In  order  that  the  stones  may  not  be  liable  to  be  broken  across, 
no  soft  stone,  such  as  the  weaker  kinds  of  sandstone  and  granular 
limestone,  should  have  a  length  greater  than  3  times  its  depth;  but 
in  harder  materials,  the  length  may  be  4  or  5  times  the  depth.  The 
breadth  in  soft  materials  may  range  from  1^  to  2  times  the  depth  ; 
in  hard  materials,  it  may  be  3  times  the  depth. 

201.  Dressing.  The  closeness  with  which  stones  fit  is  depend- 
ent solely  upon  the  accuracy  with  which  the  surfaces  in  contact  are 
wrought,  or  dressed,  and  is  of  special  importance  in  the  case  of 
bed-joints.     If  any  part  of  the  surface  projects  beyond  the  plane 


ASHLAR   MASONRY.  139 


of  the  chisel-draft,  that  projecting  part  will  have  to  bear  an  undue 
share  of  the  pressure,  the  joint  will  gape  at  the  edges, — constituting 
what  is  called  an  open  joint, — and  the  whole  will  be  wanting  in 
stability.  On  the  other  hand,  if  the  surface  of  the  bed  is  concave, 
having  been  dressed  down  below  the  plane  of  the  chisel-draft,  the 
pressure  is  concentrated  on  the  edges  of  the  stone,  to  the  risk  of 
splitting  them  off.  Such  joints  are  said  to  he  flushed.  They  are 
more  diflBcult  of  detection,  after  the  masonry  has  been  built,  than 
open  joints  ;  and  are  often  executed  by  design,  in  order  to  give  a 
neat  appearance  to  the  face  of  the  building.  Their  occurrence 
must  therefore  be  guarded  against  by  careful  inspection  during 
the  progress  of  the  stone  cutting. 

Great  smoothness  is  not  desirable  in  the  joints  of  ashlar  masonry 
intended  for  strength  and  stability  ;  for  a  moderate  degree  of  rough- 
ness adds  at  once  to  the  resistance  to  displacement  by  sliding,  and 
to  the  adhesion  of  the  mortar.  When  the  stone  has  been  dressed 
so  that  all  the  small  ridges  and  projecting  points  on  its  surface  are 
reduced  nearly  to  a  plane,  the  pressure  is  distributed  nearly  uni- 
formly, for  the  mortar  serves  to  transmit  the  pressure  to  the  small 
depressions.  '  Each  stone  should  first  be  fitted  into  its  place  dry, 
in  order  that  any  inaccuracy  of  figure  may  be  discovered  and  cor- 
rected by  the  stone-cutter  before  it  is  finally  laid  in  mortar  and 
settled  in  its  bed. 

The  thickness  of  mortar  in  the  joints  of  the  very  best  ashlar 
masonry — for  example,  the  United  States  post-office  and  custom- 
house buildings  in  the  principal  cities — is  about  ^  of  an  inch  ;  in 
first-class  railroad  masonry — for  example,  important  bridge  piers 
and  abutments,  and  large  arches — the  joints  are  from  i  to  |- 
of  an  inch.  No  cutting  should  be  allowed  after  the  stone 
has  been  set  in  mortar,  for  fear  of  breaking  the  adhesion  of  the 
mortar. 

A  chisel-draft  1^  or  2  inches  wide  is  usually  cut  at  each  exterior 
corner. 

202.  Bond.  No  side-joint  of  any  course  should  be  directly  above 
a  side- joint  in  the  course  below  ;  but  the  stones  should  overlap,  or 
break  joint,  to  an  extent  of  from  1  to  1^  times  the  depth  of  the 
course.  This  is  called  the  ho7id  of  the  masonry.  The  effect  is  that 
each  stone  is  supported  by  at  least  two  stones  of  the  course  below,  and 
assists  in  supporting  at  least  two  stones  of  the  course  above.     The 


140  STOXE    MASOXRY.  [CHAP.  YIL 

object  is  twofold  :  first,  to  distribute  the  pressure,  so  that  inequali- 
ties of  load  on  the  upper  part  of  the  structure  (or  of  resistance  at 
the  foundation)  may  be  transmitted  to  and  spread  over  an  increas- 
ing area  of  bed  in  proceeding  downwards  (or  upwards) ;  and  second, 
to  tie  the  building  together,  i.  e.,  to  give  it  a  sort  of  tenacity,  both 
lengthwise  and  from  face  to  back,  by  means  of  the  friction  of  the 
stones  where  they  overlap. 

The  strongest  bond  is  that  in  which  each  course  at  the  face  of 
the  strvicture  contains  a  header  and  a  stretcher  alternately,  the 
outer  end  of  each  header  resting  on  the  middle  of  a  stretcher  of 
the  course  below,  so  that  rather  more  than  one  third  of  the  area  of 
the  face  consists  of  ends  of  headers.  This  proportion  may  be 
deviated  from  when  circumstances  require  it,  but  in  every  case  it 
is  advisable  that  the  ends  of  headers  should  not  form  less  than  owe 
fourth  of  the  whole  area  of  the  face  of  the  structure.  A  header 
should  extend  entirely  through  the  wall,  and  should  be  over  the 
middle  of  the  stretcher  in  the  course  below. 

A  trick  of  masons  is  to  use  "blind-headers,"  or  short  stones  that 
look  like  headers  on  the  outside  but  do  not  go  deeper  into  the  wall 
than  the  adjacent  stretchers.  When  a  course  has  been  put  on  toy) 
of  these,  they  are  completely  covered  up  ;  and,  if  not  suspected, 
the  fraud  will  never  be  discovered  unless  the  weakness  of  the  wall 
reveals  it. 

Where  very  great  resistance  to  displacement  of  the  masonry  is 
required  (as  in  the  upper  courses  of  bridge  piers,  or  over  openings, 
or  where  new  masonry  is  joined  to  old,  or  where  there  is  danger  of 
unequal  settlement),  the  bond  is  strengthened  by  dowels  or  by 
cramp-irons  (§  195)  of,  say,  l^-inch  round  iron  set  with  cement 
mortar. 

203.  Backing.  Ashlar  is  usually  backed  with  rubble  masonry 
(§  213),  which  in  such  cases  is  specified  as  coursed  rubble.  Special 
care  should  be  taken  to  secure  a  good  bond  between  the  rubble 
backing  and  the  ashlar  facing.  Two  stretchers  of  the  ashlar  fac- 
ing having  the  same  width  should  not  be  placed  one  immediately 
above  the  other.  The  proportion  and  length  of  the  headers  in 
the  rubble  backing  should  be  the  same  as  in  the  ashlar  facing.  The 
"  tails  ''  of  the  headers,  or  the  parts  which  extend  into  the  rubble 
backing,  may  be  left  rough  at  the  back  and  sides;  but  their  upper 
and  lower  beds  should  be  dressed  to  the  general  plane  of  the  bed  of 


ASHLAR   MASONRY.  141 


the  course.     These  ''tails"  may  taper  slightly  in  breadth,  but  should 
not  taper  in  depth. 

The  backing  should  be  carried  up  at  the  same  time  with  the 
face-work,  and  in  courses  of  the  same  depth;  and  the  bed  of  each 
course  should  be  carefully  built  to  the  same  plane  with  that  of  the 
ashlar  facing.  The  rear  face  of  the  backing  should  be  lined  to  a 
fair  surface. 

204.  Pointing.  In  laying  masonry  of  any  character,  whether 
with  common  or  hydraulic  mortar,  the  exposed  edges  of  the  joints 
will  naturally  be  deficient  in  density  and  hardness.  The  mortar  in 
the  joints  near  the  surface  is  especially  subject  to  dislodgmeut, 
since  the  contraction  and  expansion  of  the  masonry  is  liable  either 
to  separate  the  stone  from  the  masonry  or  to  crack  the  mortar  in 
the  joint,  thus  permitting  the  entrance  of  rain-water,  which,  freezing, 
forces  the  mortar  from  the  joints.  Therefore  it  is  usual,  after  the 
masonry  is  laid,  to  refill  the  joints  as  compactly  as  possible,  to  the 
depth  of  at  least  half  an  inch,  with  mortar  prepared  especially  for 
this  purpose.     This  operation  is  called  pointing. 

The  very  best  cement  mortar  should  be  used  for  pointing,  as  the 
best  becomes  dislodged  all  too  soon.  Clear  Portland  cement  mor- 
tar is  the  best,  although  1  volume  of  cement  to  1  of  sand  is  fre- 
quently used  in  first-class  work.  The  moi-tar,  when  ready  for  use, 
should  be  rather  incoherent  and  quite  deficient  in  plasticity.  Before 
applying  the  pointing,  the  joint  should  be  well  cleansed  by  scrap- 
ing and  brushing  out  the  loose  matter,  and  then  be  well  moistened. 
Of  course,  the  cleansing  out  of  the  joints  can  be  most  easily  done 
v/hile  the  mortar  is  new  and  soft.  The  depth  to  which  the  mortar 
shall  be  dug  out  is  not  often  specified  ;  it  is  usually  cleaned  out 
about  half  an  inch  deep,  but  should  be  at  least  an  inch.  In  the 
Brooklyn  bridge  piers  the  joints  were  cleared  1^  inches' deep. 

The  mortar  is  applied  with  a  mason's  trowel,  and  the  joint  well 
calked  with  a  calking  iron  and  hammer.  In  the  very  best  Avork, 
the  joint  is  also  rubbed  smooth  with  a  steel  polishing  tool.  Walls 
should  not  be  allowed  to  dry  too  rapidly  after  pointing  ;  therefore, 
pointing  m  hot  weather  should  be  avoided. 

205.  Amount  of  Mortar.  The  amount  of  mortar  required  for 
ashlar  masonry  varies  with  the  size  of  the  blocks,  and  also  with 
the  closeness  of  the  dressing.  With  f-  to  i-inch  joints  and  12-  to 
20-inch  courses,   there  will   be  about  2  cubic  feet  of  mortar  jmt 


142  STONE   MASONRY.  [CHAP.  VII- 

cubic  yard;  with  larger  blocks  and  closer  joints,  i.  e.,  in  the  best 
masonry,  there  will  be  abont  1  cubic  foot  of  mortar  per  yard  of 
masonry.  Laid  in  1  to  2  mortar,  ordinary  ashlar  will  require  ^  to 
^  of  a  barrel  of  cement  per  cubic  yard  of  masonry. 

For  the  quantities  of  cement  and  sand  required  for  a  cubic  yard 
of  mortar  of  different  compositions,  see  page  88. 

206.  When  Employed.  Ashlar  masonry  is  used  for  piers,  abut- 
ments,, arches,  and  parapets  of  bridges;  for  hydraulic  works;  for 
facing-qnoins,  and  string  courses;  for  the  coping  of  inferior  kinds 
of  masonry  and  of  brick  work ;  and,  in  general,  for  works  in  which 
great  strength  and  stability  are  required. 

207.  Specifications  for  Ashlar.  The  specifications  for  ashlar, 
or  "  first-class  masonry  "  as  employed  on  the  railroads,  are  about 
as  follows :  * 

Ashlar  shall  consist  of  range  pitch-faced  masonry.  The  stone  shall  be  of 
durable  quality;  and  shall  be  free  from  seams,  powder  cracks,  drys,  flaws,  or 
other  imperfections. 

All  foundation  courses  sball  be  laid  with  selected,  large,  flat  stones  not  less 

than  f  inches  in  thickness,  nor  of  less  superficial  surface  than  fifteen  (15) 

square  feet. 

The  courses  shall  be  not  less  than inches  thick  nor  more  tban 

inches.^:  The  courses  shall  be  continuous  around  and  through  the  wall  ;  and 
DO  course  shall  be  thicker  than  the  one  below  it,  except  that  the  footing 
course  may  be  thinner  than  the  one  next  above.  Stretchers  shall  be  at  least 
twice  as  wide  as  thick,  and  at  least  four  times  as  long  as  thick.  Headers  shall 
be,  for  at  least  three  fourths  of  their  length,  not  less  than  twice  as  wide  as 
thick;  and  shall  extend  entirely  through  the  wall,  or  have  a  length  not  less 
than  five  times  the  thickness  of  the  course.  The  masonry  shall  consist  of 
headers  and  stretchers  alieruatiug;  at  least  one  third  §  of  the  face  of  the  wall 
shall  consist  of  headers.  Stretchers  of  the  same  width  shall  not  be  placed 
immediately  one  above  the  other  ;  but  this  shall  not  apply  to  the  ends  of 
stretchers  where  headers  come  centrally  between  stretchers.  Every  header 
shall  be  immediately  over  a  stretcher  of  the  course  next  below.  Joints  on  the 
face  of  the  wall  shall  be  broken  at  least  three  quarters  of  the  thickness  of  the 
course. 

The  beds  and  the  vertical  joints  for  12  inches  back  from  the  face  of  the 
wall  shall  be  dressed,  before  being  brought  to  the  wall,  so  as  to  form  mortar 

*  For  complete  specifications  for  railroad  and  also  other  kinds  of  masonry,  see 
Appendix  I,  page  529. 

f  Frequently  12 ;  sometimes  18. 

J  The  courses  of  the  classes  of  masonry  referred  to  above  usually  range  from 
14  to  30  inches ;  but,  of  course,  may  vary  according  to  the  circumstances,  and  for 
some  purposes  may  be  as  low  as  10  inches. 

§  Often  specified  as  one  fourth. 


SQUARE-STOKED    MASONRY.  143- 

joints  not  less  than  one  quarter  inch  nor  more  than  one  half  inch  in  thickness. 
All  stones  shall  be  laid  on  the  natural  bed.  No  part  of  a  stone  shall  extend 
beyond  the  back  edge  of  the  under  bed.  All  corners  and  batter  lines  shall 
have  a  neat  chisel-draft  one  and  one  half  inches  -wide  on  each  face.  The  pro- 
jections of  the  rock- face  must  not  exceed  four  inches  beyond  the  draft-lines  ; 
and  in  tunnel  side-walls,  the  projection  must  not  exceed  two  inches.  The 
face-edge  of  the  joiut  shall  be  pitched  to  a  straight  line. 

The  backing  shall  consist  of  stone  of  the  same  thickness  as  the  correspond- 
ing face  stone.  When  walls  exceed  four  feet  in  thickness,  there  shall  be  as 
many  headers  of  the  same  size  in  the  back  of  the  wall  as  in  the  face,  so  ar- 
ranged that  a  header  in  the  rear  of  the  wall  shall  be  between  two  headers  in 
the  front.  The  backing  shall  be  so  laid  as  to  leave  no  spaces  between  the 
stones  over  six  inches  wide,  which  spaces  shall  be  filled  with  spalls  set  in 
cement  mortar.     No  spalls  shall  be  allowed  in  the  bed  joints. 

The  coping  shall  be  formed  of  large  flat  stones,  which  shall  extend  entirely 
across  the  wall  when  the  same  is  not  more  than  six  feet  wide.  The  steps  of 
wing  walls  shall  be  capped  with  stone  covering  the  entire  step  and  extending 
under  the  step  next  above  at  least  twelve  inches.  Coping  and  step  stones  shall 
be  at  least  twelve  inches  thick,  and  have  such  projections  as  the  engineer  may 
direct  [usually  3  to  6  inches].  The  tops  and  faces  of  copings  and  step  stones 
shall  be  bush-hammered,  and  their  joints  and  beds  cut  to  one  quarter  inch 
throughout. 

208.  Sqtjared-stone  Masonry.  For  definitions  of  this  class  of 
masonry  and  its  subdivisions,  see  §  197.  The  distinction  between 
squared-stone  masonry  and  ashlar  lies  in  the  degree  of  closeness  of 
the  joints.  According  to  the  Report  of  the  Committee  of  the 
American  Society  of  Civil  Engineers,  "  when  the  dressing  on  the 
joints  is  such  that  the  distance  between  the  general  planes  of  the 
surfaces  of  adjoining  stones  is  one  half  inch  or  more,  the  stones 
properly  belong  to  this  class; "  however,  such  masonry  is  freqi;ently 
classed  as  ashlar  or  cut-stone  masonry. 

Squared-stone  masonry  is  usually  quarry-faced,  random-work, 
although  pitch-faced  range-work  is  not  uncommon.  The  quoins 
and  the  sides  of  openings  are  usually  reduced  to  a  rough-smooth 
surface  with  the  face-hammer,  the  ordinary  ax,  or  the  tooth-ax. 
This  work  is  a  necessity  where  door  or  window  frames  are  inserted; 
and  it  greatly  improves  the  general  effect  of  the  wall,  if  used 
wherever  a  corner  is  turned. 

209.  Squared-stone  masonry  is  distinguished,  on  the  one  hand, 
from  ashlar  in  having  less  accurately  dressed  beds  and  joints,  and,  on 
the  other  hand,  from  rubble  in  being  more  carefully  constructed. 
In  ordinary  practice,  the  field  covered  by  this  class  is  not  very 
definite.     The  specifications  for  "  second-class  masonry"  as  used 


144  STONE   MASONRY.  [CHAP.  VII. 

on  railroads  nsually  conform  to  the  above  description  of  qnarry-faced, 
range  sqnared-stone  masonry;  but  sometimes  this  grade  of  masonry 
is  designated  "  superior  rubble." 

210.  Amount  of  Mortar  Required.  The  amount  of  mortar 
required  for  squared-stone  masonry  varies  with  the  size  of  the 
stones  and  with  the  quality  of  the  masonry ;  as  a  rough  average, 
■one  sixth  to  one  quarter  of  the  mass  is  mortar.  When  laid  in  1  to 
2  mortar,  squared-stone  masonry  will  require  ^  to  f  of  a  barrel  of 
cement  per  cubic  yard  of  masonry. 

For  quantities  of  cement  and  sand  required  for  mortars  of 
various  compositions,  see  the  table  on  page  88. 

211.  Backing  and  Pointing.  The  statements  concerning  the 
backing  and  pointing  of  ashlar  (§§  203  and  204)  apply  substantially 
to  squared-stone  masonry.  As  the  joints  of  squared-stone  masonry 
are  thicker  than  those  of  ashlar,  the  pointing  should  be  done  pro- 
portionally more  carefully;  while  as  a  rale  it  is  done  much  more 
carelessly.  The  mortar  is  often  thrown  into  the  joint  with  a 
trowel,  and  then  trimmed  top  and  bottom  to  give  the  appearance 
of  a  thinner  joint.  Such  work  is  called  ribbon  pointing.  Trimming 
the  pointing  adds  to  the  appearance  but  not  to  the  durability. 
When  not  trimmed  it  is  called  dashed  pointing. 

212.  Specifications  for  Squared-stone  Masonry.  Squared-stone 
masonry  is  employed  for  the  piers  and  abutments  of  lighter  bridges, 
for  small  arches,  for  box-culverts,  for  basement  walls,  etc.  The 
specifications  are  about  as  follows :  * 

The  stones  shall  be  of  durable  quality;  and  shall  be  free  from  seams, 
powder  cracks,  drys,  or  other  imperfections. 

The  courses  shall  be  not  less  than  10  inches  thick. 

Stretchers  shall  be  at  least  twice  as  wide  as  thick,  and  at  least  four  times  as 
long  as  thick.  Headers  shall  be  at  least  five  times  as  long  as  thick,  and  at  least 
as  wide  as  thick.  There  shall  be  at  least  one  header  to  three  stretchers.  Joints 
on  the  face  shall  be  broken  at  least  8  inches. 

The  beds  and  vertical  joints  for  8  inches  back  from  the  face  of  the  wall 
shall  be  dressed  to  make  joints  one  half  to  one  inch  thick.  The  front  edge  of 
the  joint  shall  be  pitched  to  a  straight  line.  All  corners  and  batter-lines  shall 
be  hammer-dressed. 

The  backing  shall  consist  of  stones  not  less  in  thickness  than  the  facing. 
At  least  one  half  of  the  backing  shall  be  stones  containing  3  cubic  feet. 
The  backing  shall  be  laid  in  full  mortar   beds;  and  the  vertical  joints  shall 

*  For  complete  specifications  for  masonry  for  various  purposes,  see  Appendix  I, 
page  529. 


BUBBLE    MASONRY.  145 


also  be  filled  with  mortar.     The  spaces  between  the  large  stones  shall  be  filled 
with  spalls  set  in  mortar. 

The  coping  shall  be  formed  of  large  flat  stones  of  such  thickness  as  the 
engineer  may  direct,  but  in  no  case  to  be  less  than  eight  inches  (8).  The 
upper  surface  of  the  coping  shall  be  bush-hammered,  and  the  joints  and  beds 
shall  be  dressed  to  one  half  an  inch  (^")  throughout.  Each  stone  must  extend 
sniirely  across  the  wall  when  the  wall  is  not  more  than  four  feet  (4)  thick. 

213.  Rubble  Masonry.  For  definitions  connected  with  this 
class  of  masonry,  see  §  198. 

The  stones  used  for  rubble  masonry  should  be  prepared  by 
simply  knocking  off  all  the  weak  angles  of  the  block.  It  should  be 
cleansed  from  dust,  etc.,  and  moistened,  before  being  placed  on  its 
bed.  This  bed  is  prepared  by  spreading  over  the  top  of  the  lower 
course  an  ample  quantity  of  good,  ordinary-tempered  mortar  in 
which  the  stone  is  firmly  embedded.  The  vertical  joints  should  be 
carefully  filled  with  mortar.  The  interstices  between  the  larger 
masses  of  stone  are  filled  by  thrusting  small  fragments  or  chippings 
of  stone  into  the  mortar.  In  heavy  walls  of  rubble  masonry,  the 
precaution  should  be  observed  to  give  the  stones  the  same  position 
in  the  masonry  that  they  had  in  the  quarry,  i.  e.,  to  lay  them  on 
their  "natural  bed,"  since  stone  offers  more  resistance  to  pressure 
in  a  direction  perpendicular  to  the  quarry-bed  than  in  any  other. 
The  directions  of  the  laminas  in  stratified  stones  show  the  position 
of  the  quarry-bed. 

To  connect  the  parts  well  together  and  to  strengthen  the  weak 
points,  tliroughs  or  binders  should  be  used  in  all  the  courses,  and 
the  angles  should  be  constructed  of  cut  or  hammered  stone. 

When  carefully  executed  with  good  mortar,  rubble  possesses  all  the 
strength  and  durability  required  in  structures  of  an  ordinary  char- 
acter, and  is  much  less  expensive  than  ashlar.  The  difficulty  is  m 
getting  it  well  executed.  The  most  common  defects  are  (1)  not  bring- 
ing the  stones  to  an  even  bearing;  (3)  leaving  large  vertical  openings 
between  the  several  stones;  (3)  laying  up  a  considerable  height  of 
the  wall  dry,  with  only  a  little  mortar  on  the  face  and  back,  and 
then  pouring  mortar  on  the  top  of  the  wall;  (4)  using  insufficient 
cement,  or  that  of  a  poor  quality.  The  last  defect  is  usually  obviated 
by  furnishing  the  cement  to  the  contractor  ;  and  the  second  and 
third  defects  may  be  detected  by  probing  the  vertical  Joints  with  a 
small  steel  rod.     In  order  to  secure  good  rubble,  great  skill  and 


146  STONE   MASONET.  [CHAP.  VII. 

care  are  required  on  the  part  of  the  mason,  and  constant  watchful- 
ness on  the  part  of  the  inspector. 

A  very  stable  wall  can  be  built  of  rubble  masonry  without  any 
dressing,  except  a  draft  on  the  quoins  by  which  to  plumb  the  cor- 
ners and  carry  them  up  neatly,  and  a  few  strokes  of  the  hammer  to 
spall  off  any  projections  or  surplus  stone.  This  style  of  work  is 
not  generally  advisable,  as  very  few  mechanics  can  be  relied  upon  to 
take  the  proper  amount  of  care  in  leveling  up  the  beds  and  filling 
the  joints;  and  as  a  consequence,  one  small  stone  may  jar  loose  and 
fall  out,  resulting  probably  in  the  downfall  of  a  considerable  part  of 
the  wall.  Some  of  the  naturally  bedded  stones  are  so  smooth  and 
uniform  as  to  need  no  dressing  or  spalliug  up;  a  wall  of  such  stones 
is  very  economical,  since  there  is  no  expense  of  cutting  and  no  time 
is  lost  in  hunting  for  the  right  stone,  and  yet  strong,  massive  work 
is  assured.  However,  many  of  the  naturally  bedded  stones  have 
inequalities  on  their  surfaces,  and  in  order  to  keep  them  level  in  the 
course  it  becomes  necessary  to  raise  one  corner  by  placing  spalls  or 
chips  of  stone  under  the  bed,  and  to  fill  the  vacant  spaces  well  and 
full  with  mortar.  It  is  just  here  that  the  disadvantage  of  this  style 
of  work  becomes  apparent.  Unless  the  mason  places  these  spalls  so 
that  tlie  stone  rests  firmly,  /.  e.,  does  not  rock,  it  will  work  loose, 
particularly  if  the  structure  is  subject  to  shock,  as  the  walls  of 
cattle- guards,  etc.  Unless  these  spalls  are  also  distributed  so  as  to 
support  all  parts  of  the  stone,'  it  is  liable  to  be  broken  by  the  weight 
above  it.  A  few  such  instances  in  the  same  work  may  occasion  con- 
siderable disaster. 

One  of  the  tricks  of  masons  is  to  put  "nigger-heads"  (stones 
from  which  the  natural  rounded  surface  has  not  been  taken  off) 
into  the  interior  of  the  wall. 

214.  Rubble  masonry  h  sometimes  laid  without  any  mortar,  as 
in  slope  walls  (§  218),  paving  (§  219),  etc.,  in  which  case  it  is  called 
dry  rubble;  but  as  such  work  is  much  more  frequently  designated 
as  slope- wall  masonry  and  stone-paving,  it  is  better  to  reserve  the 
term  rubble  for  undressed  stone  laid  in  mortar.  Occasionally  box 
culverts  are -built  of  the  so-called  dry  rubble;  but  as  such  construc- 
tion is  not  to  be  commended,  there  is  no  need  of  a  term  to  desig- 
nate that  kind  of  masonry. 

215.  Amount  of  Mortar  Required.  If  rubble  masonry  is  com- 
posed of  small  and  irregular  stones,  about  one  third  of  the  mass 


BUBBLE    MASONRY.  14*! 


^ill  consist  of  mortar;  if  the  stones  are  larger  and  more  regular, 
one  fifth  to  one  quarter  will  be  mortar.  Laid  in  1  to  2  mortar, 
ordinary  rubble  requires  from  one  half  to  one  barrel  of  cement  per 
cubic  yard  of  masonry. 

For  the  amount  of  cement  and  sand  required  for  mortar  of  va- 
rious compositions,  see  the  table  on  page  88. 

216.  "When  Employed.  Rubble  masonry  of  the  quality  described 
above  is  frequently  employed  for  the  smallest  sizes  of  bridge  abut- 
ments, small  arch  culverts,  box  and  open  culverts,  foundations  of 
buildings,  etc.,  and  for  backing  for  ashlar  masonry  (§  200). 

217.  Specifications  for  Rubble  Masonry.*  The  following  re- 
quirements, if  properly  complied  with,  will  secure  what  is  generally 
known  among  railroad  engineers  as  superior  rubble. 

Rubble  masonry  shall  consist  of  coursed  rubble  of  good  quality  laid  in 
cement  mortar.  No  stone  shall  be  less  than  six  inches  (6")  in  thickness,  unless 
otherwise  directed  by  the  engineer.  No  stone  shall  measure  less  than  twelve 
inches  (13")  in  its  least  horizontal  dimension,  or  less  than  its  thickness.  At 
least  one  fourth  of  the  stone  in  the  face  shall  be  headers,  evenly  distributed 
throughout  the  wall.  The  stones  shall  be  roughly  squared  on  joints,  beds,  and 
faces,  laid  so  as  to  break  joints  and  in  full  mortar  beds.  All  vertical  spaces 
shall  be  flushed  with  good  cement  mortar  and  then  be  packed  full  with  spalls. 
No  spalls  will  be  allowed  in  the  beds.  Selected  stones  shall  be  used  at  all 
angles,  and  shall  be  neatly  pitched  to  true  lines  and  laid  on  hammer-dressed 
beds;  draft  lines  may  be  required  at  the  more  prominent  angles. 

The  top  of  parapet  walls,  piers,  and  abutments  shall  be  capped  with  stones 
extending  entirely  across  the  wall,  and  having  a  front  and  end  projection  of 
not  less  than  four  inches  (4").  Coping  stones  shall  be  neatly  squared,  and  laid 
with  joints  of  less  than  one  half  inch  (i").  The  steps  of  wing-walls  shall  be 
capped  with  stone  covering  the  entire  step,  and  extending  at  least  six  inches 
(9')  into  the  wall.  Coping  and  step  stones  shall  be  roughly  hammer- dressed 
on  top,  their  outer  faces  pitched  to  true  lines,  and  be  of  such  thickness  (not 
less  than  six  inches)  and  have  such  projections  as  the  engineer  may  direct. 

' '  The  specifications  for  rubble  masonry  will  apply  to  rabble  masonry  laid 
Iry,  except  as  to  the  use  of  the  mortar  (see  §  214)." 

218.  Slope-wall  Masonry.  A  slope-wall  is  a  thin  layer  of 
masonry  used  to  preserve  tlie  slopes  of  embankments,  excavations, 
canals,  river  banks,  etc.,  from  rain,  waves,  weather,  etc.  The  usual 
specifications  are  as  follows: — 

The  stones  must  reach  entirely  through  the  wall,  and  be  not  less  than  four 
inches  (4")  thick  and  twelve  inches  (12")  long  They  must  be  laid  with  broken 
joints;  and  the  joints  must  be  as  close  and  free  from  spalls  as  possible. 

*  For  complete  specifications  for  masonry  for  various  purposes,  see  Appendix  I. 


148  STONE    MASOXRT.  [CHAP.  VII. 


219.  Stone  Paving.  Stone  paving  is  used  for  the  inverts  of  arch 
cuiverts,  for  protecting  the  lower  end  of  arches  from  undermining, 
and  for  foundations  of  box  culverts  and  small  arches.  It  is  usually 
classed  as  dry  rubble  masonry,  although  it  is  occasionally  laid  with 
cement  mortar.     The  usual  specifications  are  about  as  follows  : 

Stone  paving  shall  be  made  of  flat  stones  from  eight  inches  (8")  to  fifteen 
inches  (15  )  in  depth,  set  on  edge,  closely  laid  and  well  bedded  in  the  soil,  and 
shall  present  an  even  top  surface. 

220.  Riprap.  Eiprap  is  stone  laid,  without  mortar,  about  the 
base  of  piers,  abutments,  etc.,  to  prevent  scour,  and  on  banks  to 
prevent  wash.  Wlien  used  for  the  protection  of  piers,  the  stones 
are  dumped  in  promiscuoiTsly,  their  size  depending  upon  the 
material  at  hand  and  the  velocity  of  the  current;  stones  of  15  to 
25  cubic  feet  each  are  frequently  employed.  When  used  for  the 
protection  of  banks,  the  riprap  is  laid  by  hand  to  a  uniform  thick- 
ness. 

221.  Strength  of  Stone  Masonry.  The  results  obtained  by 
testing  small  specimens  of  stone  (see  §  14)  are  useful  in  determin- 
ing the  relative  strength  of  different  kinds  of  stone,  but  are  of  no 
value  in  determining  the  ultimate  strength  of  the  same  stone  when 
built  into  a  masonry  structure.  The  strength  of  a  mass  of  masonry 
depends  upon  the  strengtli  of  the  stone,  the  size  of  the  blocks,  the 
accuracy  of  the  dressing,  the  proportion  of  headers  to  stretchers, 
and  the  strength  of  the  mortar.  A  variation  in  any  one  of  these 
items  may  greatly  change  the  strength  of  the  masonry. 

The  importance  of  the  mortar  as  affecting  the  strength  of 
masonry  to  resist  direct  compression  is  generally  overlooked.  The 
mortar  acts  as  a  cushion  (§  13)  between  the  blocks  of  stone,  and  if 
it  has  insufficient  strength  it  will  be  squeezed  out  laterally,  pro- 
ducing a  tensile  strain  in  the  stone;  weak  mortar  thus  causes  the 
stone  to  fail  by  tension  instead  of  by  compression.  No  experiments 
have  ever  been  made  upon  the  strength  of  stone  masonry  under  the 
conditions  actually  occurring  in  masonry  structures,  owing  to  the 
lack  of  a  testing-machine  of  sufficient  strength.  Experiments 
made  upon  brick  piers  (§  246)  12  inches  square  and  from  2  to  10 
feet  high,  laid  in  mortar  composed  of  1  volume  Portland  cement 
and  2  sand,  show  that  the  strength  per  square  inch  of  the  masonry 
is  only  about  one  sixth  of  the  strength  of  the  brick.  An  increase 
of  50  per  cent,  in  the  strength  of  the  brick  produced  no  appreciable 


STEEXGTH    OF    STOXE    MASOXEY.  149 

effect  on  the  strength  of  the  masonry;  but  the  substitution  of 
cement  mortar  (1  Portland  and  2  sand)  for  lime  mortar  (1  lime  and 
3  sand)  increased  the  strength  of  the  masonry  70  per  cent.  The 
method  of  failure  of  these  piers  indicates  that  the  mortar  squeezed 
out  of  the  joints  and  caused  the  brick  to  fail  by  tension.  Since  the 
mortar  is  the  weakest  element,  the  less  mortar  nsed  the  stronger  the 
wall;  therefore  the  thinner  the  joints  and  the  larger  the  blocks,  the 
stronger  the  masonry,  provided  the  surfaces  of  the  stones  do  not 
come  in  contact. 

It  is  generally  stated  that  the  working  strain  op  stone  masonry 
should  not  exceed  one  twentieth  to  one  tenth  of  the  strength  of  the 
stone;  but  it  is  clear,  from  the  experiments  on  the  brick  piers  re- 
ferred to  above,  that  the  strength  of  the  masonry  depends  upon  the 
strength  of  the  stone  only  in  a  remote  degree.  In  a  general  way  it 
may  be  said  that  the  results  obtained  by  testing  small  cubes  may 
vary  50  per  cent,  from  each  other  (or  say  25  per  cent,  from  the 
mean)  owing  to  undetected  differences  in  the  material,  cutting,  and 
manner  of  applying  the  pressure.  Experiments  also  show  that 
stones  crack  at  about  half  of  their  ultimate  crushing  strength. 
Hence,  when  the  greatest  care  possible  is  exercised  in  selecting  and 
bedding  the  stone,  the  safe  working  strength  of  the  stone  alone 
should  not  be  regarded  as  more  than  three  eighths  of  the  ultimate 
strength.  A  further  allowance,  depending  upon  the  kind  of  struc- 
ture, the  quality  of  mortar,  the  closeness  of  the  joints,  etc.,  should 
be  made  to  insure  safety.  Experiments  npon  even  comparatively 
large  monoliths  give  but  little  indication  of  the  strength  of  masonry. 
The  only  practicable  way  of  determining  the  actual  strength  of 
masonry  is  to  note  the  loads  carried  by  existing  structures.  How- 
ever, this  method  of  investigation  will  give  only  the  load  which  does 
not  crush  the  masonry,  since  probably  no  structure  ever  failed  owing 
to  the  crushing  of  the  masonry.  After  an  extensive  correspondence 
and  a  thorough  search  through  engineering  literature,  the  following 
list  is  given  as  showing  the  maximum  pressure  to  which  the  several 
classes  of  masonry  have  been  subjected. 

222.  Pressure  Allowed.  Early  builders  used  much  more  mas- 
sive masonry,  proportional  to  the  load  to  be  carried,  than  is  cus- 
tomary at  present.  Experience  and  experiments  have  shown  that 
such  great  strength  is  unnecessary.  The  load  on  the  monolithic 
piers  '•upporting  the  large  chiirches  in  Europe  does  not  exceed  3(? 


150  STONE   MASOXRY.  [CHAP.  VII. 

tons  per  sq.  ft.  (420  lbs.  per  sq.  in.),*  or  about  one  thirtieth  of  the 
ultimate  strength  of  the  stone  alone.  The  stone-arch  bridge  of  140 
ft.  span  at  Pont-y-Prydd,  over  the  Taff,  in  Wales,  erected  in  1750, 
is  supposed  to  have  a  pressure  of  72  tons  per  sq.  ft.  (1,000  lbs.  per 
sq.  in.)  on  hard  limestone  rubble  masonry  laid  in  lime  mortar.f 
Rennie  subjected  good  hard  limestone  rubble  in  columns  4  feet 
square  to  22  tons  per  sq.  ft.  (300  lbs.  per  sq.  in.).|  The  granite  piers 
of  the  Saltash  Bridge  sustain  a  pressure  of  9  tons  per  sq.  ft.  (125 
lbs.  per  sq.  in.). 

The  maximum  pressure  on  the  granite  masonry  of  the  towers  of 
the  Brooklyn  Bridge  is  about  28 1^  tons  per  sq.  ft.  (about  400  lbs.  per 
sq.  in.).  The  maximum  pressure  on  the  limestone  masonry  of  this 
bridge  is  about  10  tons  per  sq.  ft.  (125  lbs.  per  sq.  in.).  The  face 
stones  ranged  in  cubical  contents  from  1^  to  5  cubic  yards;  the 
stones  of  the  granite  backing  averaged  about  1^  cu.  yds.,  and  of  the 
limestone  about  1|-  cu.  yds.  per  piece.  The  mortar  was  1  volume 
of  Rosendale  cement  and  2  of  sand.  The  stones  were  rough-axed, 
or  pointed  to  |^-inch  bed-joints  and  |-inch  vertical  face-joints.§ 
These  towers  are  very  fine  examples  of  the  mason's  art. 

In  the  Rookery  Building,  Chicago,  granite  columns  about  3  feet 
square  sustain  30  tons  per  sq.  ft.  without  any  signs  of  weakness. 

In  the  Washington  Monument,  Washington,  D.  C,  the  normal 
pressure  on  the  lower  joint  of  the  walls  of  the  shaft  is  20.2  tons 
per  sq.  ft.  (280  lbs.  per  sq.  in.),  and  the  maximum  pressure  brought 
upon  any  joint  under  the  action  of  the  wind  is  25.4  tons  per  sq.  ft. 
(350  lbs.  per  sq.  in.).]] 

The  pressure  on  the  limestone  piers  of  the  St.  Louis  Bridge  was, 
before  completion,  38  tons  per  sq.  ft.  (527  lbs.  per  sq.  in.);  and  after 
completion  the  pressure  was  19  tons  per  sq.  ft.  (273  lbs.  per  sq.  in.) 
on  the  piers  and  15  tons  per  sq.  ft.  (108  lbs.  per  sq.  in.)  on  the  abut- 
ments.^ 

The  limestone  masonry  in  the  towers  of  the  Niagara  Suspension 

*  In  this  connection  it  is  convenient  to  remember  that  1  ton  per  square  foot  is 
equivalent  nearly  to  14  (exactly  13.88)  pounds  per  square  inch, 
f  The  Technograph,  University  of  Illinois,  No.  7,  p.  27. 
X  Proc.  Inst,  of  C.  E.,  vol.  x.  p.  241. 

§  F.  CoUingwood,  asst.  engineer,  in  Trans.  Am.  Soc.  of  C.  E. 
II  Report  of  Col.  T.  L.  Casey,  U.  S.  A. ,  engineer  in  charge. 
\  History  of  St.  Louis  Bridge,  pp.  370-74. 


MEASUREMENT   OF   MASONRY.  151 

Bridge  failed  under  36  tons  per  sq.  ft.,  and  were  taken  down, — how- 
ever, the  masonry  was  not  well  executed.  * 

At  the  South  Street  Bridge,  Philadelphia,  the  pressure  on  the 
limestone  rubble  masonry  in  the  pneumatic  piles  is  15.7  tons  per 
sq.  ft.  (220  lbs.  per  sq.  in. )  at  the  bottom  and  12  tons  per  sq.  ft.  at 
the  top.  ''  This  is  unusually  heavy,  but  there  are  no  signs  of  weak- 
ness."! The  maximum  pressure  on  the  rubble  masonry  (laid  in 
cement  mortar)  of  some  of  the  large  masonry  dams  is  from  11  to  14 
tons  per  sq.  ft.  (154  to  195  lbs.  per  sq.  in.).  The  Quaker  Bridge 
Dam  is  designed  for  a  maximum  pressure  of  16f  tons  per  sq.  ft. 
(230  lbs.  per  sq.  in.)  on  massive  rubble  masonry  in  best  hydraulic 
cement  mortar.  J 

223.  Safe  Pressure.  In  the  light  of  the  preceding  examples 
it  may  be  assumed  that  the  safe  load  for  the  different  classes  of 
masonry  is  about  as  follows,  provided  each  is  the  best  of  its  class  : 

Concrete 5  to  15  tons  per  square  foot. 

Rubble, 10  to  15     "      " 

Squared  stone, 15  to  20     "      "        "        " 

Limestone  ashlar,      .     .     .     .  20  to  25     "      "         "         " 

Granite  ashlar, 30     "      "        "        " 

224.  Measurement  of  Masonry.  The  method  of  determining 
the  quantity  of  masonry  in  a  strttcttire  is  frequently  governed  l)y 
trade  rules  or  local  custom,  and  these  vary  greatly  with  locality. 
Masons  have  voluminotts  and  arbitrary  rules  for  the  measurement 
of  masonry;  for  example,  the  masons  and  stone-ctttters  of  Boston 
at  one  time  adopted  a  code  of  thirty-six  complicated  rules  for  the 
measurement  of  hammer-dressed  granite.  As  an  example  of  the 
indefiuiteness  and  arbitrariness  of  all  such  rules,  we  quote  the  follow- 
ing, which  are  said  to  be  customary  in  Pennsylvania :  "  All  open- 
ings less  than  3  feet  wide  are  counted  solid.  All  openings  more 
than  3  feet  wide  are  taken  out,  but  18  inches  is  added  to  the 
running  measurement  for  every  jamb  built.  Arches  are  counted 
solid  from  the  spring  of  the  arch,  and  nothing  allowed  for  arching. 
The  corners  of  buildings  are  measured  twice.  Pillars  less  than  3  feet 
square  are  counted  on  three  sides  as  lineal  measurement,  multiplied 
by  the  fourth  side  and  depth;  if  more  than  3  feet,  the  two  opposite 

*  Trans.  Am.  Soc.  of  C.  E.,  vol.  xvii.  pp.  204^13.  t  Und.,  vol.  vli.  pp.  305-6. 

X  Engineering  Xeivs,  vol.  xix.  p.  75. 


STONE   MASONRY.  [CHAP.  VII. 


sides  are  taken;  to  each  side  18  inches  for  each  jamb  is  added  to 
lineal  measurement  tliereof ;  the  whole  multiplied  by  the  smaller  side 
and  multiplied  by  the  depth." 

A  well-established  custom  has  all  the  force  of  law,  unless  due 
notice  is  given  to  the  contrary.  The  more  definite,  and  therefore 
better,  method  is  to  measure  the  exact  solid  contents  of  the  masonry, 
and  pay  accordingly.  In  "net  measurement"  all  openings  are  de- 
ducted; in  "gross  measurement"  no  openings  are  deducted. 

The  quantity  of  masonry  is  usually  expressed  in  cubic  yards. 
The  perch  is  occasionally  employed  for  this  purpose;  but  since  the 
supposed  contents  of  a  perch  vary  from  16  to  25  cubic  feet,  the  term 
is  very  properly  falling  into  disuse.  The  contents  of  a  masonry 
structure  are  obtained  by  measuring  to  the  neat  lines  of  the  design. 
If  a  wall  is  built  thicker  than  specified,  no  allowance  is  made  for  the 
masonry  outside  of  the  limiting  lines  of  the  design;  but  if  the 
masonry  does  not  extend  to  the  neat  lines,  a  deduction  is  made  for 
the  amount  it  falls  short.  Of  course  a  reasonable  working  allow- 
ance must  be  made  when  determining  whether  the  dimensions  of 
the  masonry  meet  the  specifications  or  not. 

In  engineering  construction  it  is  a  nearly  uniform  custom  to 
measure  all  masonry  in  cubic  yards;  but  in  architectural  construc- 
tion it  is  customary  to  measure  water  tables,  string-courses,  etc., 
by  the  lineal  foot,  and  window-sills,  lintels,  etc.,  by  the  square  foot. 
In  engineering,  all  dressed  or  cut-stone  work,  such  as  copings,  bridge 
seats,  cornices,  water-tables,  etc.,  is  paid  for  in  cubic  yards,  with 
an  additional  price  per  square  foot  for  the  surfaces  that  are  dressed, 
cut,  or  bush-hammered. 

225.  Classification  of  Railroad  Masonry.  The  stone  masonry 
required  in  the  construction  of  a  railroad  is  usually  classified  about 
as  follows:  first-class  masonry,  second-class  masonry,  rubble  masonry 
(sometimes  called  third-class  masonry,  §209),  rubble  masonry  laid 
dry  (§  214),  stone  paving,  slope-walls,  and  riprap.  First-class  ma- 
sonry is  equivalent  to  ashlar  (§§  200-7) ;  this  head  generally  includes 
bridge  abutments  and  piers  of  the  larger  class,  and  arch  culverts  of 
greater  span  than  10  feet.  Sometimes  second-class  masonry  is  speci- 
fied as  squared-stone  masonry  (§§  208-12),  and  sometimes  as  superior 
rubble  (§§  213-17);  it  is  used  in  less  important  structures  than  first- 
class  masonry. 

Frequently  specifications  recognize  also  the  following  classifica- 


ESTIMATES    OF    COST.  153 


tion  :  first-class  arcli  masonry,  second-class  arch  masonry,  first-class 
bridge-pier  masonry,  second-class  bridge-pier  masonry,  and  pedestal 
masonry.  The  quality  of  work  thus  specified  is  the  same  as  for  first- 
class  and  second-class  masonry  respectively,  the  only  difference 
being  peculiar  to  the  form  of  the  masonry  structure,  as  will  be  dis- 
cussed 171  succeeding  chapters.  The  specifications  for  each  structure 
should  give  the  quantities  of  each  kind  of  masonry. 

For  complete  specifications  for  railroad  masonry,  see  Appendix  I. 

226.  Estimates  of  Cost  of  Masonry.  The  following  estimates 
of  the  cost  of  masonry,  from  Trautwine's  Engineer's  Pocket-hook,* 
are  pronounced  by  experts  to  be  as  accurate  as  such  averages  can 
be  stated,  since  every  item  is  liable  to  great  variation.  The  estimates 
are  based  on  the  assumption  that  a  mason  receives  $3.50  and  a 
laborer  $2.00  per  day  of  8  hours. 

227.  "  Quarrying. f  After  the  preliminary  expenses  of  purchas- 
ing the  site  of  a  good  quarry,  cleaning  off  the  surface  earth  and 
disintegrated  top  rock,  and  providing  the  necessary  tools,  trucks, 
cranes,  etc.,  the  total  net  expenses  fox  getting  out  the  rough  stone 
for  masonry  ready  for  delivery  may  be  roughly  estimated  thus : 
Stones  of  such  size  as  two  men  can  readily  lift,  measured  in  piles, 
will  cost  per  cubic  yard  from  i  to  ^  the  daily  wages  of  a  quarry 
laborer.  Large  stones,  ranging  from  ^  to  1  cubic  yard  each,  got  out 
by  blasting,  from  1  to  2  daily  wages  per  cubic  yard.  Larger  stones, 
ranging  from  1  to  1^  cubic  yards  each,  in  which  most  of  the  work 
must  be  done  by  wedges  in  order  that  the  individual  stones  shall 
come  out  in  tolerably  regular  shape  and  conform  to  stipulated  dimen- 
sions, from  2  to  4  daily  wages  per  cubic  yard.  The  lower  prices  are 
low  for  sandstone,  while  the  higher  ones  are  high  for  granite.  Under 
ordinary  circumstances,  about  1^  cubic  yards  of  good  sandstone  can 
be  quarried  at  the  same  cost  as  1  of  granite — or,  in  other  words, 
calling  the  cost  of  granite  1,  that  of  sandstone  will  be  f ;  hence  the 
means  of  the  foregoing  limits  may  be  regarded  as  rather  full  prices 
for  sandstone,  rather  scant  for  granite,  and  about  fair  for  limestone 
or  marble. 

228.  "  Dressing.^  In  the  first  place,  a  liberal  allowance  should 
be  made  for  waste.  Even  when  the  stone  wedges  out  handsomely 
on  all  sides  in  large  blocks  of  nearly  the  required  shape  and  size, 

*  Published  by  permission. 
+  See  Note  1,  Appendix  II. 
X  See  Notes  2  and  3,  Appendix  II. 


154  STONE    MASONRY.  [CHAP.  VII. 

from  ^  to  :|^  of  the  rough  block  will  generally  not  more  than  cover 
waste  of  dressing.  In  moderate-sized  blocks  (say  averaging  about 
^  a  cubic  yard  each)  got  out  by  blasting,  from  i  to  ^  will  not  be 
too  much  for  stone  of  medium  character  as  to  straight  splitting. 
The  last  allowance  is  about  right  for  well-scabbled  dressing.  The 
smaller  the  stones  the  greater  must  be  the  allowance  for  waste.  In 
large  operations  it  becomes  expedient  to  have  the  stones  dressed,  as 
far  as  possible,  at  the  quarry,  in  order  to  diminish  the  cost  of  trans- 
portation, which,  when  the  distance  is  great,  constitutes  an  impor- 
tant item — especially  when  by  land  and  on  common  roads. 

229.  "  Ashlar.  Average  size  of  the  stones,  say  5  feet  long,  2 
feet  wide,  and  1.4  feet  thick — or  two  such  stones  to  a  cubic  yard. 
Then,  supposing  the  stone  to  be  of  granite  or  gneiss,  the  cost  per 
cubic  yard  of  ashlar  facing  will  be  : 

"Getting  out  the  stone  from  the  quarry  by  blasting,  allow- 
ing i  for  waste  in  dressing,  1^  cubic  yards  at  f  3.00 

per  yard $4  00 

Dressing  14  sq.  ft.  of  face  at  35  cents, 4  90 

Dressing  52  sq.  ft.  of  beds  and  joints  at  18  cents,     ...       9  36 

Net  cost  of  the  dressed  stone  at  the  quarry,    .     .     .  $18  26 
Hauling  (say  1  mile),  loading,  and  unloading,     ....       1  20 

Mortar,  say, 40 

Laying,  including  scaffold,  hoisting  machinery,  etc.,       .       2  00 

Net  cost |21  86 

Profit  to  contractor,  say  15  per  cent., 3  28 

Total  cost  per  cubic  yard, $25  14 

"  Dressing  will  cost  more  if  the  faces  are  to  be  rounded  or 
moulded.  If  the  stones  are  smaller  than  we  have  assumed,  there 
will  be  more  square  feet  per  cubic  yard  to  be  dressed.  If,  in  the 
foregoing  case,  the  stones  be  perfectly  well  dressed  on  all  sides,  in- 
cluding the  back,  the  cost  per  cubic  yard  would  be  increased  about 
$10;  and  if  some  of  the  sides  be  curved,  as  in  arch  stones,  say  $12 
or  $14;  and  if  the  blocks  be  carefully  wedged  out  to  given  dimen- 
sions, $16  or  $18.  Under  these  conditions  the  net  cost  of  the 
dressed  stone  at  the  quarry  will  be  $28,  $31,  and  $35  per  cubic  yard, 
respectively. 

"If  the  stone  be  sandstone  with  good  natural  beds,  the  getting 
out  may  be  put  at  $3.00  per  cubic  yard.     Face  dressing  at  26  cents 


MARKET    PRICE    OF   STONE.  155 

pel  sq,  IL,  say  13.64  per  cu.  yd.  Beds  and  joints  at  13  cents  per 
sq.  It.,  say  ^6'.?6  per  cu.  yd.  The  total  cost,  then,  is  $19.55  instead 
of  $25.14  for  granite,  and  the  net  cost  $17.00  instead  of  the  $21.86 
per  cu.  yd.  for  granite.  The  total  cost  of  large,  well-scabbled,  ranged 
sandstone  masonry  in  mortar  may  be  taken  at  about  $10  per  cu.  yd. 
230.  "  Rubble.  With  stones  averaging  about  |-  cubic  yard  each, 
and  common  labor  at  $1  per  day,  the  cost  of  granite  ruhhle,  such 
as  is  generally  used  as  backing  for  the  foregoing  ashlar,  will  be  about 
as  follows  : 

Getting  out  the  stone  from  the  quarry  by  blasting,  allow- 
ing I  for  vaste  in  scabbling,  1|  cu.  yds.  @  $3.00,      .     $3  43 

Hauling  1  mile,  loading  and  unloading, 1  20 

Mortar  (2  cu.  ft.,  or  1.6  struck  bushels  of  quicklime,  and 
10  cu.  ft.  or  8  struck  bushels  of  sand  or  graveJ,  and 
mixing), 1  50 

Scabbling,  laying,  scaffolding,  hoisting  machinery,  etc.,         2  50 


Net  cost, $8  63 

Profit  to  contractor,  say  15  per  cent., 1  30 

Total  cost  per  cubic  yard, $9  93 

'*  Common  ruihle  of  small  stones,  the  average  size  being  such  as 
two  men  can  handle,  costs  to  get  it  out  of  the  quarry  about  80  cts. 
per  yard  of  pile,  or,  to  allow  for  waste,  say  $1.00.  Hauling  ]  mile, 
$1.00.  It  can  be  roughly  scabbled  and  laid  for  $1. 20  more.  Mortar, 
as  above,  $1.50.  Total  net  cost,  $4.70;  or  with  15  per  cent,  profit, 
$5.40,  at  the  above  wages  for  labor." 

231.  Market  Price  of  Stone.  The  average  market  quotations 
to  builders  and  contractors  for  the  year  1888  were  about  as  follows, 
f.o.h.  (free  on  board)  at  the  quarry  : 

Granite— rough $0  40  to  $0  50  per  cubic  foot. 

Limestone — common  rubble,   ...  1  00  "  1  50  per  cubic  yard. 

"            good  range  rubble,    .     .  1  50  "  2  00     "        "         " 

"            bridge  stone 08  "  10  per  cubic  foot, 

"            dimension  stone,   ...  25  "  35    "        "         " 

"             copings, 20  "  35    "        "         " 

Sandstone, 35  "  1  00  per  cubic  yard. 

232.  Cost  of  Masonry.*  TJ.  S.  Public  Buildings.  The  following 
table  gives  the  average  contract  price  during  the  past  few  years  for 
cutting  the  stone  for  the  United  States  government  buildings :  f 

*  For  additional  data,  see  Notes  1-6,  Appendix  II,  pages  511-16. 
t  American  Architect,  vol.  xxii.  pp.  6,  7. 


156 


STONE   MASOIfEY. 


[chap.   VII. 


TABLE  15. 
Cost  op  Cutting  Stone  for  U.  S.  Public  Bueldings. 


Kind  op  Surface. 

Granite. 

Marble. 

Limestone  and 
Sandstone. 

Min. 

Max. 

Min. 

Max. 

Min. 

Max. 

Beds  and  joints,  per  sq.  f  t. . . . 
Pean-hammered,  "    "    "  ... 
Plain  face,  6-cut,  "    "    "  . . . 

$0  30 
45 

$0  35 
50 
65 
75 

88 
1  10 

$0  20 
30 

10  25 
35 

$0  12 
15 

$0  15 
20 

"    8-cut,  "    "    "  ... 

"  10-cut,  "    "    "  ... 

"  12-cut,  "    "    "  ... 

Rubbed,                "    "    "  ... 

40 
50 

20 
25 

25 

Tooled,                 "    "    "  ... 

30 

The  following  table  shows  the  contract  price  for  the  masonry  of 
the  United  States  public  buildings  : 

TABLE  16. 
Cost  of  Masonry  in  U.  S.  Public  Buildings. 


Kind  of  Work. 


Random  rubble,  limestone 

"  "        sandstone 

Squared  masonry,  sandstone 

Coursed  masonry,  sandstone 

Squared  masonry,  limestone 

"  "  granite 

Rock-face  ashlar,        "      

"        "        "     and  cut-stone  granite,  avg. 

Cut  granite,  basement  and  area  walls 

Rock-face  ashlar,  and  cut  and  moulded  trim- 
mings, Stony  Point.  Mich.,  sandstone.  . 

Trimmings.  Bedford  limestone,  bid 

Rock-face  ashlar,  granite,  retaining  wall . . . 
Dressed  coping,         "  "  "   ... 

White  sandstone,— furnished  only 

Armijo        "  "  " 

Cut  and  moulded  sandstone  of  superstructure 
"         average  bid. . . . 
"     "  "       limestone,  lowest  bid  ....  . 

average  bid 

Rock-face   ashlar,    cut   and   moulded  trim- 
mings, Middlesex  brownstoue 

Cut  and  moulded,  Bedford  limestone 

"  "  .sandstone 

"       "  "  limestone 

"  "  sandstone 

"  granite,  superstructure 


Harrisburg,  Va 
Cincinnati.  O.. 
Denver,  Col. . . . 
Pittsburgh,  Pa. 

Columbus,  O. . . 
Memphis,  Tenn 
Pittsburgh,  Pa. 


Fort  Wayne,  Ind. . 

Memphis,  Tenn. . . 

Dallas,  Tex 

Denver,  Col 

Council  Bluffs,   la. 


Cost 
Date.  I    per 
Cu.  Ft. 


Rochester,  N.  Y. 
Loiiisville,  Ky. . . 

Dallas,  Tex 

Hannibal,  Mo.. . . 
Des  Moines,  la. . 
Pittsburgh,  Pa... 


1885 
1884 
1883 
1886 
1885 
1885 
1884 
1886 
1886 
1886 
1886 

1885 
1885 
1886 
1886 
1885 
1885 
1885 
1885 
1885 
1885 

1884 
1885 
1885 
1885 
1887 
1886 


$0  20 
20 
20 
35 
60 
70 
68 
30 
1  38 

1  60 

2  00 


1  52 
1  65 

1  00 

2  50 
35 
73 

1  91 

2  VI 

1  87 

2  33 

2  41 
2  00 
2  46 

1  83 

2  27 

3  00 


ACTUAL   COST.  157 


233.  Railroad  Masonry.  Tlie  following  are  tlie  average  prices 
actually  paid  in  the  construction  of  the  Cincinnati  Southern  Rail- 
road, in  1873-77  :  * 

First-class  bridge  masonry,  per  cu.  yd.,       .     .     .     .     •    .  $10  39 

Second-class  bridge  masonry,  in  cement,  per  cu.  yd.,    .  7  40 

Second-class  bridge  masonry,  dry,  per  cu.  yd.,          .     .     .  7  02 

First-class  arch  masonry,  per  cu.  yd., 11  24 

Second-class  arch  masonry,  in  cement,  per  cu.  yd.,  ...  8  61 

Second-class  arch  masonry,  (fr^/,  per  cu.  yd., 7  75 

Brick-work  in  tunnels,  per  cu.  yd 8  50 

Brick-work  in  buildings,  per  cu.  yd., 7  00 

Box-culvert  masonry,  in  cement,  per  cu.  yd., 4  89 

Box-culvert  masonry,  dry,  per  cu.  yd., 4  32 

Concrete,  percu.  yd., 5  52 

Slope  walls,  per  cu.  yd., 4  41 

Stone  paving,  per  cu.  yd. , 2  41 

234.  Tunnel  Masonry.  The  following  are  the  average  pricesf 
paid  in  1883-87  on  the  new  Croton  Aqueduct  tunnel  which  su^jplies 
New  York  City  with  water.  The  mortar  was  2  sand  to  1  Eosendale 
cement. 

Dimension-stone  masonry  (granite), $42  50 

Brick-work  lining,  per  cu.  yd., 10  14 

Brick-work  backing,  per  cu.  yd. 8  49 

Rubble  masonry,  lining,  per  cu.  yd 5  05 

Concrete  lining,  3  stone  to  1  Roscudale  cement,  per  cu.  yd.,  5  67 

Concrete  lining,  5  stone  to  1  Roseudale,  per  cu.  yd.,    .     .  5  16 

Concrete  backing,  3  stone  to  1  Roseudale,  per  cu.  yd.,       .  4  73 

Concrete  backing,  5  stone  to  1  Roseudale,  per  cu.  yd.,      .  4  22 

Fine-hammered  face  (6-cut)  for  cut  stone,  per  sq.  ft.,  ,     .  84 

Rough-poiuted  face  for  cut  stone,  per  sq.  ft.,  ....  50 
Additional   for  all  kinds  of  masonry  laid  in  Portland 

cement  mortar,  2  to  1,  per  cu.  yd., 1  78 

Additional  for  all  kinds  of  masonry  laid  in   Rosendale 

cement  mortar,  1  to  1,  per  cu.  yd., 1  20 

235.  Bridge-pier  Masonry.  The  following  are  the  details  of  the 
cost,  to  the  contractor,  of  heavy  first-class  limestone  masonry  for 
bridge-piers  erected  in  1887  by  a  prominent  contracting  firm  : 


*  Report  of  the  Chief  Engineer,  December  1,  1877,  Exhibit  3. 
t  Report  of  the  Commissioners,  Table  4. 


158  STONE   MASONET.  [CHAP.   VIL. 

Cost  of  stone  (purchased), $4  50 

Sand  and  cement, 52 

Freight, 1  79 

Laying, 1  40 

Handling  materials 65 

Derricks,  tools,  etc., 40 

Superintendence,  office  expense,  etc., 68 

Total  cost  per  cubic  yard $9  94 

The  following  data  coucerning  the  cost  in  1887  of  granite  piera 
— two  fifths  cut-stone  facing  and  three  fifths  rubble  backing — ^are 
furnislied  by  the  same  firm.     The  rock  was  very  hard  and  tough. 

Facing : — 

Quarrying,  including  opening  quarry, $3  75 

Cutting  to  dimensions, 6  75 

Laying, 1  76 

Transportation  2  miles,  superintendence,  and  general  ex- 
penses,     .         2  05 

Total  cost  per  cubic  yard,  ....         ...  $14  31 

Backing : — 

Quarrying, $3  10 

Dressing 3  60 

Laying,       1  75 

Sundries, 2  05 

Total  cost  per  cubic  yard, $10  50 

The  first-class  limestone  masonry  in  the  piers  of  the  bridges 
across  the  Missouri  at  Plattsmouth  (1879-80)  cost  the  company 
$18.60  per  cubic  yard,  exclusive  of  freight,  engineering  expenses, 
and  tools.*  The  cost  of  first-class  masonry  in  smallei  piers  usually 
ranges  from  $12  to  $14  per  cubic  yard. 

At  Chicago  in  1887  the  contract  price  for  the  masonry  in  bridge 
piers  and  abutments  was  about  as  follows :  Concrete,  1  Portland 
cement,  3  sand,  6  broken  stone,  $9.00  per  en.  yd.;  concrete,  1 
natural  cement,  3  sand,  5  broken  stone,  $6.00  per  cu.  jd.;  stone 
facing  and  coping,  $30.00  per  cu.  yd. 

236.  Arch-culvert  Masonry.  The  following  are  the  details  of 
tiie  cost  of  the  sandstone  arch  culvert  (613  cu.  yds.)  at  Nichols 
Hollow,  on  the  Indianapolis,    Decatur  and   Springfield   Railroad, 

*  Report  of  the  Chief  Engineer,  Geo.  S.  Morison. 


ACTUAL   COST. 


J53 


built  iu  1887.  Scale  of  wages  per  day  of  10  hours — foreman, 
$3.50  ;  cutters,  $3.00  ;  mortar  mixer,  11.50 ;  laborer,  $1.25  ;  water- 
boy,  50  cents  ;  carpenters,  $2.50.  f 

TABLE  17. 

Actual  Cost  op  Akch  Masonky  on  Indianapolis,  Decatur  and  Spring 

FIELD  Railroad. 


Cost. 


Items. 


Materials  : — 

Stone — 613  cu.  yds.  of  sandstone  @  $1  50 

Cement— 130  bbls.  German  Portland®  $3  17  =  $412  50 
40  "  English  "  @  3  25  =  130  00 
80    "      Louisville  "        @        96  =      28  75 

Sand— 7  car-loads®  $5  50 

Total  for  materials 

Cutting : — 

Cutters  and  helpers 

Templates,  bevels,  straight-edges,  etc 

Repairs  of  cutters'  tools 

Water-boy  

Total  for  cutting 

Laying  : — 

Masons,  110  days  @  $3.50 

Masons'  helpers 

Mortar  mixer 

Water-boy 

Arch  centers,  building  and  erecting 

Derrick,  stone  chute,  etc 

Laying  track 

Total  for  laying 

Pointing 

Grand  Total  : 

Total  for  labor 

Total  for  materials 

Total  cost  of  masonry 


$1,370  48  12  24 
11  00         01 


52  39 
11  75 


$1,445  62 


$384  87 

453  66 

121  72 

11  75 

37  65 

14  63 

7  70 


$1,032  08 


$30  00 


$2,507  60 
1,529  25 


09 
02 


$2  36 


$0  63 
74 
20 
02 
06 
02 
01 


$1  68 


$0  05 


$4  09 
2  50 


$4,036  85  $6  59 


238.  Snmmary  of  Cost.  The  following  table,  compiled  from  a 
large  amount  of  data,  will  be  convenient  for  hasty  reference.  Of 
course  any  such  table  must  be  used  with  caution,  since  such  items 
are  subject  to  great  variation. 

+  Data  furnished  by  Edwin  A.  Hill,  chief  engineer. 


160 


STONE   MASONRY. 


[chap.  VII. 


TABLE  18. 
Summary  of  Cost  of  Masonry. 


Description  of  3Iasonry. 


Arch  masonry,  first-class 

Arch  masoniy,  second-class  (iu  cement). 

Box-culvert  masonry,  iu  cement 

Brick  masonry  (see  g  258) 

Bridge  masonry,  first-class 

Bridge  masonry,  second-class  (in  cement) 

Concrete 

Coping 

Dimension-stone  masonry,  granite 

Paving 

Slope-wall  masonry 

Squared-stone  masonry 

Riprap 

Rubble,  first-class 

Rubble,  second-class  (in  cement) 


Cost  per  Cubic  Yard. 


Min. 


Max.     Average. 


$12  00 
10  00 

5  00 
10  00 
20  00 
12  00 

6  00 
14  00 
60  00 

4  00 

5  00 
10  00 

2  50 

6  00 
5  00 


110  00 
8  00 

3  50 
8  00 

14  00 
10  00 

4  00 
12  00 
50  00 

2  00 

3  00 
7  00 
1  50 

5  00 
3  00 


CHAPTER  Vlli. 
BRICK  MASONRY. 

239.  MORTAE.  Lime  mortar  is  generally  employed  lor  brick 
masonry,  particularly  in  architectural  constructions.  Many  of  the 
leading  railroads  lay  all  brick  masonry  in  cement  mortar,  and  the 
practice  should  be  followed  more  generally.  The  weakest  part  of 
a  brick  structure  is  the  mortar.  The  primary  purpose  of  the 
mortar  is  to  form  an  adhesive  substance  between  the  bricks  ;  the 
second  is  to  form  a  cushion  to  distribute  the  pressure  uniformly 
over  the  surface.  If  the  mortar  is  weaker  than  the  brick,  the 
ability  of  the  masonry  to  resist  direct  compression  is  thereby  con- 
siderably reduced.  For  the  reason,  see  §  13;  for  the  amount,  see 
the  Table  19,  page  164. 

If  the  strains  upon  a  wall  were  only  those  arising  from  a  direct 
pressure,  the  strength  of  the  mortar  would  in  most  cases  be  of 
comparatively  little  importance,  for  the  crushing  strength  of  aver- 
age quality  mortar  is  far  higher  than  the  dead  load  which  under 
ordinary  circumstances  is  put  upon  a  wall ;  but,  as  a  matter  of  fact, 
in  buildings  the  load  is  rarely  that  of  a  direct  crushing  weight, 
other  and  more  important  strains  being  developed  by  the  system  of 
construction.  Thus  the  roof  tends  to  throw  the  walls  out,  the  rafters 
being  generally  so  arranged  as  to  produce  a  considerable  outward 
thrust  against  the  wall.  The  action  of  the  wind  also  produces  aside 
strain  which  is  practically  of  more  importance  than  either  of  the 
others.  In  many  cases  the  contents  of  a  building  exert  an  outward 
thrust  upon  the  walls  ;  for  example,  barrels  piled  against  the  sides 
of  a  warehouse  produce  an  outward  pressure  against  the  walls. 

In  many  brick  constructions  the  use  of  cement  mortar  is  abso- 
lutely necessary — as,  for  example,  in  tall  chimneys,  where  the  bear- 
ing is  so  small  that  great  strength  of  the  cementing  material  is 
required. 

240.  The  thickness  of  the  mortar-joints  should  be  about  i  to  f 
of  an  inch.  Thicker  joints  are  very  common,  but  should  be  avoided. 
If  the  bricks  are  even  fairly  good,  the  mortar  is  the  weaker  part  of 

161 


162  BRICK    MASONRY.  [CHAP.  VIII. 

the  wall ;  hence  the  less  mortar  the  better.  Besides,  a  thin  layer 
of  mortar  is  stronger  under  compression  than  a  thick  one  (see  §  15). 
The  joints  should  be  as  thin  as  is  consistent  with  their  insuring  a  uni- 
form bearing  and  allowing  rapid  work  in  spreading  the  mortar.  The 
joints  of  outside  walls  should  be  thin  in  order  to  decrease  the  dis- 
integration by  weathering.  The  joints  of  inside  walls  are  usually 
made  from  |  to  ^  inch  thick. 

Brick  should  not  be  merely  laid,  but  every  one  should  be  rubbed 
and  pressed  down  in  such  a  manner  as  to  force  the  mortar  into  the 
pores  of  the  bricks  and  produce  the  maximum  adhesion  ;  with  quick- 
setting  cement  this  is  still  more  important  than  with  lime  mortar. 
For  the  best  work  it  is  specified  that  the  brick  shall  be  laid  with  a 
^' shove  joint ;"  that  is,  that  the  brick  shall  first  be  laid  so  as  to 
project  over  the  one  below,  and  be  pressed  into  the  mortar,  and 
then  be  shoved  into  its  final  position. 

Lime  mortar  is  liable  to  work  out  of  the  joints,  owing  to  the 
action  of  the  elements  and  to  changes  of  temperature.  Hence  it 
is  customary  either  (1)  to  lay  the  face  in  mortar  containing  more 
lime  than  that  used  for  the  interior,  or  (2)  to  lay  the 
face  in  a  mortar  containing  more  or  less  cement,  or 
"^  (3),  in  rare  cases,  to  point  the  joints  with  neat  cement 
mortar.  Whatever  the  kind  of  mortar  used,  the  finish 
of  the   face  of  the   joint   is   important.     The   most 

Fia.  47.  -  durable  joint  is  finished  as  shown  in  Fig.  47,  although, 
unfortunately  for  durability,  it"  is  customary  to  make  the  slope  in 
the  opposite  direction. 

241.  Since  brick  have  great  avidity  for  water,  it  is  best  to 
dampen  them  before  laying.  If  the  m.ortar  is  stiff  and  the  brick 
dry,  the  latter  absorb  the  water  so  rapidly  that  the  mortar  does 
not  set  properly,  and  will  crumble  in  the  fingers  when  dry.  Neglect 
in  this  particular  is  the  cause  of  most  of  the  failures  of  brick-work. 
Since  an  excess  of  water  in  the  brick  can  do  no  harm,  it  is  best  to 
thoroughly  drench  them  with  water  before  laying.  Lime  mortar  is 
sometimes  made  very  thin,  so  that  the  brick  will  not  absorb  all  the 
water.  This  process  interferes  Avith  the  setting  of  the  mortar,  and 
particularly  with  the  adhesion  of  the  mortar  to  the  brick.  Watery 
mortar  also  contracts  excessively  in  drying  (if  it  ever  does  dry), 
which  causes  undue  settlement  and,  possibly,  cracks  or  distortion, 
Wetting  the  brick  before  laying  will  also  remove  the  dust  from  the 
surface,  which  otherwise  would  prevent  perfect  adhesion. 


BOND. 


163 


1  '  1    1  '  1    1  '  1    1  '  1  1      1    1 

1  '  1    1  '  1    1    1    1    1  1     II 

1  '  1    1  '  1    1    1    1    1  1      II 

1    1    1     1    1    1    1    1   1      II 

1  '  1    1  '  1    1  '  1    1  '  1  1     1    1 

Fig.  48. — English  Bond. 


242.  SONO.  The  bricks  used  in  a  given  wall  being  of  uniform 
size  are  laid  according  to  a  uniform  system,  which  is  called  the  bond 
of  the  brick-work.  As  in  ashlar  masonry,  so  in  brick-work,  a  lieader 
is  a  brick  whose  length  lies  perpendicular  to  the  face  of  the  wall; 
and  a  stretcher  is  one  whose  length  lies  parallel  with  the  face. 
Brick  should  be  made  of  such  a  size  that  two  headers  and  a  mortar- 
joint  will  occupy  the  same  length  as  a  stretcher. 

243.  English  Bond.     This  consists  in  laying  entire  courses  of 

headers  and  stretchers,  which  some- 
times alternate,  as  in  Fig.  48;  but 
generally  only  one  course  of  headers 
is  laid  for  every  two,  three,  four,  etc., 
courses  of  stretchers.  In  ordinary 
practice  the  custom  is  to  lay  four  to  six 
courses  of  stretchers  to  one  of  head- 
ers. The  stretchers  bind  the  walls 
together  lengthwise  ;  the  headers,  crosswise.  The  proportionate 
numbers  of  the  courses  of  headers  and  stretchers  should  depend  on 
the  relative  importance  of  transverse  and  longitudinal  strength. 
The  proportion  of  one  course  of  headers  to  two  of  stretchers  is  that 
which  gives  equal  tenacity  to  the  wall  lengthwise  and  crosswise. 

In  building  brick-work  in  English  bond,  it  is  to  be  borne  in 
mind  that  there  are  twice  as  many  vertical  or  side  Joints  in  a  course 
of  headers  as  there  are  in  a  course  of  stretchers ;  and  that  unless 
in  laying  the  headers  great  care  be  taken  to  make  these  joints  very 
thin,  two  headers  will  occupy  a  little  more  space  than  one  stretcher, 
and  the  correct  breaking  of  the  joints — exactly  a  quarter  of  a  brick — ■ 
will  be  lost.  This  is  often  the  case  in  carelessly  built  brick-work,  in 
which  at  intervals  vertical  joints  are  seen  nearly  or  exactly  above 
each  other  in  successive  courses. 

244.  Flemish  Bond.  This  consists  of  a  header  and  a  stretcher 
alternately  in  each  course,   so  placed 

that  the  outer  end  of  each  header 
lies  on  the  middle  of  a  stretcher  in 
the  course  below  (Fig.  49).  The 
number  of  vertical  joints  in  each 
course  is  the  same,  so  that  there  is  no 
risk  of  the  correct  breaking  of  the 
joints  by  a  quarter  of  a  brick  being 
lost;  and  the  wall  presents  a  neater  appearance  than  one  built  in 


Fig.  49.— Flemish  Bond. 


164 


BEICK   MASONRY. 


[chap.  Till. 


English    bond.       The    latter,    however,    when   correctly   built,    is 
stronger  and  more  stable  than  Flemish  bond. 

245.  Hoop-iron  Bond.  Pieces  of  hoop-iron  are  frequently  laid 
flat  in  the  bed-joints  of  brick-work  to  increase  its  longitudinal 
tenacity,  about  2  inches  of  the  ends  of  each  piece  being  bent  down 
and  inserted  into  the  vertical  joints.  Although  thin  strips  of  iron 
are  generally  employed,  it  would  be  better  to  use  thicker  pieces  ;  the 
value  of  the  iron  for  this  purpose  depends  wholly  upon  the  rigidity 
of  the  ends  which  are  turned  down,  and  this  will  vary  about  as 
the  square  of  the  thickness.  The  strip  of  iron  should  be  nearly 
as  thick  as  the  mortar-joint.  This  means  of  strengthening  masonry 
is  frequently  employed  over  openings  and  to  connect  interior  brick 
walls  Avith  stone  fronts. 

246.  Compressive  Strength  of  Brick  Masonry.  Experi- 
ments at  AVatertown,  Mass.,  with  the  United  States  testing-machine, 
upon  piers  12  inches  square  and  from  1  ft.  4  in.  to  10  ft.  high,  gave 
results  as  follows  :* 


TABLE  19. 

Strength  of  Brick  Masonrt  compared  with  that  op  the  Brick  and 

THE  Mortar. 


« 

B-2-fe 

b 

g  S  o 

O    . 

z;  B 

w  a 

Pu, 

T  P!  Z 

H 

U  n 

n 

«ss 

HO 

?> 

Strength  of  the 

z^ 

b 

«^  . 

Pier  in  terms 

"Z  ^ 

a 

°u 

O  H  Z; 

OF  THE  Strength 

5f: 

s 

W  o 

^"^ 

OF  THE  Brick. 

Hh    b 

Composition  op  the  Mortab. 

u 

0" 

Sa« 

a  ^ 

6 

B. 

5^ 

"ksS 

B  a 

1<5 

w 

f  as 

fc  m  a  -< 

5^ 

u 

°  5     « 

o  s 

H 

o 

«»< 

n  o  ^iri 

a  a 

Bi 

5  m 

&  CD  B  « 

g  a  J  H 
g  n      K 

ft. 

a 

52 

S  ==  K  K 

Min, 

Max. 

Mean. 

« 

^ 

P 

02 

m 

1 

1  lime,  3  sand 

15 

1,508 

124 

.06 

.18 

.10 

}9, 

2 

2  mortar  (1  lime,  3  sand),  1  Rosen- 

dale  cement 

1 

1,646 

183 

.11 

9 

3 

2  mortar  (1   lime,  3  sand),  1   Port- 

land cement 

1 
1 

8 

1,411 
1,972 
2,544 

192 
162 
545 

.09 
.13 
.17 

7 

4 

1  Rosendale  cement,  2  sand 

1  Portland  cement,  2  sand  

^9, 

5 

.10 

.27 

4.7 

fi 

Clear  Rosendale 

521 
3,483 

7 

Clear  Portland  cement 

1 

2,375 

.16 

0  7 

*  Report  on  "  Tests  of  Metals,  etc.,"  for  the  year  ending  June  30, 1884,  pp.  69-122. 


COMPRESSIVE    STRENGTH.  165 

The  brick  had  an  average  strength  of  nearly  15,000  lbs.  per  sq. 
in.,  tested  flatwise  between  steel.  The  mortar  was  14^  months  old 
when  it  was  tested.  The  piers  were  built  by  a  common  mason,  with 
only  ordinary  care;  and  they  were  from  a  year  and  a  half  to  two 
years  old  when  tested.  Their  strength  varied  with  their  height; 
and  in  a  general  way  the  experiments  show  that  the  strength  of  a 
prism  10  ft.  high,  laid  in  either  lime  or  cement  mortar,  is  about  two 
thirds  that  of  a  1-foot  cube.  A  deduction  derived  from  so  few 
experiments  (22  in  all)  is  not,  however,  conclusive.  The  different 
lengths  of  the  piers  tested  occurred  in  about  equal  numbers.  The 
piers  began  to  show  cracks  at  one  half  to  two  thirds  of  their  ultimate 
strength. 

In  attempting  to  draw  conclusions'  from  any  experiments,  it 
must  be  borne  in  mind  continually  that  the  result  of  a  single  trial 
may  possibly  be  greatly  in  error.  In  this  case  this  precaution  is 
very  important,  since  the  difference  between  experiments  apparently 
exactly  alike  was  in  some  cases  as  much  as  50  per  cent.  A  great 
variation  in  the  results  is  characteristic  of  all  experiments  on  stone, 
brick,  mortar,  etc.  Except  on  the  ground  of  a  variation  in  ex- 
periments, it  is  difficult  to  explain  why  mortar  No.  4  is  weaker  than 
No.  2,  while  the  masonry  is  stronger  ;  or  why  the  masonry  of  No.  5 
is  stronger  than  that  of  No.  7. 

Of  course  the  apparent  efficiency  of  the  masonry,  as  given  in  the 
table,  depends  upon  the  manner  in  which  the  strengths  of  the 
brick  and  mortar  were  determined,  as  well  as  upon  the  method  of 
testing  the  masonry.  For  example,  if  the  brick  had  been  tested  on 
end  the  apparent  efficiency  of  the  masonry  would  have  been  con- 
siderably more  ;  or  if  the  mortar  had  been  tested  in  thin  sheets  the 
strength  of  the  masonry  relative  to  that  of  the  mortar  would  not 
have  been  so  great.* 

247.  Some   German   experimentsf  gave  results  as  in  the  table 

*  It  should  be  mentioned  that  the  mortar  with  which  these  piers  were  built  appears 
to  be  much  weaker  than  similar  mortar  under  like  conditions.  (Compare  page  72, 
and  pages  126,  166, 188, 197  of  the  Report  of  Tests  of  Metals,  etc.,  made  at  Watertown 
in  1884.)  Ordinarily,  mortar  is  eight  to  ten  times  as  strong  in  compression  as  in 
tension,  whereas  the  first  six  mortars  in  the  preceding  table  were  but  little  stronger 
in  compression  than  such  mortar  should  have  been  in  tension.  The  officer  in  charge 
is  " unable  to  offer  any  explanation.  The  cement  was  bought  on  the  market;  the 
maker's  name  is  not  known.  The  cement  was  not  tested."  However,  the  experi- 
ments are  consistent  with  themselves,  and  therefore  show  relative  strengths  correctly. 
+  Van  Nostrand's  Engin'g  Mag.,  vol.  xxxiv.  p.  240,  from  the  Abstracts  of  the 
Inst,  of  C.  E.  (London),  vol.  79,  p.  376. 


166 


BRICK    MASOISTRY. 


[chap.    VIII. 


below.  It  is  not  stated  how  the  strength  of  the  brick  or  of  the 
masonry  was  determined.*  The  term  cement  refers  to  Portland 
cement.  According  to  the  building  regulations  of  Berlin,  the  safe 
load  for  brick  masonry  is  one  tenth  of  the  results  in  the  table. 

TABLE  20. 
Relative  Strength  op  Brick  and  Brick  Masonry. 


;Avkrage  Crush- 
ing Strength 
OF  Brick,  in  lbs. 

PER  SQ.  IN. 

Ultimate  Strength,  in  lbs.  per  sq.  in.,  of 
Brick-work  with  Mortar  composed  op— 

Kind  op  Brick. 

1  Lime, 

2  Sand. 

7  Lime, 
1  Cement, 
16  Sand. 

1  Cement, 
6  Sand. 

1  Cement, 
3  Sand. 

Clinker  stock 

5,390 
3,669 
2,930 
2,759 
2,617 
1,195 

2,370 
1,620 
1,290 
1,210 
1,150 
580 

2,590 
1,760 
1.390 
1,320 
1,250 
570 

2,960 
2,020 
1,610 
1,520 
1,440 
650 

3  410 

Selected     "     

2,820 
1  850 

Ordinary  "     

Perforated 

1  710 

Porous  

1,650 
750 

Porous  perforated 

Table  19  shows  conclusively  that  the  strength  of  brick  masonry 
is  mainly  dependent  upon  the  strength  of  the  mortar.  An  in- 
crease of  50  per  cent,  in  the  strength  of  the  brick  shows  no 
appreciable  effect  on  the  strength  of  the  masonry.  Notice, 
however,  that  the  masonry  in  the  fifth  line  of  Table  19  is  70  per 
cent,  stronger  than  that  in  the  first,  due  to  the  difference  between 
a  good  Portland  cement  mortar  and  the  ordinary  lime  mortar. 
In  Table  20  notice  that  brick  laid  in  a  1  to  3  Portland  cement 
mortar  is  nearly  50  per  cent,  stronger  than  in  a  1  to  2  lime 
mortar.  Similar  experiments  f  show  that  masonry  laid  in  mortar 
composed  of  1  part  Rosendale  cement  and  2  parts  sand  is  56 
per  cent,  stronger  than  when  laid  in  mortar  composed  of  1  part 
lime  and  4  parts  sand.  A  member  of  the  Institute  of  Civil  Engi- 
neers (London)  says|  that  brick-work  laid  in  lime  is  only  one  fourth 
as  strong  as  when  laid  in  clear  Portland  cement.  Probably  the  dif- 
ference in  durability  between  cement  mortar  and  lime  mortar  is 
considerably  greater  than  their  difference  in  strength. 

*  If  the  strength  of  the  brick  (in  any  line  of  the  table)  be  represented  by  100,  that 
of  the  masonry  is  44,  48,  55,  and  63,  respectively,  which  shows  that  the  values  in  the 
table  were  not  derived  directly  from  experiments. 

t  Report  of  Experiments  on  Building  Materials  for  the  City  of  Philadelphia  with 
the  U.  S.  testing-machine  at  Watertown,  Mass.,  pp.  32,  38. 

X  Proc.  Inst,  of  C.  E.,  vol.  xvii.  p.  441. 


TRANSVERSE    STRENGTH.  167 

248.  Pressure  allowed  in  Practice.  The  pressure  at  the  base  of 
a  brick  shot-tower  in  Baltimore,  246  feet  high,  is  estimated  at  6-^ 
tons  per  sq.  ft.  (about  90  lbs.  per  sq.  in.).  The  pressure  at  the  base 
of  a  brick  chimney  at  Glasgow,  Scotland,  468  ft.  high,  is  estimated 
at  9  tons  per  sq.  ft.  (about  125  lbs.  per  sq.  in.);  and  in  heavy  gales 
this  is  increased  to  15  tons  per  sq.  ft.  (210  lbs.  per  sq.  in.)  on  the 
leeward  side.  The  leading  Chicago  architects  allow  10  tons  per  sq. 
ft.  (140  lbs.  per  sq.  in.)  on  the  best  brick- work  laid  in  1  to  2  Port- 
land cement  mortar  ;  8  tons  for  good  brick-work  in  1  to  2  Rosendale 
cement  mortar  ;  and  5  tons  for  ordinary  brick- work  in  lime  mortar. 
Ordinary  brick  piers  have  been  known  to  bear  40  tons  per  sq.  ft. 
(560  lbs.  per  sq.  in.)  for  several  days  without  any  sign  of  failure. 

Tables  19  and  20  appear  to  show  that  present  practice  is  very 
conservative  with  regard  to  the  pressure  allowed  on  brick  masonry. 
According  to  Table  19  (page  164),  the  ultimate  strength  of  the  best 
brick  laid  in  ordinary  lime  mortar  is  110  tons  per  sq.  ft.;  if  laid 
in  1  to  2  Portland  cement  mortar,  180  tons  ;  and  by  Table  20  (page 
166)  the  strength  of  ordinary  brick  in  1  to  2  lime  mortar  is  100  tons 
per  sq.  ft.,  and  in  1  to  3  Portland  cement  mortar  140  to'^s.  From 
the  above,  it  would  seem,  that  reasonably  good  brick  laid  in  good 
lime  mortar  should  be  safe  under  a  pressure  of  20  tons  per  sq.  ft., 
and  that  the  best  brick  in  good  Portland  cement  mortar  should  be  safe 
under  30  tons  per  sq.  ft.  The  nominal  pressure  allowed  upon  brick 
masonry  depends  upon  the  kind  of  materials  employed  ;  the  degree  of 
care  with  which  it  is  executed  ;  whether  it  is  for  a  temporary  or  per- 
manent, an  important  or  unimportant  structure  ;  and,  it  may  be 
added,  the  care  with  which  the  nominal  maximum  load  is  estimated. 

249.  Transverse  Strength  of  Brick  Masonry.  Masonry  is 
seldom  employed  where  any  strain  except  direct  compression  will 
come  upon  it,  but  sometimes  it  is  subject  to  transverse  strain.  The 
transverse  strength  of  brick-work  depends  theoretically  upon  the 
tensile  strength  of  the  brick  and  upon  the  adhesion  and  cohesion 
of  the  mortar,  but  practically  the  strength  of  the  mortar  deter- 
mines the  strength  of  the  masonry.  For  example,  in  the  case  of 
a  high  wall  whose  upper  portion  is  overthrown  by  a  lateral  force  or 
pressure  of  any  kind,  the  failure  is  due  either  (1)  to  the  breaking  of 
the  adhesion  in  the  bed- joints  and  of  the  cohesion  of  the  side-jofnts, 
or  (2)  to  the  ru^jture  of  the  mortar  in  the  bed-joints  alone.  The 
latter  method  of  failure,  however,  is  improbable,  since  the  cohesion 


J  68  BRICK    MASONRY.  [CHAP.  VUL 

of  cement  mortars  is  always  much  greater  than  their  adhesion  (com- 
pare §§  134  and  137);  and  hence,  in  estimating  the  resistance  of  the 
wall  to  overturning,  it  becomes  necessary  to  fix  values  for  both  the 
cohesive  and  adhesive  strength  of  the  mortar  at  the  time  when  the 
structure  is  first  exposed  to  the  action  of  the  lateral  force  or  pres- 
sure, and  also  to  ascertain  the  relative  areas  of  beds  and  side-joints 
in  the  assumed  section  of  rupture.  In  good  brick-work  the  aggre- 
gate area  of  the  side-joints,  in  any  section  parallel  to  tlie  beds,  will 
amount  to  about  one  seventh  of  the  total  area  of  such  section. 
Hence,  when  the  masonry  is  liable  to  be  subjected  to  transverse 
strains  the  adhesive  strength  of  the  mortar  is  more  important  than 
its  cohesive  strength. 

The  adhesion  of  mortar  to  brick  or  stone  has  already  been  dis- 
cussed (§  137).  While  the  experiments  uniformly  show  a  relatively 
low  adhesive  power,  it  is  well  known  that  when  old  walls  are  de- 
molished the  adhesion  of  even  common  lime  mortar  is  found  to  be 
very  considerable.  Although  the  adhesive  power  of  mortar  may  be 
small  as  compared  with  its  tensile  strength,  good  brick  masonry  has 
a  considerable  transverse  strength. 

Experiments  made  under  the  author's  direction  *  indicate  that 
brick  beams  bonded  as  regular  masonry  have  a  modulus  of  rupture 
equal  to  about  twice  the  tensile  strength  of  the  mortar  when  built 
with  ordinary  care,  and  about  three  times  when  built  with  great  care. 
When  the  beams  are  constructed  as  piers,  i.  e.,  with  no  interlocking 
action,  the  modulus  of  rupture  is  about  equal  to  the  tensile  strength 
of  the  mortar. 

250.  Application.  To  illustrate  the  practical  application  of  the 
fact  that  brick-work  has  a  transverse  strength,  let  it  be  required  to 
compute  the  strain  which  may  come  upon  a  lintel,  or  girder  used 
to  support  a  brick  wall  over  an  opening,  f 

Let   H  =  the  height,  in  feet,  of  the  wall  above  the  opening ; 
H„  =  the  height,  in  feet,  of  the  wall  that  produces  a  maxi- 
mum strain  on  the  lintel ; 
Hs  =  the  height,  in  feet,  of  the  masonry  when  it  will  just 
support  itself  over  the  opening  ; 
8  =  the  span,  in  feet ; 
t  =  the  thickness,  in  feet,  of  the  wall ; 

*  The  Technogkaph,  University  of  Illinois,  No.  7  (1892-93),  pp.  29-37. 
t  The  principle  of  the  following  computations  is  from  an  editorial  in  Engineering 
(London),  vol.  xiv.  pp.  44  and  72. 


TRAXSYERSE    STRENGTH.  169 

R  =■  the  modulus  of  rupture,  in  pounds  per  square  inch, 
of  the  brick-work  ; 

pr=  the  weight,  in  pounds,  of  a  cubic  foot  of  the  wall. 
W  varies  from  100  to  140  pounds,  and  for  conven- 
ience is  here  assumed  to  be  144  ;  the  error  is  always 
on  the  safe  side  ; 

Ml  =  the  bending  moment  on  the  lintel,  in  pounds  per 
square  inch. 

Consider  the  masonry  as  a  beam  fixed  at  both  ends  and 
loaded  uniformly.  Then,  by  the  principles  of  the  resistance  of 
materials,  when  the  masonry  is  just  self-supporting,  one  twelfth 
of  the  weight  of  the  wall  above  the  opening  muUvplied  by  the 
span  is  equal  to  one  sixth  of  the  tensile  strength  multiplied  by 
the  thickness  and  also  the  square  of  the  depth  of  the  wall. 
The  weight  of  the  wall  above  the  opening  is  W  8  H^  t.     Hence 


^{WSHJ)S=\{lUR)tH:, (1) 

or 

"'-is (^) 

Notice  that  the  weight  of  the  wall  over  any  given  opening 
increases  as  the  height,  while  the  resistance  increases  as  the, 
square  of  the  height.  The  height  for  which  the  masonry  isl 
self-supporting  is  given  by  equation  (2) ,  for  a  height  greater' 
than  Hg  the  masonry  would  be  more  than  self-supporting  ;  and 
for  a  height  less  than  H^  the  masonry  would  need  extraneous 
support. 

To  find  the  height  of  the  wall  producing  a  maximum  stress 
in  the  lintel,  notice  that  the  bending  moment  on  the  lintel  is 
equal  to  the  moment  of  the  load  minus  the  moment  of  the 
resistance  of  the  brick-work  over  the  opening ;  or,  in  algebraic 
language, 

Ml  =  -^{WSHt)  S  -  U^UE)iH\ 


170  BRICK    MASONRY.  [CHAP.  VIII. 

Differentiating  the  above  equation,  regarding  Jf,  and  H  as  the 
variables,  and  finding  the  maximum  value  of  H  in  the  usual  way, 
we  get 


The  fact  that  the  value  of  H^  in  equation  (3)  is  one  half  of 
that  of  Hg  in  equation  (2),  shows  that  the  maximum  stress  on 
the  lintel  occurs  when  the  height  of  the  wall  is  half  of  its  self- 
supporting  height,  at  which  time  one  half  of  the  wall  will  be 
self-supporting  and  one  half  will  require  extraneous  support. 
Or,  in  other  words,  the  greatest  stress  on  a  lintel  due  to  a  wall 
of  any  height  Avill  not  be  greater  than  that  due  to  a  distributed 
load  of 

i  WH„,St  =  i  W^.  St  =  nearly  18  '^  pounds.     .     (4) 

251.  Examplex.  To  apply  the  above  formula,  assume  that  it  is 
proposed  to  cut  a  10-foot  opening  through  an  old  brick  wall,  and 
that  it  is  desirable  to  know  whether  the  brick-work  will  be  self-sup- 
porting, the  wall  rising  40  feet  above  the  top  of  the  opening.  Sub- 
stituting the  above  data  in  equation  (2)  gives 

(10)' 
40  =  \-w^ ',     or     i?  =  1.25  lbs.  per  sq.  in. 

Hence,  to  be  self-supporting  across  the  opening,  the  wall  must  be 
capable  of  supjsorting  a  tensile  strain  of  1.25  pounds  per  square 
inch.  It  would  be  poor  lime  mortar  that  would  not  bear  eight  or 
ten  times  this.  Notice  that  if  the  wall  were  only  4  feet  high  over 
the  opening,  instead  of  40  feet,  as  above,  the  strength  required 
would  be  12.5  pounds  ]Der  square  inch. 

For  another  illustration,  assume  that  a  brick  wall  1  foot  thick 
is  to  be  built  over  a  10-foot  opening,  and  that  we  wish  to  know 
whether  a  timber  10  inches  deep  and  12  inches  wide  will  sustain  the 
load.  Assuming  the  beam  as  being  fixed  at  the  ends,  the  timber 
will  sustain  a  uniformly  distributed  load  of  10  tons  with  a  deflection 
of  one  twelfth  of  an  inch.  This  is  equivalent  to  the  entire  weight 
of  the  wall  when  14  feet  high.     If  the  wall  is  to  be  carried  higher 


TRAXSTERSE    STREiJinTH.  171 

than  this,  the  girder  must  be  supported  temporarily,  or  time  must 
be  given  for  the  mortar  to  set. 

However,  before  the  wall  is  14  feet  above  the  opening,  the  brick- 
work at  the  bottom  will  have  attained  some  strength,  and  therefore 
the  load  on  the  girder  will  not  be  as  great  as  above.  The  average 
strength  of  the  brick-work  will  always  be  at  least  the  average  between 
the  strength  at  the  top  and  the  bottom  ;  that  is,  the  average  strength 
will  always  be  more  than  half  of  that  at  the  bottom.  Since  10  tons 
is  the  maximum  load  allowed  on  the  girder,  and  since  the  maximum 
load  which  comes  upon  it  is  half  of  the  entire  weight  of  the  masonry 
above  the  opening,*  the  timber  will  receive  its  maximum  load  when 
the  wall  is  twice  14  feet,  or  28  feet,  above  the  opening.  The  masonry 
may  be  run  up  28  feet  without  necessitating  any  extraneous  support 
for  the  lintel,  provided  time  enough  is  allowed  for  the  mortar  to 
develop  the  average  tensile  strength  found  by  substituting  in  (4) 
the  maximum  load  allowed  on  the  girder,  and  solving  for  R.  Mak- 
ing this  substitution  gives 

18  riOV  1 
20000  =  — ^-~ — ,  from  which  R  =  0.90  lb.  per  sq.  in. 
ft 

With  an  average  strength  of  0.90  lb.  per  sq.  in.,  the  wall  will 
become  self-supporting  when  55  feet  above  the  opening. 

252.  Custom  differs  as  to  the  manner  of  estimating  the  pressure 
on  a  girder  due  to  a  superincumbent  mass  of  masonry.  One  extreme 
consists  in  assuming  the  masonry  to  be  a  fluid,  and  taking  the  load 
on  the  lintel  as  the  weight  of  all  the  masonry  above  the  opening. 
The  opposite  extreme  consists  in  assuming  the  pressure  to  be  the 
weight  of  the  masonry  included  in  a  triangle  of  which  the  open- 
ing is  the  base  and  whose  sides  make  45°  with  this  line.  Both  of 
these  methods  differ  materially  from  the  one  discussed  above  ;  and 
neither  is  defensible.  As  the  wall  is  several  days  in  building,  the 
masonry  first  laid  attains  considerable  streugth  before  the  wall  is 
completed;  and  hence,  owing  to  the  cohesion  of  the  mortar,  the  final 
weight  on  the  girder  can  not  be  equal  to  or  compared  with  any  fluid 
Folume. 

The  principle  involved  in  the  second  method  would  be  applicable 

*  See  discussion  of  equation  (3),  above. 


172  BRICK    MASONRY.  [CHAP.  VIII. 

to  a  wall  composed  wholly  of  perfectly  smootli  bricks.  In  a  dry 
wall,  the  angle  which  the  side  lines  make  with  the  base  would 
depend  upon  the  bond  and  upon  the  relative  length  and  breadth  of 
the  bricks.     Assuming  the  boundary  lines  to  make  an  angle  of  45° 

with  the  base,  the  method  gives  a  load  -^  times     that    (§   250) 

which  takes  account  of  the  transverse  strength  of  the  masonry,  i.  e., 
the  frictional  and  tensile  resistance  of  the  wall.  If  E  is  relatively 
large  and  S  is  small,  this  fraction  will  be  more  than  unity,  under 
which  conditions  the  second  method  is  safe.  But  if  E  is  small  and 
;iS^  is  large,  then  this  fraction  is  less  than  one,  Avhich  shows  that 
under  these  conditions  the  second  method  is  unsafe. 

The  method  of  §  250  is  quite  simple  and  perfectly  general.  The 
substantial  correctness  of  this  method,  illustrated  in  §  251,  is 
proven  by  the  fact  that  large  openings  are  frequently  cut  through 
walls  without  providing  any  extraneous  support ;  and  also  by  the 
fact  that  walls  are  frequently  supported  over  openings  on  timbers 
entirely  inadequate  to  carry  the  load  if  the  masonry  did  not  have 
considerable  strength  as  a  beam.  The  discussion  in  §  251  also  makes 
clear  why  frequently  a  temporary  support  is  sufficient.  After  the 
masonry  has  been  laid  a  short  time,  the  strength  of  the  mortar 
causes  it  to  act  as  a  beam.  The  discussion  also  shows  the  advantage 
of  using  cement  mortar  (or  better,  quick-setting  cement  mortar) 
when  it  is  desired  that  the  masonry  shall  early  become  self-sup- 
porting. 

253.  Measukement  of  Beick-WORK.  The  method  of  determin- 
ing the  quantity  of  brick  masonry  is  governed  by  voluminous  trade 
rules  or  by  local  customs,  which  are  even  more  arbitrary  than  those 
for  stone  masonry  (§  224,  which  see). 

The  quantity  is  often  computed  in  perches,  but  there  is  no  uni- 
formity of  understanding  as  to  the  contents  of  a  perch.  ■  It  ranges 
from  16|  to  25  cubic  feet. 

Brick-work  is  also  often  measured  by  the  square  rod  of  exterior 
surface.  No  wall  is  reckoned  as  being  less  than  a  brick  and  a  half 
in  thickness  (13  or  13^  inches),  and  if  thicker  the  measurement  is 
still  expressed  in  square  rods  of  this  standard  thickness.  Unfor- 
tunately the  dimensions  adopted  for  a  square  rod  are  variable,  the 
following  values  being  more  or  less  customai-y  :  16^  feet  square  or 


DATA   FOR   ESTIMATES,  173 

272i  square  feet,  18  feet  square  or  324  square  feet,  and  16-j  square 
feet. 

The  volume  of  a  brick  is  sometimes  used  as  a  unit  in  stating:  the 
contents  of  a  wall.  The  contents  of  the  Avail  are  found  by  multi- 
plying the  number  of  cubic  feet  in  the  wall  by  the  number  of  brick 
which  it  is  assumed  make  a  cubic  foot ;  but  as  the  dimensions  of 
brick  vary  greatly  (see  §  62),  this  method  is  objectionable.  A  cubic 
foot  is  often  assumed  to  contain  20  brick,  and  a  cubic  yard  600. 
The  last  two  quantities  are  frequently  used  interchangeably,  although 
the  assumed  volume  of  the  cubic  yard  is  thirty  times  that  of  the 
cubic  foot. 

Brick-work  is  also  sometimes  measured  by  allowing  a  certain 
number  of  brick  to  each  superficial  foot,  the  number  varying  with 
the  thickness  of  the  wall.  A  4-inch  wall  (thickness  =  width  of  one 
brick)  is  frequently  assumed  to  contain  7  bricks  per  sq.  ft. ;  a  9-inch 
wall  (thickness  =  width  of  two  bricks),  14  bricks  per  sq.  ft.;  a  13- 
inch  wall  (thickness  =  width  of  three  bricks),  21  bricks  per  sq.  ft., 
€tc. ;  the  number  of  brick  j^er  square  foot  of  the  face  of  the  wall 
being  seven  times  the  thickness  of  the  wall  in  terms  of  the  width  of 
a  brick. 

254.  The  only  relief  from  such  arbitrary,  uncertain,  and  indefi- 
nite customs  is  to  specify  that  the  masonry  will  be  paid  for  by  the 
cubic  yard, — gross  or  net  measurement,  according  to  the  structure 
or  the  preference  of  the  engineer  or  architect. 

In  engineering  the  uniform  custom  is  to  measure  the  exact  solid 
contents  of  the  wall. 

255.  Data  for  Estimates.  Number  of  Brick  Required.  Since 
the  size  of  brick  varies  greatly  (§  62),  it  is  impossible  to  state  a  rule 
which  shall  be  equally  accurate  in  all  localities.  If  the  brick  be  of 
standard  size  (8jX4x25-  inches),  and  laid  with  ^-  to  f-inch  joints, 
a  cubic  yard  of  masonry  will  require  about  410  brick;  or  a  thousand 
brick  will  lay  about  2j  cubic  yards.  If  the  joints  are  \-  to  f-inch,  a 
cubic  yard  of  masonry  will  require  about  495  brick;  or  a  thousand 
brick  will  lay  about  2  cubic  yards.  "With  face  brick  (8f  X  4^  x  2i 
inches)  and  ^-inch  joints,  a  cubic  yard  of  masonry  will  require  about 
496  brick;  or  a  thousand  face  brick  will  lay  about  2  cubic  yards. 

In  making  estimates  for  the  number  of  bricks  required,  an  al- 
lowance must  be  made  for  breakage,  and  for  waste  in  cutting  brick 
to  fit  angles,  etc.     With  good  brick,  in  massive  work  this  allowance 


17-i  BRICK   MASONRY.  [CHAP.  VIII. 

need  not  exceed  1  or  2  per  cent.;  but  in  buildings  3  to  5  per  cent, 
is  none  too  much. 

256.  Amount  of  Mortar  Required.  The  proportion  of  mortar 
to  brick  will  vary  with  the  size  of  the  brick  and  with  the  thickness 
of  the  joints.  With  the  standard  size  of  brick  (8^x4x2^  inches), 
a  cubic  yard  of  masonry,  laid  with  |-  to  f-inch  joints,  will  require 
from  0.35  to  0.40  of  a  cubic  yard  of  mortar;  or  a  thousand  brick 
will  require  0.80  to  0.90  of  a  cubic  yard.  If  the  joints  are  i  to  f 
inch,  a  cubic  yard  of  masonry  will  require  from  0.25  to  0.30  of  a 
cubic  yard  of  mortar;  or  a  thousand  brick  will  require  from  0.45  to 
0.55  of  a  cubic  yard.  If  the  joints  are  ^  of  an  inch,  a  cubic  yard  of 
masonry  will  require  from  0.10  to  0.15  of  a  cubic  yard  of  mortar; 
or  a  thousand  brick  will  require  from  0.15  to  0.20  of  a  cubic  yard. 

With  the  above  data,  and  the  table  on  page  86,  the  amount  of 
cement  and  sand  required  for  a  specified  number  of  brick,  or  for  a 
given  number  of  yards  of  masonry,  can  readily  be  determined. 

257.  Labor  Required.  "  A  bricklayer,  with  a  laborer  to  keep  him 
supplied  with  materials,  will  lay  on  an  average,  in  common  house- 
walls,  about  1,500  bricks  per  day  of  10  working  hours;  in  the  neater 
outer  faces  of  brick  buildings,  from  1,000  to  1,200;  in  good  ordinary 
street  fronts,  from  800  to  1,000  ;  and  in  the  very  finest  lower-story 
faces  used  in  street  fronts,  from  150  to  300  according  to  the  number 
of  angles,  etc.  In  plain  massive  engineering  work,  he  should  aver- 
age about  2,000  bricks  per  day,  or  4  cu.  yds.  of  masonry ;  and  in 
large  arches,  about  1,500,  or  3  cu.  yds.''* 

In  the  United  States  Government  buildings  the  labor  per  thou- 
sand, including  tools,  etc.,  is  estimated  at  seven  eighths  of  the  wages 
for  ten  hours  of  mason  and  helper. 

Table  21,  opposite,  f  gives  the  actual  labor,  per  cubic  yard,  re- 
quired on  some  large  and  important  jobs. 

258.  Cost.  In  the  construction  of  the  Cincinnati  Southern  R.  E., 
during  1873-77,  the  brick  lining  of  tunnels  cost  S8.50  per  cu.  yd.; 
brick- work  in  buildings,  $7.00.1  The  average  price  paid  for  the 
brick-work  in  the  new  Croton  Aqueduct  tunnel,  which  supplies  New 
York  City  with  water,  was,  including  everything,  $10.14  per  cu.  yd. 


*  Trautwine's  Engineer's  Pocket-Book,  p.  671. 

+  Trans.  Am.  Soc.  of  C.  E. 

X  Report  of  the  Chief  Engineer,  Dec.  1,  1877,  Exhibit  3. 


BPECIFICATIONS.  175 


TABLE  21. 
Labor  required  for  Brick  Masonry. 


Location  and  Description  op  the  Masonry. 


Work  required,  ih 
Days  per  Cubic  Tasd 


High  Bridge  Enlargement,  N.  Y.  City — 

Lining  wall  and  flat  arches  laid  with  very  close  joints. 

Washington  (D.  C.)  Aqueduct — 

Circular  conduit,  9  feet  in  diameter  with  walls  12 
inches  thick 


St.  Louis  Water  Works — 

Semi-circular  conduit,  6  feet  in  diameter. 


Kew  York  City  Storage  Reservoir — 

Lining  of  gate-house  walls  and  arches — rough  work . . 


0.714 

0.439 
0.364 

0.304 


for  lining,  and  18.49  for  backing.  The  mortar  was  composed  of 
1  part  Eosendale  natural  cement  and  2  parts  of  sand.* 

In  Chicago  in  188?.  the  price  of  brick  laid  in  lime  in  interior 
walls  was  about  ^11  per  thousand,  equivalent  to  about  87  per  cu.  yd. 
The  wages  of  masons  were  from  45  to  50  cents  per  hour,  and  of 
common  labor  from  20  to  25  cents  per  hour. 

259.  Specifications  for  Brick  Masonry.  For  Buildings. 
There  is  not  even  a  remote  approach  to  uniformity  in  the  specifica- 
tions for  the  brick-work  of  buildings.  Ordinarily  the  specifications 
for  the  brick  masonry  are  very  brief  and  incomplete.  The  following 
conform  closely  to  ordinary  construction.  Of  course,  a  higher  grade 
of  workmanship  can  be  obtained  by  more  stringent  specifications.! 

The  brick  in  the  exterior  walls  must  be  of  good  quality,  hard-burned;  fine, 
compact,  and  uniform  in  texture  ;  regular  in  shape,  and  uniform  in  size.| 
One  fourth  of  the  brick  in  the  interior  walls  may  be  what  is  known  as  soft 
or  salmon  brick  (see  2,  §  56).  The  brick  must  be  thoroughly  wet  before 
being  laid.  The  joints  of  the  exterior  walls  shall  be  from  i  to  |  inch  thick.§ 
The  joints  of  interior  division-walls  may  be  from  |  to  i  inch  thick.  The 
mortar  shall  be  composed  of  1  part  of  fresh,  well-slaked  lime  and  2|  to  3  parta 

*  Report  of  the  Aqueduct  Commission,  1883-87,  Table  4. 

t  For  specifications  for  masonry  for  various  purposes,  see  Appendix  I. 

J  See  §  57,  page  37. 

§  For  the  best  work,  omit  this  item  and  insert  the  following  :  T?ie  outside  woBm 
thdU,  be  faced  with  the  best  pressed  brick  of  uniform  color,  laid  in  colored  mortar,  wUk 
joints  not  exceeding  one  eighth  of  an  inch  in  thickness.  Face  brick  are  made  a  llttl» 
larger  (§  62)  than  ordinary  brick  to  compensate  for  the  thinner  joints. 


176  BRICK   MASOISTRY.  [CHAP.  VIII. 

of  clean,  sharp  sand.*  The  lime-paste  and  the  sand  shall  be  thoroughly 
mixed  before  being  used.  The  joints  shall  be  well  filled  with  the  above 
mortar  ;  no  grout  shall  be  used  in  the  work.  The  bond  must  consist  of  live 
courses  of  stretchers  to  one  of  headers,  and  shall  be  so  arranged  as  to  thor- 
oughly bind  the  exterior  and  interior  portions  of  the  wall  to  each  other. 

The  contractor  must  furnish,  set  up,  and  take  away  his  own  scaffolding  ; 
he  must  build  in  such  strips,  plugs,  blocks,  scantling,  etc.,  as  are  required  for 
securing  the  wood-work ;  and  must  also  assist  in  placing  all  iron-work,  as 
beams,  stairways,  anchors,  bed-plates,  etc.,  connected  with  the  brick-work. 

260.  For  Sewers.  The  following  are  the  specifications  employed, 
in  1885,  in  tlie  construction  of  brick  sewers  in  Washington,  D.  C.  : 

"  The  best  quality  of  whole  new  brick,  burned  hard  entirely  through,  free 
from  injurious  cracks,  with  true  even  faces,  and  with  a  crushing  strength  of 
not  less  than  5,000  pounds  per  square  inch,  shall  be  used,  and  must  be  thor- 
oughly wet  by  immersion  immediately  before  laying.  Every  brick  is  required 
to  be  laid  in  full  mortar  joints,  on  bottom,  sides,  and  ends,  which  for  each 
brick  is  to  be  performed  by  one  operation.  In  no  case  is  the  joint  to  be  made 
by  working  in  mortar  after  the  brick  has  been  laid.  Every  second  course  shall 
be  laid  with  a  line,  and  joints  shall  not  exceed  three  eighths  of  an  inch.  The 
brick-work  of  the  arches  shall  be  properly  bonded,  and  keyed  as  directed  by 
the  engineer.  No  portion  of  the  brick-work  shall  be  laid  drj'  and  afterwards 
grouted. 

"  The  mortar  shall  be  composed  of  cement  and  dry  sand,  in  the  proportion 
of  300  pounds  of  cement  and  2  barrels  of  loose  sand,  thoroughly  mixed  dry, 
and  a  sufficient  quantity  of  water  afterwards  added  to  form  a  rather  stiff  paste 
It  shall  be  used  within  an  hour  after  mixing,  and  not  at  all  if  once  set. 

"The  cement  shall  be  of  the  best  quality,  freshlj^  burned,  and  equal  in 
every  respect  to  the  Round  Top  or  Shepardstown  cement,  manufactured  upon 
the  formula  of  the  engineer-commissioner  of  the  District  of  Columbia,  capable 
of  being  worked  for  twenty  minutes  in  mortar  without  loss  of  strength,  and 
shall  be  tested  in  such  manner  as  the  engineer  may  direct.  After  being  mixed 
with  water,  allowed  to  set  in  air  for  twenty-four  hours,  and  then  immersed  in 
water  for  six  days,  the  tensile  strength  must  be  as  follows  : 

Neat  cement 95  lbs.  per  sq.  in 

One  part  cement  and  one  part  sand. ....... .56   "      "    "     " 

"       "  "  "    two  parts   "    22   "      "    "     " 

"    three  "       "   12   "      "    "     " 

'The  sand  used  shall  be  clean,  sharp,  free  from  loam,  vegetable  matter,  or 
Bther  dirt,  and  capable  of  giving  the  above  results  with  the  cement. 

"  The  water  shall  be  fresh  and  clean,  free  from  earth,  dirt,  or  sewerage. 

*  For  masonry  that  is  to  be  subjected  to  a  heavy  pressure,  omit  this  item  and 
insert  the  following  ;  The  mortar  must  be  composed  of  1  part  lime-paste,  1  part  cement, 
and  2  parts  of  clean,  sharp  sand.  Or,  if  a  heavier  pressure  is  to  he  resisted,  specify 
that  some  particular  £rrade  of  cement  mortar  is  to  be  used.     '.See  §§  246  and  247.» 


SPECIFICATIOXS.  177 


"  Tight  mortar-boxes  shall  be  provided  by  the  contractor,  and  no  mortar 
shall  be  made  except  in  such  boxes. 

"The  proportions  given  are  intended  to  form  a  mortar  in  which  every 
particle  of  sand  shall  be  enveloped  by  the  cement ;  and  this  result  must  be 
attained  to  the  satisfaction  of  the  engineer  and  under  his  direction.  The 
thorough  mixing  and  incorporation  of  all  materials  (preferably  by  machine 
labor)  will  be  insisted  upon.  If  by  hand  labor,  the  dry  cement  and  sand  shall 
be  turned  over  with  shovels  by  skilled  workmen  not  less  than  six  times  before 
the  water  is  added.  After  adding  the  water,  the  paste  shall  again  be  turned 
over  and  mixed  with  shovels  by  skilled  workmen  not  less  than  three  times  be- 
fore it  is  used." 

261.  For  Arches.  The  specifications  for  the  brick  arch  masonry 
on  the  Atchison,  Topeka  and  Santa  Fe  Railroad  are  as  follows  : 

"The  bricks  must  be  of  the  best  quality  of  smooth,  hard-burnt,  paving 
bricks,  well  tempered  and  moulded,  of  the  usual  size,  compact,  well  shaped, 
free  from  lime,  cracks,  and  other  imperfections,  and  must  stand  a  pressure 
of  4,000  pounds  per  square  inch  without  crushing.  No  bats  will  be  allowed 
in  the  work  except  for  making  necessary  closures.  All  bricks  will  be  culled 
on  the  ground  after  delivery,  and  selected  in  strict  accordance  with  these 
specifications. 

"The  mortar  must  be  made  of  1  measure  of  good  natural  hydraulic  cement 
and  2  measures  of  clean,  sharp  sand — or  such  other  proportion  as  may  be 
prescribed  by  the  engineer — well  mixed  together  with  clean  water,  in  clean 
mortar-beds  constructed  of  boards,  and  must  be  used  immediately  after  being 
mixed. 

"  The  brick  must  be  laid  flush  in  cement  mortar,  and  must  be  thoroughly 
wet  when  laid.  All  joints  and  beds  must  be  thoroughly  filled  with  mortar  so 
as  to  leave  no  empty  spaces  whatever  in  the  masonry  of  the  walls  and  arches, 
which  must  be  solid  throughout.  The  thickness  of  mortar- joints  must  be  as 
follows  :  In  the  walls  and  in  the  arch  between  bricks  of  the  same  ring,  not  less 
than  three  eighths  of  an  inch  (f ")  nor  more  than  one  half  inch  (|").  In  the  arch 
between  rings,  not  less  than  one  half  inch  (|")  nor  more  than  five  eighths  of 
an  inch  (f").  Each  brick  is  to  be  driven  into  place  by  blows  of  a  mallet.  The 
bricks  must  be  laid  in  the  walls  with  the  ordinary  English  bond,  five  stretcher 
courses  to  one  header  course.  They  must  be  laid  in  the  arch  in  concentric 
rings,  each  longitudinal  line  of  bricks  breaking  joints  with  the  adjoining 
lines  in  the  same  ring  and  in  the  ring  under  it.  No  headers  to  be  used  in 
the  arch." 

262.  Bbice  vs.  Stone  Masonry.  Brick  masonry  is  not  much 
used,  except  in  the  walls  of  buildings,  in  lining  tunnels,  and  in  con- 
structing sewers,  the  general  opinion  being  that  brick-work  is  in 
every  way  inferior  to  stone  masonry.  This  belief  may  have  been 
well  founded  when  brick  was  made  wholly  by  hand,  by  inexpert 
operatives,  and  imperfectly  burned  in  the  old-time  kilns,  the  prod- 


178  BRICK    MASONRY.  [CHAP.  VIII, 

uct  being  then  generally  poor  ;  but  things  have  changed,  and  since 
the  manufacture  of  brick  has  become  a  business  conducted  on  a 
large  scale  by  enterprising  men,  with  the  aid  of  a  variety  of  machines 
and  improved  kilns,  the  product  is  more  regular  in  size  and  quality 
and  stronger  than  formerly.  Brick  is  rapidly  displacing  stone  for 
the  largest  and  best  buildings  in  the  cities,  particularly  in  Chicago 
•  and  St.  Petersburg,  where  the  vicissitudes  of  the  climate  try  masonry 
very  severely.  There  are  many  engineering  structures  in  which 
brick  could  be  profitably  employed  instead  of  stone  ;  as,  for  example, 
the  walls  of  box-culverts,  cattle-guards,  etc.,  and  the  less  important 
bridge  piers  and  abutments,  particularly  of  highway  bridges. 

Brick-work  is  superior  to  stone  masonry  in  several  respects,  as 
follows  :  1.  In  many  localities  brick  is  cheaper  than  stone,  since 
the  former  can  be  made  near  by  while  the  latter  must  be  shipped. 
2.  As  brick  can  be  laid  by  less  skillful  masons  than  stone,  it  costs 
less  to  lay  it.  3.  Brick  is  more  easily  handled  than  stone,  and  can 
be  laid  without  any  hoisting  apparatus.  4.  Brick  requires  less  fit- 
ting at  corners  and  openings.  5.  Brick  masonry  is  less  liable  to 
great  weakness  through  inaccurate  dressing  or  bedding.  6.  Brick- 
work resists  fire  better  than  limestone,  granite,  or  marble,  sand- 
stone being  the  only  variety  of  stone  that  can  compare  with  brick 
in  this  respect.  7.  Good  brick  stands  the  efEect  of  weathering  and 
of  the  acids  in  the  atmosphere  better  than  sandstones,  and  in  dura- 
bility even  approaches  some  of  the  harder  stones  (see  §§  31-33). 
8.  All  masonry  fails  when  the  mortar  in  its  joints  disintegrates  or 
becomes  dislodged;  therefore  brick  masonry  will  endure  the  vicissi- 
tudes of  the  weather  as  Avell  as  stone  masonry,  or  even  better,  since 
the  former  usually  has  thinner  joints. 

Brick-work  is  not  as  strong  as  ashlar  masonry,  but  costs  less ;. 
while  it  is  stronger  and  costs  more  than  ordinary  rubble. 

263.  Beick  Masonry  Impervious  to  Water.  It  sometimes  be- 
comes necessary  to  prevent  the  percolation  of  water  through  brick 
walls.  A  cheap  and  effective  process  has  not  yet  been  discovered, 
and  many  expensive  trials  have  proved  failures.  The  following 
account*  gives  the  details  of  two  experiments  that  were  entirely  suc- 
cessful. 

"  The  face  walls  of  the  back  bays  of  the  gate-houses  of  the  new 

*  Abstract  of  a  paper  by  Wm.  L.  Dearborn,  in  Trans.  Am.  Soc.  of  C.  E. ,  toI.  L 
pp.  303-8' 


MASONRY  IMPEEVIOUS  TO   WATER.  179 

Croton  reservoir,  located  north  of  Eighty-sixth  Street,  in  Centi'al 
Park,  Xew  York  City,  were  built  of  the  best  quality  of  hard-burnt 
brick,  laid  in  mortar  composed  of  hydraulic  cement  of  New  York 
[Ulster  Co.  Rosendale]  and  sand  mixed  in  the  proportion  of  one 
measure  of  cement  to  two  of  sand.  The  space  between  the  walls  was 
4  feet,  and  was  filled  with  concrete.  The  face  walls  were  laid  up 
with  great  care,  and  every  precaution  was  taken  to  have  the  joints 
well  filled  and  to  insure  good  work.  The  Avails  are  12  inches  thick 
and  40  feet  high;  and  the  bays,  when  full,  generally  have  36  feet  of 
water  in  them. 

"  When  the  reservoir  was  first  filled  and  the  water  let  into  the 
gate-houses,  it  was  found  to  filter  through  these  walls  to  a  consider- 
able amount.  As  soon  as  this  was  discovered  the  water  was  drawn 
out  of  the  bays,  with  the  intention  of  attempting  to  remedy  or  pre- 
vent this  infiltration.  After  carefully  considering  several  modes  of 
accomplishing  the  object  desired,  I  [Dearborn]  came  to  the  conclu- 
sion to  try  '  Sylvester's  Process  for  Repelling  Moisture  fi-om  Exter 
nal  Walls.' 

"  The  process  consists  in  using  two  washes  or  solutions  for  cov- 
ering the  surface  of  the  walls — one  composed  of  Castile  soap  and 
water,  and  one  of  alum  and  water.  The  proportions  are  three 
quarters  of  a  pound  of  soap  to  one  gallon  of  water,  and  half  a  pound 
of  alum  to  four  gallons  of  water,  both  substances  to  be  perfectly 
dissolved  in  water  before  being  used.  The  walls  should  be  perfectly 
clean  and  dry,  and  the  temperature  of  the  air  not  below  50°  Fahr. 
when  the  comjjositions  are  applied, 

"  The  first,  or  soap-wash,  should  be  laid  on,  when  boiling  hot, 
with  a  flat  brush,  taking  care  not  to  form  a  froth  on  the  brick-work. 
This  wash  should  remain  24  hours,  so  as  to  become  dry  and  hard 
before  the  second,  or  alum,  wash  is  applied,  which  should  be  done 
in  the  same  manner  as  the  first.  The  temperature  of  this  wash, 
when  applied,  maybe  60°  or  70°  Fahr.;  and  this  also  should  remain 
24  hours  before  a  second  coat  of  the  soap-wash  is  put  on. 
These  coats  are  to  be  applied  alternately  until  the  walls  are  made 
impervious  to  water.  The  alum  and  soap  thus  combined  form  an 
insoluble  compound,  filling  the  pores  of  the  masonry  and  entirely 
"oreventing  the  water  from  entering  the  walls. 

"Before  applying  these  compositions  to  the  walls  of  the  bays 
some  experiments  were  made  to  test  the  absorption  of  water  by 


ISO  BKICK   MASONEY.  [CHAP.  YUt 

bricks  under  pressure  after  being  covered  with  these  washes,  in 
order  to  determine  how  many  coats  the  walls  would  require  to  render 
them  impervious  to  waiter.  To  do  this,  a  strong  wooden  box  large 
enough  to  hold  two  bricks  was  made,  put  together  with  screws,  and 
in  the  top  was  inserted  a  1-inch  pipe  40  feet  long.  In  this  box 
were  placed  two  bricks,  after  being  made  j)erfectly  dry,  which  were 
then  covered  with  a  coat  of  each  of  the  washes,  as  before  directed, 
and  weighed.  They  were  then  subjected  to  a  column  of  water  40 
feet  high  ;  and  after  remaining  a  sufficient  length  of  time  they  were 
taken  out  and  weighed  again,  to  ascertain  the  amount  of  water  they 
had  absorbed.  The  bricks  were  then  dried,  and  again  coated  with 
the  washes  and  weighed,  and  subjected  to  pressure  as  before,  this 
operation  being  repeated  until  the  bricks  were  found  not  to  absorb 
any  water.  Four  coatings  rendered  the  bricks  impenetrable  under 
the  pressure  of  a  40-foot  head.  The  mean  weight  of  the  bricks  (dry) 
before  being  coated  was  3^  lbs. ;  the  mean  absorption  was  one  half- 
pound  of  water.     A  hydrometer  was  used  in  testing  the  solutions. 

''As  this  experiment  was  made  in  the  fall  and  winter  (1863), 
after  the  temporary  roofs  were  put  on  to  the  gate-house,  artificial 
heat  had  to  be  resorted  to  to  dry  the  walls  and  keep  the  air  at  a 
proper  temperature.  The  cost  was  10  cents  per  sq.  ft.  As  soon  as 
the  last  coat  had  become  hard,  the  water  was  let  into  the  bays,  and 
the  walls  were  found  to  be  perfectly  impervious  to  water,  and  they 
remain  so  in  1870,  after  about  6^  years. 

264.  "The  brick  arch  of  the  footway  of  High  Bridge  is  the  arc 
of  a  circle,  29^  feet  radius,  and  is  12  inches  thick;  the  width  on  top 
is  17  feet,  and  the  length  covered  is  1,381  feet.  The  first  two 
courses  of  the  brick  of  the  arch  are  composed  of  the  best  hard-burnt 
brick,  laid  edgewise  in  mortar  composed  of  1  part,  by  measure,  of 
hydraulic  cement  of  New  York  [Eosendale  natural]  and  2  parts  of 
sand.  The  top  of  these  bricks,  and  the  inside  of  the  granite 
coping  against  which  the  two  top  courses  of  brick  rest,  was  covered, 
when  perfectly  dry,  with  a  coat  of  asphalt  one  half  an  inch  thick, 
laid  on  when  the  asphalt  was  heated  to  a  temperature  of  from  360° 
to  518°  Fahr.  On  top  of  this  was  laid  a  course  of  brick  flatwise, 
dipped  in  asphalt,  and  laid  when  the  asphalt  was  hot;  and  the  joints 
were  run  full  of  hot  asphalt.  On  top  of  this,  a  course  of  pressed 
brick  was  laid  flatwise  in  hydraulic  cement  mortar,  forming  the 
paving  and  floor  of  the  bridge. 


EFFLORESCENCE.  181 


"  The  area  of  the  bridge  covered  with  asphalted  brick  was  23,065 
sq.  ft.  There  were  used  94,200  lbs.  of  asphalt,  33  barrels  of  coal  tar, 
10  cu.  yds.  of  sand,  and  93,800  bricks.  The  asphalt  was  the  Trini- 
dad variety  ;  and  was  mixed  with  10  per  cent.,  by  measure,  of  coal 
tar,  and  25  per  cent,  of  sand.  The  time  occupied  was  109  days  of 
masons,  and  148  days  of  laborers.  Two  masons  and  two  laborers 
will  melt  and  spread,  of  the  first  coat,  1,650  sq.  ft.  per  day.  The 
total  cost  of  this  coat  was  5^  cents  per  sq.  ft.,  exclusive  of  duty  on 
asphalt. 

''There  were  three  grooves,  2  inches  wide  by  4  inches  deep, 
made  entirely  across  the  brick  arch  immediately  under  the  first 
coat  of  asphalt,  thus  dividing  the  arch  into  four  equal  parts.  The 
grooves  were  filled  with  elastic  paint  cement.  This  arrangement 
was  intended  to  guard  against  the  evil  effects  of  the  contraction  of 
the  arch  in  winter  ;  for,  since  it  was  expected  to  yield  slightly  at 
these  points  and  at  no  other,  the  elastic  cement  would  prevent  any 
leakage  there.  The  entire  experiment  has  proved  a  very  successful 
one,  and  the  bridge  has  remained  perfectly  tight. 

*'In  proposing  the  above  plan  for  working  the  asphalt  with  the 
brick-work,  the  object  was  to  avoid  depending  on  a  large  continuous 
surface  of  asphalt,  as  is  usual  in  covering  arches,  which  very  fre- 
quently cracks  from  the  greater  contraction  of  the  asphalt  than  that 
of  the  masonry  with  which  it  is  in  contact,  the  extent  of  the  asphalt 
on  this  work  being  only  about  one  quarter  of  an  inch  to  each  brick. 
This  is  deemed  to  be  an  essential  element  in  the  success  of  the  im- 
pervious covering." 

265.  Effloeescence.  Masonry,  particularly  in  moist  climate 
or  in  damp  places — as  cellar  walls, — is  frequently  disfigured  by  the 
formation  of  a  white  efflorescence  on  the  surface.  This  deposit 
generally  originates  with  the  mortar,  but  frequently  spreads  over 
the  entire  face  of  the  wall.  The  water  which  is  absorbed  by  the 
mortar  dissolves  the  salts  of  soda,  potash,  magnesia,  etc.,  contained 
in  the  lime  or  cement,  and  on  evaporating  deposits  these  salts  as  a 
white  efflorescence  on  the  surface.  With  lime  mortar  the  deposit 
is  frequently  very  heavy,  particularly  on  plastering ;  and,  usually, 
it  is  heavier  with  natural  than  with  Portland  cement.  The  efflo- 
rescence sometimes  originates  in  the  brick,  particularly  if  the  brick 
was  burned  with  sulphurous  coal,  or  was  made  from  clay  contain- 
ing iron  pyrites;  and  when  the  brick  gets  wet,  the  water  dissolves 


182  BRICK    MASONRY.  [CHAP.   VIII. 

the  sulphates  of  lime  and  magnesia,  and  on  evaporating  leaves  the 
crystals  of  these  salts  on  the  surface.  Frequently  the  efflorescence 
on  the  brick  is  due  to  the  absorption  by  the  brick  of  the  impreg- 
nater  -^-ater  from  the  mortar. 

This  efflorescence  is  objectionable  because  of  the  unsightly  ap- 
pearance which  it  often  produces,  and  also  because  the  crystalliza- 
tion of  these  salts  within  the  pores  of  the  mortar  and  of  the  brick 
or  stone  causes  disintegration  which  is  in  many  respects  like  frost. 

As  a  preventive,  Gillmore  recommends*  the  addition  of  100  lbs. 
of  quicklime  and  8  to  12  lbs,  of  any  cheap  animal  fat  to  each  barrel 
of  cement.  The  lime  is  simply  a  vehicle  for  the  fat,  which  should 
be  thoroughly  incorporated  with  the  lime  before  slaking.  The  ob- 
ject of  the  fat  IS  to  saponify  the  alkaline  salts.  The  method  is  not 
entirely  satisfactory,  since  the  deposit  is  only  made  less  prominent 
and  less  effective,  and  not  entirely  removed  or  prevented. 

The  efflorescence  may  be  entirely  prevented,  whatever  its  origin, 
by  applying  Sylvester's  washes  (see  §  263)  to  the  entire  external  sur- 
faces of  the  wall ;  and,  since  usually  the  efflorescence  is  due  to  the 
water  absorbed  by  the  mortar,  it  can  generally  be  prevented,  and 
can  always  be  much  diminished,  by  using  mortar  which  is  itself  im- 
pervious to  water  (see  §  141).  The  latter  is  the  cheaper  method, 
particula.rly  if  the  impervious  mortar  be  used  only  for  the  face  of 
the  joints.  If  the  wall  stands  in  damp  ground,  one  or  more  of  the 
horizontal  joints  just  above  the  surface  should  be  laid  in  impervious 
mortar,  or  better,  the  brick  for  several  courses  should  be  rendered 
impervious  and  be  laid  in  impervious  mortar  to  j)revent  the  wall's 
absorbmg  moisture  from  below. 

*  "Limes,  Hydraulic  Cements,  and  Mortars,"  p,  296. 


F»ART  III. 

FOUNDATIONS. 


CHAPTER  IX. 
INTRODUCTORY. 

265.  Definitions.  The  term  foundation  is  ordinarily  used  in- 
differeii  Jy  for  either  the  lower  courses  of  a  structure  of  masonry  or 
the  artjficial  arrangement,  whatever  its  character,  on  which  these 
courses  rest.  For  greater  clearness,  the  term  foundation  will  here 
be  restricted  to  the  artificial  arrangement,  whether  timber  or  mason- 
ry, Avhich  supports  the  main  structure  ;  and  the  prepared  surface 
upon  which  this  artificial  structure  rests  will  be  called  the  heel  of 
the  foundation.  There  are  many  cases  in  which  this  distinction 
can  not  be  adhered  to  strictly. 

267.  Importance  of  the  Subject.  The  foundation,  whether 
for  the  more  important  buildings  or  for  bridges  and  culverts,  is  the 
most  critical  part  of  a  masonry  structure.  The  failures  of  works  of 
masonry  due  to  faulty  workmanship  or  to  an  insufficient  thickness 
of  the  walls  are  rare  in  comparison  with  those  due  to  defective 
foundations.  "When  it  is  necessar}^  as  so  frequently  it  is  at  the 
present  day,  to  erect  gigantic  edifices — as  high  buildings  or  long- 
span  bridges — on  Aveak  and  treacherous  soils,  the  highest  construc- 
tive skill  is  required  to  supplement  the  weakness  of  the  natural 
foundation  by  such  artificial  preparations  as  will  enable  it  to  sustain 
such  massive  and  costly  burdens  with  safety. 

Probably  no  branch  of  the  engineer's  art  requires  more  ability 
and  skill  than  the  construction  of  foundations.  The  conditions 
governing  safety  are  generally  capable  of  being  calculated  with 
as  much  practical  accuracy  m  this  as  in  any  other  part  of  a  con- 

183 


184  FOUNDATION'S:   I]S"TRODUCTOET.  [CHAP.  IX. 


struction  ;  but,  unfortunately,  practice  is  frequently  based  upon 
empirical  rules  rather  than  upon  a  scientific  application  of  funda- 
mental principles.  It  is  unpardonable  that  any  liability  to  danger 
or  loss  should  exist  from  the  imperfect  comprehension  of  a  subject 
of  such  vital  importance.  Ability  is  required  in  determining  the 
conditions  of  stability ;  and  gi-eater  skill  is  required  m  fulfilling 
these  conditions,  that  the  cost  of  the  foundation  may  not  be  pro- 
portionally too  great.  The  safety  of  a  structure  may  be  imperiled, 
or  its  cost  unduly  increased,  according  as  its  foundations  are  laid 
with  insufficient  stability,  or  with  provision  for  security  greatly  in 
excess  of  the  requirements.  The  decision  as  to  what  general  method 
of  procedure  will  probably  be  best  in  any  particular  case  is  a  ques- 
tion that  can  be  decided  with  reasonable  certainty  only  after  long 
experience  in  this  branch  of  engineering  ;  and  after  having  decided 
upon  the  general  method  to  be  followed,  there  is  room  for  the 
exercise  of  great  skill  in  the  means  employed  to  secure  the  desired 
end.  The  experienced  engineer,  even  with  all  the  information 
which  he  can  derive  from  the  works  of  others,  finds  occasion  for  the 
use  of  all  his  knowledge  and  best  common  sense. 

The  determination  of  the  conditions  necessary  for  stability  can 
be  reduced  to  the  application  of  a  few  fundamental  principles  Avhich 
may  be  studied  from  a  text-book  ;  but  the  knowledge  required  to 
determine  beforehand  the  method  of  construction  best  suited  to  the 
case  in  hand,  together  with  its  probable  cost,  comes  only  by  personal 
experience  and  a  careful  study  of  the  experiences  of  others.  The 
object  of  Part  III.  is  to  classify  the  principles  employed  in  con- 
structing foundations,  and  to  give  such  brief  accounts  of  actual 
practice  as  will  illustrate  the  applications  of  these  principles. 

268.  Plan  of  Proposed  Discussion.  In  a  general  way,  soils 
may  be  divided  into  three  classes  :  (1)  ordinary  soils,  or  those  which 
are  capable,  either  in  their  normal  condition  or  after  that  condition 
has  been  modified  by  artificial  means,  of  sustaining  the  load  that  is 
to  be  brought  upon  them  ;  (2)  compressible  soils,  or  those  that  are 
incapable  of  directly  supporting  the  given  pressure  with  any  reason- 
able area  of  foundation  ;  and  (3)  semi-liquid  soils,  or  those  in  which 
the  fluidity  is  so  great  that  they  are  incapable  of  supporting  any 
considerable  load.  Each  of  the  above  classes  gives  rise  to  a  special 
method  of  constructing  a  foundation. 

1.  With  a  soil  of  the  first  class,  the  bearing  power  may  be  in- 


PLAN    OF    PEOPOSED    DISCUSSION".,  185 

creased  by  compacting  the  surface  or  by  drainage  ;  or  the  area  of 
the  foundation  may  be  increased  by  the  use  of  masonry  footing 
courses,  inverted  masonry  arches,  or  one  or  more  layers  of  timbers, 
railroad  rails,  iron  beams,  etc.  Some  one  of  these  methods  is  or- 
dinarily employed  in  constructing  foundations  on  land  ;  as,  for 
example,  for  buildings,  bridge  abutments,  sewers,  etc.  Usually  all 
of  these  methods  are  inapplicable  to  bridge  piers,  i.  e.,  for  founda- 
tions under  water,  owing  to  the  scouring  action  of  the  current  and 
also  to  the  obstruction  of  the  channel  by  the  greatly  extended  base 
of  the  foundation. 

2.  With  compressible  soils,  the  area  of  contact  may  be  increased 
by  suppoi'ting  the  structure  upon  piles  of  wood  or  iron,  which  are 
sustained  by  the  friction  of  the  soil  on  their  sides  and  by  the  direct 
pressure  on  the  soil  beneath  their  bases.  This  method  is  frequently 
employed  for  both  buildings  and  bridges. 

3.  A  semi-fluid  soil  must  generally  be  removed  entirely  and  the 
structure  founded  upon  a  lower  and  more  stable  stratum.  This 
method  is  specially  applicable  to  foundations  for  bridge  piers. 

There  are  many  cases  to  which  the  above  classification  is  not 
strictly  applicable. 

For  convenience  in  study,  the  construction  of  foundations  will 
be  discussed,  in  the  three  succeeding  chapters,  under  the  heads 
Ordinary  Foundations,  Pile  Foundations,  and  Foundations  under 
Water.  However,  the  methods  employed  in  each  class  ar«  not 
entirely  distinct  from  those  used  in  the  others. 


CHAPTER  X. 
ORDINARY  FOUNDATIONS. 

269.  In  this  chapter  will  be  discussed  the  method  of  construct- 
ing the  foundations  for  buildings,  bridge  abutments,  culverts,  or, 
in  general,  for  any  structure  founded  upon  dry,  or  nearly  dry, 
ground.  This  class  of  foundations  could  appropriately  be  called 
Foundations  for  Buildings,  since  these  are  the  most  numerous  of  the 
class. 

This  chapter  is  divided  into  three  articles.  The  first  treats  of 
the  soil,  and  includes  (a)  the  methods  of  examining  the  site  to  de- 
termine the  nature  of  the  soil,  (b)  a  discussion  of  the  bearing  power 
of  different  soils,  and  (c)  the  methods  of  increasing  the  bearing 
power  of  the  soil.  The  second  article  treats  of  the  method  of  de- 
signing the  footing  courses,  and  includes  (a)  the  method  of  deter- 
mining the  load  to  be  supported,  and  (b)  the  method  of  increasing 
the  area  of  the  foundation.  The  third  contains  a  few  remarks  con- 
cerning the  practical  work  of  laying  the  foundation. 

Art.  1.  The  Soil. 

270.  Examination  of  the  Site.  The  nature  of  the  soil  to  be 
built  upon  is  evidently  the  first  subject  for  consideration,  and  if  it 
has  not  already  been  revealed  to  a  considerable  depth,  by  excava- 
tions for  buildings,  wells,  etc.,  it  will  be  necessary  to  make  an  ex- 
amination of  the  subsoil  preparatory  to  deciding  upon  the  details 
of  the  foundation.  It  will  usually  be  suJBficient,  after  having  dug 
the  foundation  pits  or  trenches,  to  examine  the  soil  with  an  iron 
rod  or  a  post-auger  from  3  to  5  feet  further,  the  depth  depending 
upon  the  nature  of  the  soil,  and  the  weight  and  importance  of  the 
intended  structure. 

In  soft  soil,  soundings  40  or  50  feet  deep  can  be  made  by  driving 
a  small  (say  f-inch)  gas-pipe  with  a  hammer  or  maul  from  a  tem- 
porary scaffold,  the  height  of  which  will  of  course  depend  upon  the 
length  of  the  sections  of  the  pipe.    If  samples  of  the  soil  are  desired, 

186 


ART.  1.]  THE    SOIL.  187 

use  a  2-inch  pipe  open  at  the  lower  end.  If  much  of  this  kind  of 
work  is  to  be  done,  it  is  advisable  to  fit  up  a  hand  pile-driving 
machine  (see  §  335),  using  a  block  of  wood  for  the  dropping  weight. 
Borings  50  to  100  feet  deep  can  be  made  very  expeditiously  in 
common  soil  or  clay  with  a  common  wood-auger  turned  by  men, 
with  levers  2  or  3  feet  long.  The  auger  will  bring  up  samples  suf- 
ficient to  determine  the  nature  of  the  soil,  but  not  its  compactness, 
since  it  will  probably  be  comj^ressed  somewhat  in  being  cut  off. 

When  the  testing  must  be  made  through  sand  or  loose  soil,  it 
may  be  necessary  to  drive  down  an  iron  tube  to  prevent  the  soil 
from  falling  into  the  hole.  The  sand  may  be  removed  from  the 
inside  of  the  tube  with  an  auger,  or  with  the  "  sand-pump"  used  in 
digging  artesian  wells.  When  the  subsoil  is  composed  of  various 
strata  and  the  structure  demands  extraordinary  precaution,  borings 
must  be  made  with  the  tools  employed  for  boring  artesian  wells.* 

271.  If  the  builder  desires  to  avoid,  on  the  one  hand,  the  unnec- 
essarily costly  foundations  which  are  frequently  constructed,  or,  on 
the  other  hand,  those  insufficient  foundations  evidences  of  which 
are  often  seen,  it  may  be  necessary,  after  opening  the  trenches,  to 
determine  the  supporting  power  of  the  soil  by  applying  a  test  load. 

In  the  case  of  the  capitol  at  Albany,  'N.  Y.,  the  soil  was  tested 
by  applying  a  measured  load  to  a  square  foot  and  also  to  a  square 
yard.  The  machine  used  was  a  mast  of  timber  12  inches  square, 
held  vertical  by  guys,  with  a  cross-frame  to  hold  the  weights.  For 
the  smaller  area,  a  hole  3  feet  deep  was  dug  in  the  blue  clay  at  the 
bottom  of  the  foundation,  the  hole  being  18  inches  square  at  the 
top  and  14  inches  at  the  bottom.  Small  stakes  were  driven  into 
the  ground  in  lines  radiating  from  the  center  of  the  hole,  the  tops 
being  brought  exactly  to  the  same  level ;  then  any  change  in  the 
surface  of  the  ground  adjacent  to  the  hole  could  readily  be  detected 
and  measured  by  means  of  a  straight-edge.  The  foot  of  the  mast 
was  placed  in  the  hole,  and  weights  applied.  No  change  in  the 
surface  of  the  adjacent  ground  was  observed  until  the  load  reached 
5.9  tons  per  sq.  ft.,  when  an  uplift  of  the  surrounding  earth  was 
noted  in  the  form  of  a  ring  with  an  irregularly  rounded  surface, 
the  contents  of  which,  above  the  previous  surface,  measured  0.09 
cubic  feet.     Similar  experiments  were  made  by  applying  the  load  to 

*  For  illustrations  of  tools  for  this  purpose,  see  Engineering  News,  vol.  21,  p.3S4. 


188  ORDINAEY   FOUNDATIONS.  [CHAP.  X. 

a  square  yard  with  essentially  the  same  results.  The  several  loads 
were  allowed  to  remain  for  some  time,  and  the  .settlements  observed.* 
Similar  experiments  were  made  in  connection  with  the  construc- 
tion of  the  Congressional  Library  Building,  Washington,  D.  C,  with 
a  frame  which  rested  upon  4  foot-plates  each  a  foot  square.  The 
frame  could  be  moved  from  place  to  place  on  wheels,  and  the  test 
was  applied  at  a  number  of  places. 

272.  Bearing  Power  of  Soils.  It  is  scarcely  necessary  to  say 
that  soils  vary  greatly  in  their  bearing  power,  ranging  as  they 
do  from  the  condition  of  hardest  rock,  through  all  intermediate 
stages,  to  a  soft  or  semi-liquid  condition,  as  mud,  silt,  or  marsh. 
The  best  method  of  determining  the  load  which  a  specific  soil  will 
bear  is  by  direct  experiment  (§  271);  but  good  judgment  and  ex- 
perience, aided  by  a  careful  study  of  the  nature  of  the  soil — its  com- 
pactness and  the  amount  of  water  contained  in  it — will  enable  one  to 
determine,  with  reasonable  accuracy,  its  probable  supporting  power. 
The  following  data  are  given  to  assist  in  forming  an  estimate  of  the 
load  which  may  safely  be  imposed  upon  different  soils. 

273.  Rock.  The  ultimate  crushing  strength  of  stone,  as  deter- 
mined by  crushing  small  cubes,  ranges  from  180  tons  per  square 
foot  for  the  softest  stone — such  as  are  easily  worn  by  running  water 
or  exposure  to  the  weather — to  1,800  tons  per  square  foot  for  the 
hardest  stones  (see  page  11).  The  crushing  strength  of  slabs,  i.  e., 
of  prisms  of  a  less  height  than  width,  increases  as  the  height  de- 
creases. A  prism  one  half  as  high  as  wide  is  about  twice  as  strong 
as  a  cube  of  the  same  material.  If  a  slab  be  conceived  as  being  made 
up  of  a  number  of  cubes  placed  side  by  side,  it  is  easy  to  see  wliy 
the  slab  is  stronger  than  a  cube.  The  exterior  cubes  prevent  the 
detachment  of  the  disk-like  pieces  (Fig.  1,  page  G)  from  the  sides  of 
the  interior  cubes ;  and  hence  the  latter  are  greatly  strengthened, 
which  materially  increases  the  strength  of  the  slab.  In  testing 
cubes  and  slabs  the  pressure  is  applied  uniformly  over  the  entire 
upper  surface  of  the  test  specimen  ;  and,  reasoning  from  analogy, 
it  seems  probable  that  when  the  pressure  is  applied  to  only  a  small 
part  of  the  surface,  as  in  the  case  of  foundations  on  rock,  the  strength 
will  be  much  greater  than  that  of  cubes  of  the  same  material. 

The  table  on  page  190  contains  the  results  of  experiments  made 

*  W.  J.  McAlpine,  the  engineer  in  charge,  in  Trans.  Am.  Soc.  C.  E.,  vol.  ii.  p.  287. 


AET.   1,]  THE    SOIL.  189 

by  the  author,  and  shows  conclusively  that  a  unit  of  material  has  a 
much  greater  power  of  resistance  Avhen  it  forms  a  portion  of  a  larger 
mass  than  when  isolated  in  the  manner  customary  in  making  ex- 
periments on  crushing  strength. 

The  ordinary  '^  crushing  strength"  given  in  next  to  the  last 
column  of  Table  22  was  obtained  by  crushing  cubes  of  the  identical 
materials  employed  in  the  other  experiments.  The  concentrated 
pressure  was  applied  by  means  of  a  hardened  steel  die  thirty-eight 
sixty-fourths  of  an  inch  in  diameter  (area  =  0,277  sq.  in.).  All  the 
tests  were  made  between  self-adjusting  parallel  plates  of  a  hydro- 
static testing-machine.  No  packing  was  used  in  either  series  of 
experiments  ;  that  is  to  say,  the  pressed  surfaces  were  the  same  in 
both  series.  However,  the  block  of  limestone  7  inches  thick  (Ex- 
periments Nos.  8  and  13)  is  an  exception  in  this  respect.  This 
block  had  been  sawed  out  and  was  slightly  hollow,  and  it  was 
thought  not  to  be  worth  while  to  dress  it  down  to  a  plane.  As  pre- 
dicted before  making  the  test,  the  block  split  each  time  in  the  di- 
rection of  the  hollow.  If  the  bed  had  been  flat,  the  block  would 
doubtless  have  shown  a  gi-eater  strength.  The  concentrated  pres- 
sure was  generally  applied  near  the  corner  of  a  large  block,  and 
hence  the  distance  from  the  center  of  the  die  to  the  edge  of  the 
block  is  to  the  nearest  edge.  Frequently  the  block  had  a  ragged 
edge,  and  therefore  these  distances  are  only  approximate.  The 
quantity  in  the  last  column — ''Ratio" — is  the  crushing  load  per 
square  inch  for  concentrated  pressures  divided  bv  the  crushing  load 
per  square  inch  for  uniform  pressure. 

The  experiments  are  tabulated  in  an  order  intended  to  show  that 
the  strength  under  concentrated  pressure  varies  (1)  with  the  thick- 
ness of  the  block  and  (2)  with  the  distance  between  the  die  and 
the  edge  of  the  material  being  tested.  It  is  clear  that  the  strength 
increases  very  rapidly  with  both  the  thickness  and  the  distance  from 
the  edge  to  the  point  where  the  pressure  is  applied.  Therefore  we 
conclude  that  the  compressive  strength  of  cubes  of  a  stone  gives 
little  or  no  idea  of  the  ultimate  resistance  of  the  same  material  when 
in  thick  and  extensive  layers  in  its  native  bed. 

274.  The  safe  bearing  power  of  rock  is  certainly  not  less  than 
one  tenth  of  the  ultimate  crushing  strength  of  cubes;  that  is  to  say, 
the  safe  bearing  power  of  solid  rock  is  not  less  than  18  tons  per  sq. 
ft.  for  the  softest  rock  and  180  for  the  strongest.     It  is  safe  to  say 


190 


OEDINART   FOUNDATIONS. 


[chap.  X. 


TABLE  22. 

Compressive  Strength  when  the  Pressure  is  applied  on  only  a  Part 
OF  the  Upper  Surface. 


6 

9 

10 

7 
11 
12 

8 
13 

14 


Material. 


Lime  Mortar 
Marble 

Brick 

Limestone. . . 
Sandstone. .  . 
Limestone. . . 

Sandstone. . . 
Limestone. . . 


o 

o 

s 
§ 

P3 

H 

o 

Q 

•«) 

m 

b 

te 

Ed 

Eh 

z; 

°^  id 

g 

td  o 

O 

D3 

hW 

d 

E-i 

o 

^ 

4  in. 

3  in. 

4 

1     " 

3   " 

4 

2     " 

2    " 

3 

21   " 

2   " 

11 

3     " 

2    " 

4 

3     " 

2    " 

2 

4     " 

2    " 

3 

7     " 

2   " 

2 

8     " 

2   " 

2 

3     " 

3   " 

1 

3     " 

4   " 

1 

4     " 

2   " 

3 

4     " 

3    " 

3 

4     " 

4   " 

1 

7     " 

2   " 

2 

7     " 

4    " 

1 

K  n 

E"  u  ^ 

o  ^  S 

Z-i  g 

Ed  C^ 
SEd<l 

H  C  = 

o 


8,610 
18,050 
36.100 
11,801 
31,046 
51,600 
75,361 
64,077 

51,600 
59,204 

75,810 

75,361 
102,900 
111,188 

64,077 

87,720 


S    » 

g  S  « 

«     o 

CO  Bi  h 

<<  ^ 

O  P  M 

H 

Z  ©"2 

Eci 

K^"S 

o 

£  oi  t»  03 

d 

^ 

o 

3 

1,340 

y 

10,500 

8 

10,100 

18 

2,654 

8 

3,4.53 

8 

3,696 

2 

4,671 

5 

8,453 

3 

3,696 

2 

4,761 

'■   5 

1    " 

8,458 

"       1 

« 


2.7 

1.7 

8.6 

5.1 

9.0 

14.0 

16.0 

18.5 

14.0 
16.0 
20.5 

16.0 
22.0 
24.0 

18.5 
25.0 


Clay,  which  for  years  has  safely  carried,  without  appreciable  settlement, 
buildings  concentrating  1|  to  2  tons  per  square  foot  (20  to  28  pounds 
per  square  inch),  when  tested  in  the  form  of  cubes  was  crushed  with  4 
to  8  pounds  per  square  inch.     In  this  case  the  average  "  ratio"  is  4.3. 


that  almost  any  rock,  from  the  hardness  of  granite  to  that  of  a  soft 
crumbling  stone  easily  worn  by  exposure  to  the  weather  or  to  run- 
ning water,  when  well  bedded  will  bear  the  heaviest  load  that  can 
be  brought  upon  it  by  any  masonry  construction. 

It  scarcely  ever  occurs  in  practice  that  rock  is  loaded  with  the 
full  amount  of  weight  which  it  is  capable  of  sustaining,  as  the  extent 
of  base  necessary  for  the  stability  of  the  structure  is  generally  suffi- 
cient to  prevent  any  undue  pressure  coming  on  the  rock  beneath. 

275.  Clay.  The  clay  soils  vary  from  slate  or  shale,  which  will 
support  any  load  that  can  come  upon  it,  to  a  soft,  damp  clay  which 
will  squeeze  out  in  every  direction  when  a  moderately  heavy  pros- 


AET.   1.]  THE    SOIL.  191 

sure  is  brought  upon  it.  Foundations  on  clay  should  be  laid  at 
such  depths  as  to  be  unaffected  by  the  weather  ;  since  clay,  at  even 
considerable  depths,  will  gain  and  lose  considerable  water  as  the 
seasons  change.  The  bearing  power  of  clayey  soils  can  be  very 
much  improved  by  drainage  (§  285),  or  by  preventing  the  penetra- 
tion of  water.  If  the  foundation  is  laid  upon  undrained  clay,  care 
must  be  taken  that  excavations  made  in  the  immediate  vicinity  do 
not  allow  the  clay  under  pressure  to  escape  by  oozing  away  from 
under  the  building.  When  the  clay  occurs  in  strata  not  horizontal, 
great  care  is  necessary  to  prevent  this  flow  of  the  soil.  When  coarse 
sand  or  gravel  is  mixed  with  the  clay,  its  supporting  power  is  greatly 
increased,  being  greater  in  proportion  as  the  quantity  of  these 
materials  is  greater.  When  they  are  present  to  such  an  extent  that 
the  clay  is  just  sufficient  to  bind  them  together,  the  combination 
will  bear  as  heavy  loads  as  the  softer  rocks. 

276.  The  following  data  on  the  bearing  power  of  clay  will  be  of 
assistance  in  deciding  upon  the  load  that  may  safely  be  imposed 
upon  any  particular  clayey  soil.  From  the  experiments  made  in 
connection  with  the  construction  of  the  capitol  at  Albany,  N.  Y., 
as  described  in  §  271,  the  conclusion  was  drawn  that  the  extreme 
supporting  power  of  that  soil  was  less  than  6  tons  per  sq.  ft.,  and 
that  the  load  which  might  be  safely  imposed  upon  it  was  2  tons  per 
sq,  ft.  "  The  soil  was  blue  clay  containing  from  60  to  90  per  cent, 
of  alumina,  the  remainder  being  fine  siliceous  sand.  The  soil  con- 
tains from  27  to  43,  usually  about  40,  per  cent,  of  water  ;  and  vari- 
ous samples  of  it  weighed  from  81  to  101  lbs.  per  cu.  ft."  In  the 
case  of  the  Congressional  Library  (§  271),  the  ultimate  supporting- 
power  of  ''yellow  clay  mixed  with  sand"  was  134-  tons  per  sq.  ft.;. 
and  the  safe  load  was  assumed  to  be  2^  tons  per  sq.  ft.  Experi- 
ments made  on  the  clay  under  the  piers  of  the  bridge  across  the 
Missouri  at  Bismarck,  with  surfaces  1^  inches  square,  gave  an  aver- 
age ultimate  bearing  power  of  15  tons  per  sq.  ft.* 

The  stiffer  varieties  of  what  is  ordinarily  called  clay,  when  kept 
dry,  will  safely  bear  from  4  to  6  tons  per  sq.  ft.;  but  the  same  clay, 
if  allowed  to  become  saturated  with  water,  can  not  be  trusted  to 
bear  more  than  2  tons  per  sq.  ft.  At  Chicago,  the  load  ordinarily 
put  on  a  thin  layer  of  clay  (hard  above  and  soft  below,  resting  on  a 

*  Report  of  the  engineer,  Geo.  S.  Morison. 


192  ORDINARY    FOUXDATIONS.  [CHAP.   X. 

thick  stratum  of  quicksand)  is  14^  to  2  tons  per  sq.  ft. ;  and  the  set- 
tlement, which  usually  reaches  a  maximum  in  a  year,  is  about  1 
inch  per  ton  of  load.  Experience  in  central  Illinois  shows  that,  if 
the  foundation  is  carried  down  below  the  action  of  frost,  the  clay 
«ubsoil  will  bear  1|-  to  2  tons  per  sq.  ft.  without  appreciable  settling. 
Jiankine  gives  the  safe  load  for  comjiressible  soils  as  1^  to  If  tons 
per  sq.  ft. 

277.  Sand.  The  sandy  soils  vary  from  coarse  gravel  to  fine  sand. 
The  former  when  of  sufficient  thickness  forms  one  of  the  firmest 
and  best  foundations ;  and  the  latter  when  saturated  with  water 
is  practically  a  liquid.  Sand  when  dry,  or  wet  sand  when  prevented 
from  spreading  laterally,  forms  one  of  the  best  beds  for  a  founda- 
tion. Porous,  sandy  soils  are,  as  a  rule,  unaffected  by  stagnant 
water,  but  are  easily  removed  by  running  water  ;  in  the  former  case 
they  present  no  difficulty,  but  in  the  latter  they  require  extreme 
care  at  the  hands  of  the  constructor,  as  will  be  considered  later. 

278.  Compact  gravel  or  clean  sand,  in  beds  of  considerable 
thickness,  protected  from  being  carried  away  by  water,  may  be 
loaded  with  8  to  10  tons  per  sq.  ft.  with  safety.  In  an  experiment 
in  France,  clean  river-sand  compacted  in  a  trench  supported  100 
tons  per  sq.  ft.  Sand  well  cemented  with  clay  and  compacted,  if 
protected  from  water,  will  safely  carry  4  to  6  tons  per  sq.  ft. 

The  piers  of  the  Cincinnati  Suspension  Bridge  are  founded  on  a 
bed  of  coarse  gravel  12  feet  below  low-water,  although  solid  lime- 
stone was  only  12  feet  deeper  ;  if  the  friction  on  the  sides  of  the 
pier*  be  disregarded,  the  maximum  pressure  on  the  gravel  is  4  tons 
per  sq.  ft.  The  piers  of  the  Brooklyn  Suspension  Bridge  are  founded 
44  feet  below  the  bed  of  the  river,  upon  a  layer  of  sand  2  feet  thick 
resting  upon  bed-rock  ;  the  maximum  pressure  is  about  5^  tons 
per  sq.  ft. 

At  Chicago  sand  and  gravel  about  15  feet  below  the  surface  are 
successfully  loaded  with  2  to  2^  tons  per  sq.  ft.  At  Berlin  the  safe 
load  for  sandy  soil  is  generally  taken  at  2  to  2^  tons  per  sq.  ft.  The 
Washington  Monument,  Washington,  D.  C,  rests  upon  a  bed  of 
very  fine  sand  two  feet  thick  underlying  a  bed  of  gravel  and  bowl- 
ders; the  ordinary  pressure  on  certain  parts  of  the  foundation  is 
not  far  from  11  tons  per  sq,  ft.,  Avhich  the  wind  may  increase  to 
nearly  14  tons  per  sq.  ft. 

*  For  the  amount  of  such  friction,  see  §§  418-19  and  §  455. 


AfeT.   1.]  THE   SOIL.  193 

279.  Semi-Liquid  Soils.  With  a  soil  of  this  class,  as  mud,  silt, 
or  quicksand,  it  is  customary  (1)  to  remove  it  entirely,  or  (2)  to 
sink  piles,  tubes,  or  caissons  through  it  to  a  solid  substratum,  or 
\'d)  to  consolidate  the  soil  by  adding  sand,  earth,  stone,  etc.  The 
method  of  performing  these  operations  will  be  described  later.  Soils 
of  a  soft  or  semi-liquid  character  should  never  be  relied  upon  for  a 
foundation  when  anything  better  can  be  obtained  ;  but  a  heavy 
superstructure  may  be  supported  by  the  upward  pressure  of  a  semi- 
iiquid  soil,  in  the  same  way  that  water  bears  up  a  floating  body. 

According  to  Rankine,*  a  building  will  be  supported  Avhen  the 

(1  I  SI  11  ty  \^ 
: 1  per  unit  of  area,  in  which  ex- 
pression 7V  is  the  weight  of  a  unit  volume  of  the  soil,  h  is  the  depth 
of  immersion,  and  a  is  the  angle  of  repose  of  the  soil  li  a  =  5°, 
then  according  to  the  preceding  relation  the  supporting  power  of 
the  soil  is  1.4  w  h  per  unit  of  area  ;  it  a  =  10°,  it  is  2.0  to  h  ;  and 
if  «  =  15°,  it  IS  2.9  w  h.  The  weight  of  soils  of  this  class,  i.  e., 
mud,  silt,  and  quicksand,  varies  from  100  to  130  lbs.  per  cu.  ft. 
Eankine  gives  this  formula  as  being  applicable  to  any  soil ;  but  since 
it  takes  no  account  of  cohesion,  for  most  soils  it  is  only  roughly  ap- 
proximate, and  gives  results  too  small.  The  following  experiment 
seems  to  show  that  the  error  is  considerable.  '*' A  10-foot  square 
base  of  concrete  resting  on  mud,  whose  angle  of  repose  was  5  to  1 
[a  =11^°],  bore  700  lbs.  per  sq.  ft."f  This  is  2^  times  the  result 
by  the  above  formula,  using  the  maximum  value  of  w. 

Large  buildings  have  been  securely  founded  on  quicksand  by 
making  the  base  of  the  immersed  part  as  large  and  at  the  same  time 
as  light  as  possible.  Timber  in  successive  layers  (§  309)  or  grillage 
on  piles  (§  320)  is  generally  used  in  such  cases.  This  class  of  foun- 
dations is  frequently  required  in  constructing  sewers  in  water-bear- 
ing sands,  and  though  apparently  presenting  no  difficulties,  such 
foundations  often  demand  great' skill  and  ability. 

280.  It  is  difficult  to  give  results  of  the  safe  bearing  power  of 
soils  of  this  class.  A  considerable  part  of  the  supporting  power  is 
derived  from  the  friction  on  the  vertical  sides  of  the  foundation ; 
hence  the  bearing  power  depends  in  part  upon  the  area  of  the  side 
surface  in  contact  with  the  soil.     Furthermore,  it  is  difficult  to  de- 

*  See  Rankine's  Civil  Engineering,  p.  379. 
+  Proc.  Inst,  of  C.  E.,  vol.  xviii.  p.  493. 


194 


ORDINARY    FOUNDATIONS. 


TCHAP.  X. 


termine  the  exact  suijporting  power  of  a  plastic  soil,  since  a  consid- 
erable settlement  is  certain  to  take  place  with  the  lanse  of  time. 
The  experience  at  New  Orleans  with  alluvial  soil  an*^  »  f'*™'  experi- 
ments* that  have  been  made  on  quicksand  seem  to  indicate  mat 
with  a  load  of  ^  to  1  ton  per  square  foot  the  settlement  will  not  be 
excessive. 

281.  Bearing  Power:  Summary.     Gathering  together  the  results 
of  the  preceding  discussion,  Ave  have  the  following  table  • 

TABLE  23. 
Safe  Bearing  Power  of  Soils. 


Kind  of  Material. 

Safe  Bearing  Power 
IN  Tons  pee  Sq.  Ft. 

Min. 

Max. 

Rock — the  hardest — in  thick  layers,  in  native  bed  (§  274) 
*'    equal  to  best  ashlar  masonry  (^  274) 

200 
25 
15 

5 

4 

2 

1 

8 

4 

2 

0.5 

30 

"        "      "     ' '    brick          "            "     

20 

"        "      "  poor     "             "            "     

10 

Clay  in  thick  beds  always  dry  (§  276)  

6 

"      "       "         "    moderately  dry  (§  276) 

4 

"      soft  (§  276) 

2 

Gravel  and  coarse  sand,  well  cemented  (§  278) 

10 

Sand,  compact  and  well  cemented,            "      

"      clean,  dry "      

6 

4 

Quicksand,  alluvial  soils,  etc.  (§  280) 

1 

•  282.  Conclusion.  It  is  well  to  notice  that  there  are  some  prac- 
tical considerations  that  modify  the  pressure  which  may  safely  be 
put  upon  a  soil.  For  example,  the  pressure  on  the  foundation  of 
a  tall  chimney  should  be  considerably  less  than  that  of  the  low  mas- 
sive foundation  of  a  fire-proof  vault.  In  the  former  case  a  slight 
inequality  of  bearing  power,  and  consequent  unequal  settling,  might 
endanger  the  stability  of  the  structure  ;  while  in  the  latter  no  seri- 
ous harm  would  result.  The  pressure  per  unit  of  area  should  be 
less  for  a  light  structure  subject  to  the  passage  of  heavy  loads — as, 

*  Trans.  Am.  Soc.  of  C.  E.,  vol.  xiv.  p.  182 ;  Engineering,  vol.  xx.  p.   103  ;  Proc, 
Inst,  of  C.  E.,  vol.  xvii.  p.  443 ;  Cleeman's  Railroeid  Practice,  pp.  103-4. 


AET.  1.]  THE   SOIL.      •  195 

for  example,  a  railroad  viaduct — than  for  a  heavy  structure  subject 
only  to  a  quiescent  load,  since  the  shock  and  jar  of  the  moving  load 
are  far  more  serious  than  the  heavier  quiescent  load. 

The  determination  of  the  safe  bearing  power  of  soils,  particular- 
ly when  dealing  with  those  of  a  semi-liquid  character,  is  not  the 
only  question  that  must  receive  careful  attention.  In  the  founda- 
tions for  buildings,  it  may  be  necessary  to  provide  a  safeguard 
against  the  soil's  escaping  by  being  pressed  out  laterally  into  excava- 
tions in  the  vicinity.  In  the  foundations  for  bridge  abutments,  it 
maybe  necessary  to  consider  what  the  effect  will  be  if  the  soil  around 
the  abutment  becomes  thoroughly  saturated  with  water,  as  it  may 
during  a  flood;  or  what  the  effect  will  be  if  the  soil  is  deprived  of 
its  lateral  support  by  the  washing  away  of  the  soil  adjacent  to  the 
abutment.  The  provision  to  prevent  the  wash  and  undermining 
action  of  the  stream  is  often  a  very  considerable  part  of  the  cost  of 
the  structure.  The  prevention  of  either  of  these  liabilities  is  a  prob- 
lem by  itself,  to  the  solution  of  which  any  general  discussion  will 
contribute  but  little. 

283.  IMPEOVING  THE  BEAEING  POWEE  OF  THE  SOIL.  When 
the  soil  directly  under  a  proposed  structure  is  incapable,  in  its  nor 
mal  state,  of  sustaining  the  load  that  will  be  brought  upon  it,  the 
bearing  power  may  be  increased  (1)  by  increasing  the  depth  of  the 
foundation,  (2)  by  draining  the  site,  (3)  by  compacting  the  soil,  or 
(4)  by  adding  a  layer  of  sand. 

284.  Increasing  the  Depth.  The  simplest  method  of  increas- 
mg  the  bearing  power  is  to  dig  deeper.  Ordinary  soils  will  bear 
more  weight  the  greater  the  depth  reached,  owing  to  their  becom- 
ing more  condensed  from  the  superincumbent  weight.  Depth  is 
especially  important  with  clay,  since  it  is  then  less  liable  to  be  dis- 
placed laterally  owing  to  other  excavations  in  the  immediate  vicin- 
ity, and  also  because  at  greater  depths  the  amount  of  moisture  in  it 
will  not  vary  so  much. 

In  any  soil,  the  bed  of  the  foundation  should  be  below  the  reach 
of  frost.  Even  a  foundation  on  bed-rock  should  be  below  the  frost 
line,  else  water  may  get  under  the  foundation  through  fissures,  and, 
freezing,  do  damage. 

285.  Drainage.  Another  simple  method  of  increasing  the  bear- 
ing power  of  a  soil  is  to  drain  it.  The  water  may  find  its  wa}'  to 
the  bed  of  the  foundation  down  the  side  of  the  wall,  or  by  percola- 


196  ORDI]*ARY    FOUXDATIONS.  [CHAP.   X. 

tion  througli  the  soil,  or  througli  a  seam  of  sand.  In  most  cases 
the  bed  ean  be  sufficiently  drained  by  covering  it  with  a  layer  of 
gravel — the  thickness  depending  upon  the  plasticity  of  the  soil, — 
and  then  surrounding  the  building  with  a  tile-drain  laid  a  little 
below  the  foundation.  In  extreme  cases,  it  is  necessary  to  enclose 
the  entire  site  with  a  puddle-wall  to  cut  off  drainage  water  from  a 
higher  area. 

286.  Springs.  In  laying  foundations,  springs  are  often  met 
with,  and  sometimes  prove  very  troublesome.  The  water  may  be 
excluded  from  the  foundation  pit  by  driving  sheet  piles,  or  by  plug- 
ging the  spring  with  concrete.  If  the  flow  is  so  strong  as  to  wash 
the  cement  out  before  it  has  set,  a  heavy  canvas  covered  with  pitch, 
etc.,  upon  which  the  concrete  is  deposited,  is  sometimes  used  ;  or 
the  water  may  be  carried  away  in  temporary  channels,  until  the 
concrete  in  the  artificial  bed  shall  have  set,  when  the  water-ways 
may  be  filled  with  semi-fluid  cement  mortar.  Below  is  an  account 
of  the '  method  of  stopping  a  very  troublesome  spring  encountered 
in  laying  the  foundation  of  the  dry-dock  at  the  Brooklyn  Navy 
Yard. 

"The  dock  is  a  basin  composed  of  stone  masonry  resting  on 
piles.  The  foundation  is  42  feet  below  the  surface  of  the  ground 
and  37  feet  below  mean  tide.  In  digging  the  pit  for  the  founda- 
tion, springs  of  fresh  water  were  discovered  near  the  bottom,  which, 
proved  to  be  very  troublesome.  The  upward  pressure  of  the  water 
was  so  great  as  to  raise  the  foundation,  however  heavily  it  was  loaded. 
The  first  indication  of  undermining  by  these  springs  was  the  settling 
of  the  piles  of  the  dock  near  by.  In  a  day  it  made  a  cavity  in  which 
a  pole  was  run  down  20  feet  below  the  foundation  timbers.  Into 
this  hole  were  thrown  150  cubic  feet  of  stone,  which  settled  10  feet 
during  the  night ;  and  50  cubic  feet  more,  thrown  in  the  following 
day,  drove  the  spring  to  another  place,  where  it  burst  through  a 
bed  of  concrete  2  feet  thick.  This  new  cavity  was  filled  with 
concrete,  but  the  precaution  was  taken  of  putting  in  a  tube  so  as  to 
permit  the  water  to  escape  ;  still  it  burst  through,  and  the  opera- 
tion was  repeated  several  times,  until  it  finally  broke  out  through  a 
heavy  body  of  cement  14  feet  distant.  In  this  place  it  undermined 
the  foundation  piles.  These  were  then  driven  deeper  by  means  of 
followers ;  and  a  space  of  1,000  square  feet  around  the  spring  was 
then  planked,  forming  a  floor  on  which  was  laid  a  layer  of  brick  in 


ART.   1.]  THE    SOIL.  197 

dry  cemeut,  and  on  that  a  layer  of  brick  set  in  mortar,  and  the 
foundation  was  completed  over  all.  Several  vent-holes  were  left 
through  the  floor  and  the  foundation  for  the  escape  of  the  water. 
The  work  was  completed  in  1851,  and  has  stood  well  ever  since."  * 

287.  Consolidating  the  Soil.  A  soft,  clayey  soil  can  be  greatly 
improved  by  spreading  a  thin  layer  of  sand,  dry  earth,  or  broken 
stone  over  the  bed  of  the  foundation  and  pounding  it  into  the  soil. 
If  the  soil  is  very  soft,  compacting  the  surface  will  be  insufficient ; 
in  this  case  the  soil  may  be  consolidated  to  a  considerable  depth  by 
driving  short  piles  into  it.  For  this  purpose  small  piles — say  6 
feet  long  and  6  inches  in  diameter — serve  better  than  large  ones ; 
and  they  can  be  driven  with  a  hand-maul  or  by  dropping  a  heavy 
block  of  wood  with  a  tackle  attached  to  any  simple  frame,  or  by  a 
hand  pile-driver  (§  335).  They  may  be  driven  as  close  together  as 
necessai'y,  although  2  to  4  feet  in  the  clear  is  usually  sufficient. 
The  latter  method  of  compacting  the  soil  is  far  more  efficient  than 
pounding  the  surface.  In  the  case  of  impact  upon  earth,  the  im- 
mediate layers  are  compressed  at  once,  and  by  their  inertia  and 
adhesion  to  the  surrounding  soil  they  intercept  the  effect  of  the 
blow,  and  thus  prevent  the  consolidation  of  the  lower  strata.  Even 
though  the  effect  of  a  blow  is  not  communicated  to  any  considerable 
depth,  the  heavy  masses  of  masonry  make  themselves  felt  at  great 
depth,  and  hence  for  heavy  buildings  it  is  necessary  to  consolidate 
the  lower  strata.  This  can  be  done  most  easily  and  most  efficiently 
by  driving  piles  (see  Art.  2). 

In  this  connection  it  is  necessary  to  remember  that  clay  is  com- 
pressible, while  sand  is  not.  Hence  this  method  of  consolidating 
soils  is  not  applicable  to  sand,  and  is  not  very  efficient  in  soils 
largely  composed  of  it. 

288.  Sand  Piles.  Experiments  show  that  in  compacting  the 
yoil  by  driving  piles,  it  is  better  to  withdraw  them  and  immediately 
fill  the  holes  with  sand,  than  to  allow  the  wooden  piles  to  remain. 
This  advantage  is  independent  of  the  question  of  the  durability  of 
the  wood.  When  the  wooden  pile  is  driven,  it  compresses  the  soil 
an  amount  nearly  or  quite  equal  to  the  volume  of  the  pile,  and 
when  the  latter  is  withdrawn  this  consolidation  remains,  at  least 
temporarily.     If  the  hole  is  immediately  filled  with  sand  this  com- 

*  DelafieWs  Foundations  in  Compressible  Soils,  p.  14 — a  pamphlet  published  br 
the  Engineer's  Department  of  the  U.  S.  Army. 


198  ORDINARY    FOUXDATIONS.  [CHAP.  X. 

pression  is  retained  permanently,  and  the  consolidation  may  be  still 
farther  increased  by  ramming  the  sand  in  in  thin  layers,  owing  to 
the  ability  of  the  latter  to  transmit  pressure  laterally.  And  further, 
the  sand  pile  will  support  a  greater  load  than  the  wooden  pile;  for, 
since  the  sand  acts  like  innumerable  small  arches  reaching  from 
one  side  of  the  hole  to  the  other,  more  of  the  load  is  transmitted  to 
the  soil  on  the  sides  of  the  hole.  To  secure  the  best  results,  the 
sand  should  be  fine,  sharp,  clean,  and  of  uniform  size. 

289.  When  the  piles  are  driven  primarily  to  compact  the  soil, 
it  is  customary  to  load  them  and  also  the  soil  between  them,  either 
by  cutting  the  piles  off  near  the  surface  and  laying  a  tight  platform 
of  timber  on  top  of  them  (see  §  320),  or  by  depositing  a  bed  of  con- 
crete between  and  over  the  heads  of  the  piles  (see  §  319). 

If  the  soil  is  very  soft  or  composed  largely  of  sand,  this  method 
is  ineffective;  in  which  case  long  piles  are  driven  as  close  together 
as  is  necessary,  the  supporting  power  being  derived  either  from  the 
resting  of  the  piles  upon  a  harder  substratum  or  from  the  buoyaiicy 
due  to  immersion  in  the  semi-liquid  soil.  This  method  of  securing 
a  foundation  by  driving  long  piles  is  very  expensive,  and  is  seldom 
resorted  to  for  buildings,  since  it  is  generally  more  economical  to 
increase  the  area  of  the  foundation. 

290.  Layers  of  Sand.  If  the  soil  is  very  soft,  it  may  be  ex- 
cavated and  replaced  by  sand.  The  method  of  using  sand  for  j^iles 
has  been  described  in  §  288,  which  see.  The  opportunities  for  the 
use  of  sand  in  foundations  are  numerous,  and  the  employment  of 
it  would,  in  many  constructions,  promote  economy  and  stability. 
The  simplest  method  of  using  sand  for  this  purpose  is  to  excavate 
a  trench  or  pit  to  the  proper  depth,  and  fill  it  by  depositing  succes- 
sive layers  of  sand,  each  of  which  should  be  thoroughly  settled  by 
a  heavy  beetle  before  laying  the  next.  To  cause  the  sand  to  pack 
firmly,  it  should  be  slightly  moistened  before  being  placed  in  the 
trench. 

Sand,  when  used  in  this  wa}',  possesses  the  valuable  proj^erty  of 
assuming  a  new  position  of  equilibrium  and  stability  should  the 
soil  on  which  it  is  laid  yield  at  any  of  its  points  ;  not  only  does  this 
take  place  along  the  base  of  the  sand  bed,  but  also  along  its  edges 
or  sides.  The  bed  of  sand  must  be  thick  enough  to  distribute  the 
pressure  on  its  upper  surface  over  the  entire  base.  The4-e  is  no  way 
of  telling  what  this  thickness  should  be,  except  by  trial. 


ART.  2.]  DESIGNING  THE   FOOTING.  199 

291.  The  followiug  examples,  cited  by  Trautwine,*  are  interest- 
ing as  showing  the  surprising  effect  of  even  a  thin  layer  of  sand 
or  gravel  : 

"  Some  portions  of  the  circular  brick  aqueduct  for  supplying 
Boston  with  water  gave  a  great  deal  of  trouble  when  its  trenches 
passed  through  running  quicksands  and  other  treacherous  soils. 
Concrete  was  tried,  but  the  wet  quicksand  mixed  itself  with  it  and 
hilled  it.  Wooden  cradles,  etc.,  also  failed  ;  and  the  difficulty  was 
overcome  by  simply  depositing  in  the  trenches  about  two  feet  in 
depth  of  strong  gravel. 

"Smeaton  mentions  a  stone  bridge  built  upon  a  natural  bed  of 
gravel  only  about  2  feet  thick,  overlying  deep  mud  so  soft  that  an 
iron  bar  40  feet  long  sank  to  the  head  by  its  own  weight.  Although 
a  wretched  precedent  for  bridge  building,  this  example  illustrates 
the  bearing  power  of  a  thick  layer  of  well-compacted  gravel." 


Art.  2.  Designing  the  Footing. 

292.  Load  to  be  Supported.     The  first  step  is  to  ascertain  the 

load  to  be  supported  by  the  foundation.  This  load  consists  of  three 
parts  :  (1)  the  building  itself,  (2)  the  movable  loads  on  the  floors 
and  the  snoAV  on  the  roof,  and  (3)  the  part  of  the  load  that  may  be 
transferred  from  one  part  of  the  foundation  to  the  other  by  the 
force  of  the  wind. 

293.  The  weight  of  the  building  is  easily  ascertained  by  calcu- 
lating the  cubical  contents  of  all  the  various  materials  in  the  struct- 
ure. If  the  weight  is  not  equally  distributed,  care  must  be  taken 
to  ascertain  the  proportion  to  be  carried  by  each  part  of  the  foun- 
dation. For  example,  if  one  vertical  section  of  the  wall  is  to  con- 
tain a  number  of  large  windows  while  another  will  consist  entirely 
of  solid  masonry,  it  is  evident  that  the  pressure  on  the  foundation 
under  the  first  section  will  be  less  than  that  under  the  second. 

In  this  connection  it  must  be  borne  in  mind  that  concentrated 
pressures  are  not  transmitted,  undiminished,  through  a  solid  mass 
in  the  line  of  application,  but  spread  out  in  successively  radiating 
lines  ;  hence,  if  any  considerable  distance  intervenes  between  the 
foundation  and  the  point  of  application  of  this  concentrated  load, 

*  Engineer's  Pocket-book  (ed.  1885),  p.  634. 


200 


OKDINARY   POUND ATIOifS. 


[chap.  X, 


the  pressure  will  be  nearly  or  quite  uniformly  distributed  over  the 
entire  area  of  the  base.  The  exact  distribution  of  the  pressure  can 
not  be  computed. 

The  following  data  will  be  useful  in  determining  the  weight  of 
the  structure  ■ 


TABLE  24. 
Weight  of  Masonry. 


Kind  of  Masonry. 


Weight 


LBS.  PER  CU.  FT 


Brick-work,  pressed  brick,  thin  joints , , 

"  ordinary  quality 

"  soft  brick,  thick  joints 

Concrete,  1  cement,  3  sand,  and  6  broken  stone 

Granite — 6  per  cent,  more  than  the  corresponding  limestone. . 
Limestone,  ashlar,  largest  blocks  and  thinnest  joints 

"  "       12"  to  20"  courses  and  |-  to  |-inch  joints.. . 

"  squared-stone  (see  §  208) 

"  rubble,  best 

"  "        rough 

Mortar,  1  Rosendale  cement  and  2  sand 

' '       common  lime,  dried 

Sandstone — 14  per  cent,  less  than  the  corresponding  limestone 


145 
125 
100 
140 

160 
155 
148 
142 
136 
116 
100 


Ordinary  lathing  and  plastering  weighs  about  10  lbs.  per  sq.  ft. 
The  weight  of  floors  is  approximately  10  lbs.  per  sq.  ft.  for  dwell- 
ings ;  25  lbs.  per  sq.  ft.  for  public  buildings  ;  and  40  or  50  lbs.  per 
sq.  ft.  for  warehouses.  The  weight  of  the  roof  varies  with  the  kind 
of  covering,  the  span,  etc.  A  shingle  roof  may  be  taken  at  10  lbs. 
per  sq.  ft.,  and  a  roof  covered  with  slate  or  corrugated  iron  at  25 
lbs.  per  sq.  ft. 

294.  The  movable  load  on  the  floor  depends  upon  the  nature  of 
the  building.  For  dwellings,  it  does  not  exceed  10  lbs.  per  sq.  ft. ; 
for  large  office  buildings,  it  is  usually  taken  at  30  lbs.  per  sq.  ft. ; 
for  churches,  theatres,  etc.,  the  maximum  load — a  crowd  of  people 
— may  reach  100  lbs.  per  sq.  ft. ;  for  stores,  warehouses,  factories. 


ART.  2.]  DESIGNIXG   THE    FOOTIXG.  201 

etc.,  the  load  -svill  be  from  100  to  400  lbs.  per  sq.  ft.,  according  to 
the  purposes  for  which  they  are  used. 

The  preceding  loads  are  the  ones  to  be  used  in  determining  the 
strength  of  the  floor,  and  not  in  designing  the  footings;  for  there 
is  no  probability  that  each  and  every  square  foot  of  floor  will  have 
its  maximum  load  at  the  same  time.  The  amount  of  moving  load 
to  be  assigned  in  any  particular  case  is  a  matter  of  judgment.  At 
Chicago  in  designing  tall  steel-skeleton  office  buildings,  it  is  the 
practice  to  assume  that  nearly  all  of  the  maximum  live  load  reaches 
the  girders,  that  a  smaller  per  cent,  reaches  the  columns,  and  that 
no  live  load  reaches  the  footings.  -In  many  cities  the  building  law 
specifies  the  live  load  to  be  assumed  as  reaching  the  footing. 

Attention  must  be  given  to  the  manner  in  which  the  weight  of 
the  roof  and  floors  is  transferred  to  the  walls.  For  example,  if  the 
floor  joists  of  a  warehouse  run  from  back  to  front,  it  is  evident  that 
the  back  and  front  walls  alone  will  carry  the  weight  of  the  floors 
and  of  the  goods  placed  upon  them,  and  this  will  make  the  pressure 
upon  the  foundation  under  them  considerably  greater  than  under 
the  other  walls.  Again,  if  a  stone-front  is  to  be  carried  on  an  arch 
or  on  a  girder  having  its  bearings  on  piers  at  each  side  of  the  build- 
ing, it  is  manifest  that  the  weight  of  the  whole  superincumbent 
structure,  instead  of  being  distributed  equally  on  the  foundation 
under  the  front,  will  be  concentrated  on  that  part  of  the  founda- 
tion immediately  under  the  piers. 

295.  The  pressure  of  the  wind  against  towers,  tall  chimneys, 
etc.,  will  cause  a  concentration  of  the  weight  of  the  structure  upon 
one  side  of  the  foundation.  The  maximum  liorizontal  pressure  of 
the  wind  is  usually  taken  as  50  lbs.  per  sq.  ft.  on  a  flat  surface  per- 
pendicular to  the  wind,  and  on  a  cylinder  at  about  30  lbs.  per  sq. 
ft.  of  the  projection  of  the  surface.  The  pressure  upon  an  inclined 
surface,  as  a  roof,  is  about  1  lb.  per  sq.  ft.  per  degree  of  inclination 
to  the  horizontal.  For  example,  if  the  roof  has  an  inclination  of 
30°  with  the  horizontal,  the  pressure  of  the  wind  will  be  about  30 
lbs.  per  sq.  ft. 

The  effect  of  the  wind  will  be  considered  in  §§  301-4. 

296.  Akea  Required.  Having  determined  the  pressure  which 
may  safely  be  brought  upon  the  soil,  and  having  ascertained  the 
weight  of  each  part  of  the  structure,  the  area  required  for  the  foun- 
dation is  easily  determined  by  dividing  the  latter  by  the  former. 


202  ORDINARY   FOUNDATIONS  [CHAP.  X. 

Then,  having  found  the  area  of  foundation,  the  base  of  the  struct- 
ure must  be  extended  by  footings  of  masonry,  concrete,  timber, 
etc.,  so  as  to  (1)  cover  that  area  and  (2)  distribute  the  pressure  uni- 
formly over  it.  The  two  items  will  be  considered  in  inverse  order. 
297.  Center  of  Pressure  and  Center  of  Base.  In  construct- 
ing a  foundation  the  object  is  not  so  much  to  secure  an  absolutely 
unyielding  base  as  to  secure  one  that  will  settle  as  little  as  possible, 
and  uniformly.  All  soils  will  yield  somewhat  under  the  pressure  of 
any  building,  and  even  masonry  itself  is  compressed  by  the  weight 
of  the  load  above  it.  The  pressure  per  square  foot  should,  there- 
fore, be  the  same  for  all  parts  of  the  building,  and  particularly  of 
the  foundation,  so  that  the  settlement  may  be  uniform.  This  can 
be  secured  only  when  the  axis  of  the  load  (a  vertical  line  through 
the  center  of  gravity  of  the  weight)  passes  through  the  center  of 
the  area  of  the  foundation.  If  the  axis  of  pressure  does  not  coincide 
exactly  with  the  axis  of  the  base,  the  ground  will  yield  most  on  the 
side  which  is  pressed  most ;  and  as  the  ground  yields,  the  base  as- 
sumes an  inclined  position,  and  carries  the  lower  part  of  the  struct- 
ure with  it,  thus  producing  unsightly  cracks,  if  nothing  more. 

The  coincidence  of  the  axis  of  pressure  with  the  axis  of  resist- 
ance is  of  first  importance.  This  principle  is  self-evident,  and  yet 
the  neglect  to  observe  it  is  the  most  frequent  cause  of  failure  in  the 
foundations  of  buildings. 

Fig.  50  is  an  example  of   the  way  in  which  this  principle  is 

violated.  The  shaded  portion 
represents  a  heavily  loaded  exte- 
rior wall,  and  the  light  portion  a 
lightly  loaded  interior  wall.  The 
foundations  of  the  two  walls  are 
rigidly  connected  together  at 
their  intersection.  The  center 
•  of  the  load  is  under  the  shaded 
Fig.  50.  scction,  and  the   center   of   the 

area  is  at  some  point  farther  to  the  left ;  consequently  the  exterior 
wall  is  caused  to  incline  outward,  producing  cracks  at  or  near  the 
corners  of  the  building.  Doubtless  the  two  foundations  are  con- 
nected in  the  belief  that  an  increase  of  the  bearing  surface  is  of  first 
importance ;  but  the  true  principle  is  that  the  coincidence  of  the 
axis  of  pressure  with  the  axis  of  resistance  is  the  most  important. 


AKT.  2.] 


DESIGNIKG  THE   FOOTIJSTG. 


203 


Fig.  51  is  another  illustration  of  the  same  principle.  The  foun- 
dation is  continuous  under  the  opening, 
and  hence  the  center  of  the  foundation  is  to 
the  left  of  the  center  of  pressure ;  conse- 
quently the  wall  inclines  to  the  right,  pro- 
ducing cracks,  usually  over  the  opening.* 

298.  The  center  of  the  load  can  be  made 
to  fall  inside  of  the  center  of  foundation  by 
extending  the  footings  outwards,  or  by  cur- 
tailing the  foundations  on  the  inside.  The 
latter  finds  exemplification  in  the  properly 
constructed  foundation  of  a  wall  containing  a  number  of  openings. 
For  example,  in  Fig.  52,  if  the  foundation  is  uniform  under  the 
entire  front,  the  center  of  pressure  must  be 
outside  of  the  center  of  the  base  ;  and  conse- 
quently the  two  side  walls  will  incline  outward, 
and  show  cracks  over  the  openings.  If  the 
width  of  the  foundation  under  the  openings 
be  decreased,  or  if  this  part  of  the  foundation 
be  omitted  entirely,  the  center  of  pressure 
will  fall  inside  of  the  center  of  base  and  the 
walls  will  tend  to  incline  inwards,  and  hence 
be  stable. 

299.  Conclusions.  One  conclusion  to  be 
drawn  from  the  above  examples  is  that  the 
foundation  of  a  wall  should  never  be  connected  with  that  of  another 
wall  either  much  heavier  or  much  lighter  than  itself.  Both  are 
equally  objectionable. 

A  second  conclusion  is  that  the  axis  of  the  load  should  strike  a 
little  inside  of  the  center  of  the  area  of  the  base,  to  make  sure  that 
it  will  not  be  outside.  Any  inward  inclination  of  the  wall  is  ren- 
dered impossible  by  the  interior  walls  of  the  building,  the  fioor- 
beams,  etc. ;  while  an  outward  inclination  can  be  counteracted  only 
by  anchors  and  the  bond  of  the  masonry.  A  slight  deviation  of  the 
axis  of  the  load  outward  from  the  center  of  the  base  has  a  marked 
effect,  and  is  not  easily  counteracted  by  anchors. 

*  For  an  account  showing  the  violation  of  this  principle  in  the  construction  of 
the  Cooper  Institute  Building,  New  York  City,  and  the  method  used  to  remedy  it,  see 
Hanitary  U/igineer,  vol.  xiL  pp.  465-68. 


Fig.  52. 


204 


ORDIN"ARY   FOUNDATIONS. 


[chap.  X. 


The  above  conclusions  may  be  summarized  in  the  following 
principle  :  All  foundalions  should  be  so  constructed  as  to  cornpress 
tlie  ground  slightly  concave  npivards,  rather  than  convex  iip- 
zoards.  On  even  slightly  compressible  soils,  a  small  difEerence  in 
the  pressure  on  the  foundation  will  be  sufficient  to  cause  the  bed  to 
become  convex  upwards.  At  Chicago,  an  omission  of  1  to  2  per 
cent,  of  the  weight  (by  leaving  openings)  usually  causes  sufficient 
convexity  to  prod-uce  unsightly  cracks.  With  very  slight  differences 
of  pressure  on  the  foundation,  it  is  sufficient  to  tie  the  building 
together  by  careful  bonding,  by  hoop-iron  built  in  over  openings, 
and  by  heavy  bars  built  in  where  one  wall  joins  another. 

300.  Independent  Piers.  The  art  of  constructing  founda- 
tions on  compressible  soil  has  been  brought  to  a  high  degree  of 
development  by  the  architects  of  Chicago.  The  special  feature  of 
the  practice  in  that  city  is  what  is  called  "the  method  of  independ- 
ent piers  ;"  that  is,  each  tier  of  columns,  each  pier,  each  wall,  etc. , 
has  its  own  independent  foundation,  the  area  of  which  is  propor- 
tioned to  the  load  on  that  part.*  The  interior  walls  are  fastened  to 
the  exterior  ones  by  anchors  which  slide  in  slots.  For  a  detailed 
account  of  the  methods  employed  in  one  of  the  best  and  largest 
buildings  erected  there,  see  Sanitary  Engineer,  Dec.  10,  1885. 

301.  Effect  of  the  Wind.  Overturning.  The  preceding  dis- 
cussion refers  to  the  total  weight  that  is  to  come  upon  the  foun- 

jj  jp    dation.     The  pressure  of  the  wind  against  towers, 

tall  chimneys,  etc. ,  transfers  the  point  of  applica- 
tion of  the  load  to  one  side  of  the  foundation.  The 
method  of  computing  the  position  of  the  center 
of  the  pressure  on  the  foundation  under  the  action 
of  the  wind  is  illustrated  in  Fig.  53,  in  which 
ABED  represents  a  vertical  section  of  the 

tower; 
rt  is  a  point  horizontally  opposite  the  center  of 
the  surface  exposed  to  the  pressure  of  the 
wind  and  vertically  above  the  center  of  grav- 
FiG.  53.  ity  of  the  tower; 


*  This  method  was  first  made  known  to  the  public  by  Frederick  Bauman,  of  Chi- 
cago, in  a  pamphlet  entitled  "  The  Method  of  Constructing  Foundations  on  Isolated 
Piers,"  published  by  him  in  187'2.  The  above  examples  and  principles  are  from  that 
pamphlet. 


AKT.  2.]  DESIGNING  THE   FOOTING.  20( 


C  is  the  position  of  the  center  of  pressure  when  there  is  no  wind  ; 

IN  is  the  center  when  the  wind  is  acting. 
For  convenience,  let 

P  =  the  maximum  pressure  on  the  foundation,  per  unit  of  area; 

2)  =  the  pressure  of  the  wind  per  unit  of  area  (see  §  295); 

H  =  the  total  pressure  of  the  wind  against  the  exposed  surface ; 

W  =  the  weight  of  that  part  of  the  structure  above  the  section 
considered, — in  this  case,  A  B  ; 

S  =  the  area  of  the  horizontal  cross  section ; 

/  =  the  moment  of  inertia  of  this  section ; 

I  =  the  distance  A  B ; 

h  =  the  distance  a  C ; 

d  =  the  distance  iV^  C; 
M  =  the  moment  of  the  wind. 

"When  there  is  no  horizontal  force  acting,  the  load  on  ^  ^  is 
uniform ;  but  when  there  is  a  horizontal  force  acting — as,  for  ex- 
ample, the  wind  blowing  from  the  right, — the  pressure  is  greatest 
near  A  and  decreases  towards  B.  To  find  the  law  of  the  variation 
of  this  pressure,  consider  the  tower  as  a  cantilever  beam.  The 
maximum  pressure  at  A  will  be  that  due  to  the  weight  of  the  tower 
plus  the  compression  due  to  flexure  ;  and  the  pressure  at  B  will  be 
the  compression  due  to  the  weight  minus  the  tension  due  to  flexure. 

W 

The  uniform  pressure  due  to  the  weight  is  —^.    The  strain  at  A  due 


to  flexure  is,  by  the  principles  of  the  resistance  of  materials, 
Then  the  maximum  pressure  per  unit  of  area  at  A  is 


Ml 


and  the  minimum  pressure  at  B  is 

S        21 ^'^f 

Equations  (1)  and  (2)  are  perfectly  general ;  they  are  applicable 
to  any  cross  section,  and  also  to  any  system  of  horizontal  and  ver- 
tical forces.  In  succeeding  chapters  they  will  be  emploved  in 
finding  the  unit  pressure  in  masonry  dams,  bridge  piers,  arches, 
etc. 


306 


OEDIK^ARY  FOUXDATIONS. 


[chap.  X. 


The  value  of  /  in  the  above  formulas  is  given  in  Fig.  54  for  the 
sections  occurring  most  frequently  in  practice.  Xotice  that  /  is  th© 
dimension  parallel  to  the  direction  of  the  wind,  and  J. the  dimen- 
sion perpendicular  to  the  direction  of  the  wind. 


I-^\,bP 


Fig.  54. 


I  =  ^n{l*-  h^) 


302.  If  the  area  of  the  section  A  B,  Fig.  53,  is  a  rectangle, 
S  =  I  b,  and  I  =  -^^b  P.  Substituting  these  values  in  equation  (1) 
gives 

p=K+n- (3) 


The  moment  of  the  wind,  M,  is  equal  to  the  product  of  its  total 
pressure,  H,  and  the  distance,  //,  of  the  center  of  pressure  above 
the  horizontal  section  considered;  or  M=  H.h.  H  is  equal  to 
the  pressure  per  unit  of  area,  jh  miilfiplied  by  the  area  of  the  sur- 
face exposed  to  the  pressure  of  the  wind.  Substituting  the  above 
value  of  M  in  equation  (3)  gives 

W  .   GH.h 

^4) 


P  = 


lb 


bV 


To  still  further  simplify  the  above  formula,  notice  that  Fig.  53 
gives  the  proportion 

H:  W::NC:aC, 
from  which 

H.aC=  W.NC; 

or,  changing  the  nomenclature, 

Hh=  Wd. 

Notice  that  the  last  relation  can  also  be  obtained  directly  by  the 
principle  of  moments.  Substituting  the  value  of  H.  h,  as  above,  in 
equation  (4)  gives 

^  ~  Ib^    bV  ' ^^' 

which  is  a  convenient  form  for  practical  application. 


ART.  2.]  DESIGNING   THE   FOOTING.  207 

An  examination  of  equation  (5)  shows  that  when  d  ==.  N C  =  \l, 
the  maximum  pressure  at  A  is  twice  the  average.  Notice  also  that 
under  these  conditions  the  pressure  at  B  is  zero.  This  is  equiva- 
lent to  what  is  known,  in  the  theory  of  arches,  as  the  principle  of 
the  middle  third.  It  shows  that  as  long  as  the  center  of  pressure 
lies  in  the  middle  third,  the  maximum  pressure  is  not  more  than 
twice  the  average  pressure,  and  that  there  is  no  tension  at  B. 

The  above  discussion  of  the  distribution  of  the  pressure  on  the 
foundation  is  amply  sufficient  for  the  case  in  hand  ;  however,  the 
subject  is  discussed  more  fully  in  the  chapter  on  Stability  of  Masonry 
Dams  (see  Chapter  XIII). 

303.  The  average  pressure  per  unit  on  A  B  has  already  been 
adjusted  to  the  safe  bearing  power  of  the  soil,  and  if  the  maximum 
pressure  at  A  does  not  exceed  the  iiltimate  bearing  power,  the  occa- 
sional maximum  pressure  due  to  the  wind  will  do  no  harm  ;  but  if 
this  maximum  exceeds  or  is  dangerously  near  the  ultimate  strength 
of  the  soil,  the  base  must  be  widened. 

304.  Sliding.  The  pressure  of  the  wind  is  a  force  tending  to 
slide  the  foundation  horizontally.  This  is  resisted  by  the  friction 
caused  by  the  weight  of  the  entire  structure,  and  also  by  the  earth 
around  the  base  of  the  foundation,  and  hence  there  is  no  need,  in 
this  connection,  of  considering  this  manner  of  failure. 

305.  Designing  the  Footing.  The  term  footing  is  usually  un- 
derstood as  meaning  the  bottom  course  or  courses  of  masonry  which 
extend  beyond  the  faces  of  the  Avail.  It  will  be  used  here  as  apply- 
ing to  the  material — whether  masonry,  timber,  or  iron — employed 
to  increase  the  area  of  the  base  of  the  foundation.  "Whatever  the 
character  of  the  soil,  footings  should  extend  beyond  the  face  of  the 
wall  (1)  to  add  to  the  stability  of  the  structure  and  lessen  the  dan- 
ger of  the  work's  being  thrown  out  of  plumb,  and  (2)  to  distribute 
the  weight  of  the  structure  over  a  larger  area  and  thus  decrease 
the  settlement  due  to  the  compression  of  the  gi'ound.  To  serve 
the  first  purpose,  footings  must  be  securely  bonded  to  the  body  of 
the  wall;  and  to  produce  the  second  effect,  they  must  have  sufficient 
strength  to  resist  the  transverse  strain  to  which  they  are  exposed. 
In  ordinary  buildings  the  distribution  of  the  weight  is  more  impor- 
tant than  adding  to  the  resistance  to  overturning,  and  hence  only 
the  former  will  be  considered  here. 

The  area  of  the  foundation  may  be  increased  until  the  inherent 


208  OKDINAET   FOUXDATIOXS.  [CHAP.  X. 

bearing  power  of  the  area  covered  is  suflBcient  to  support  the  load 
(1)  by  extending  the  bottom  courses  of  masonry,  or  (2)  by  the  use 
of  one  or  more  layers  of  timbers,  railroad  rails,  or  steel  I-beams,  or 
(3)  by  resting  the  structure  upon  inverted  masonry  arches. 

306.  Off-sets  of  Masonry  Footings.  The  area  of  the  foundation 
having  been  determined  and  its  center  having  been  located  -with 
reference  to  the  axis  of  the  load  (§  29T),  the  next  step  is  to  deter- 
mine how  m.uch  narrower  each  footing  course  may  be  than  the  one 
nrxt  below  it.  The  projecting  part  of  the  footing  resists  as  a  beam 
fixed  at  one  end  and  loaded  uniformly.  The  load  is  the  pressure 
on  the  earth  or  on  the  course  next  below.  The  off-set  of  such  a 
course  depends  upon  the  amount  of  the  pressure,  the  transverse 
strength  of  the  material,  and  the  thickness  of  the  course. 

To  deduce  a  formula  for  the  relation  between  these  quantities, 
let 

P  =  the  pressure,  in  tons  per  square  foot,  at  the  bottom  of  the 
footing  course  under  consideration  ; 

a  =  the  modulus  of  rupture  of  the  material,  in  pounds  jDer 
square  inch  ; 

p  =  the  greatest  possible  projection  of   the  footing  course,  in 

inches ; 
t  =  the  thickness  of  the  footing  course,  in  inches. 

The  part  of  the  footing  course  that  projects  beyond  the  one  above 
it,  is  a  cantilever  beam  uniformly  loaded.  From  the  principles  of 
the  resistance  of  materials,  we  know  that  the  upward  pressure  of  the 
earth  against  the  part  that  projects  multiplied  by  one  half  of  the 
length  of  the  projection  is  equal  to  the  continued  product  of  one 
sixth  of  the  modulus  of  rupture  of  the  material,  the  breadth  of  the 
footing  course,  and  the  square  of  the  thickness.  Expressing  this 
relation  in  the  above  nomenclature  and  reducing,  we  get  the  for- 
mula 

-or,  with  sufficient  accuracy, 

P  =  lt\^ (7) 

Hence  the  projection  available  with  any  given  thickness,  or  the 
thickness  required  for  any  given  projection,  may  easily  be  computed 


ART.  2.] 


DE3IGXIXG   THE    FOOTIXG. 


209 


by  equation  (7).     Notice  that  with  the  off-set  given  by  the  above 
iormula  the  stone  would  be  on  the  point  of  breaking. 

307.  The  margin  to  be  allowed  for  safety  will  depend  upon  the 
care  used  in  computing  the  loads,  in  selecting  the  materials  for  the 
footing  courses,  and  in  bedding  and  placing  them.  If  all  the  loads 
have  been  allowed  for  at  their  probable  maximum  value,  and  if  the 
material  is  to  be  reasonably  uniform  in  quality  and  laid  with  care, 
then  a  comparatively  small  margin  for  safety  is  sufficient ;  but  if 
all  the  loads  have  not  been  carefully  computed,  and  if  the  job  is  to 
be  done  by  an  unknown  contractor,  and  neither  the  material  nor 
the  work  is  to  be  carefully  inspected,  then  a  large  margin  is  neces- 
sary. As  a  general  rule,  it  is  better  to  assume,  for  each  particular 
case,  a  factor  of  safety  in  accordance  with  the  attendant  conditions 
of  the  problem  than  blindly  to  use  the  results  deduced  by  the 
application  of  some  arbitrarily  assumed  factor.  The  following  table 
is  given  for  the  convenience  of  those  who  may  wish  to  use  10  as  a 
factor  of  safety. 

TABLE  25. 

Safe  Off-set  for  Masonry  Footing  Courses,  in  Terms  of  the  Thick- 
ness OP  THE  Course,  using  10  as  a  Factor  of  Safety. 
For  limitations,  see  §  308. 


Kind  of  Stone. 


Blue-stone  Uaggiug  (see  page  13). 

Granite  (see  page  13) 

Limestone  (see  page  13) 

Sandstone  (see  page  13) 

Slate  (see  page  13) 


Be«t  Hard  Brick  (see  pages  40,  41) 

Hard  Brick  (see  pages  40,  41) 

( 1  Portland ) 

Concrete  (see  page  112c)  -<2  sand  [ 
(3  pebbles  ) 
(iPortiand     ,         , 

Concrete  (see  page  112t))  ^  3  sand         <-  *     -  ^'^^ 
( 5  pebbles 


4  weeks 
old 


old 


R,  IN   LBS. 
PERSQ.  IN. 


2,700 
1,800 
1,500 
1,200 
5,400 

1,500 
800 

150 


80 


Off-set    for    a    Pres- 
sure, IN   TONS  pbr   sq. 
FT.,  ON  THE  Bottom  op 
THE  Course,  op 


0.5 


3.6 
2.9 

2.7 
2.6 
5.0 

2.7 
1.9 

0.8 
0.6 


1.0 


2.6 

2.1 
1.9 
1.8 
3.6 

1.9 
1.4 

0.6 
0.4 


2.0 


1.8 
1.5 
1.3 
1.3 
2.5 

1.3 

0.8 

0.4 
0.3 


To  illustrate  the  method  of  using  the  preceding  table,  assume 
that  it  is  desired  to  determine  the  off-set  for  a  limestone  footing 
course  when  the  pressure  on  the  bed  of  the  foundation  is  1  ton  per 
square  foot,  using  10  as  a  factor  of  safety.     In  the  table,  opposite 


210  ORDIXAEY   FOUNDATIOXS.  [CHAP.  X» 

limestone,  in  next  to  the  last  column,  we  find  the  quantity  1.9. 
This  shows  that,  under  the  conditions  stated,  the  off-set  may  be  1.9 
times  the  thickness  of  the  course. 

The  values  in  the  table  agree  very  well  with  the  practice  of  the 
principal  architects  and  engineers  for  hammer-dressed  stones  laid 
in  good  cement  mortar. 

If  it  is  desired  to  use  any  other  factor  of  safety,  it  is  only  neces- 
sary to  substitute  for  R,  in  the  preceding  formula,  the  desired  frac- 
tional part  of  that  quantity  as  given  in  the  second  column  of  the- 
above  table.  For  examj^le,  assume  that  it  is  necessary  to  use  lime- 
stone in  the  foundation,  and  that  it  is  required  to  draw  in  the  foot- 
ing courses  as  rapidly  as  possible.  Assume  also  that  the  pressure, 
P,  on  the  base  of  the  foundation  is  2  tons  per  square  foot.  If  the 
limestone  is  of  the  best,  and  if  it  is  laid  with  great  care,  it  will  be 
sufficient  to  use  4  as  a  factor  of  safety.  Under  these  conditions, 
equation  (7)  as  above  gives 


That  is,  the  projection  may  be  2.3  times  the  thickness  of  the  course. 

308.  Strictly,  the  above  computations  are  correct  only  for  the 
lower  off-set,  and  then  only  when  the  footing  is  composed  of  stones 
whose  thickness  is  equal  to  the  thickness  of  the  course  and  which 
project  less  than  half  their  length,  and  which  are  also  well  bedded. 
The  resistance  of  two  or  more  courses  to  bending  varies  as  the  square 
of  their  depth,  and  the  bending  due  to  the  uniform  pressure  on  the 
base  will  also  increase  as  the  square  of  the  sum  of  the  projections, 
and  therefore  the  successive  off-sets  should  be  proportional  to  the 
thickness  of  the  course;  or,  in  other  words,  the  values  as  above  are 
applicable  to  any  course,  provided  no  stone  projects  more  than  half 
its  length  beyond  the  top  course. 

The  preceding  results  will  be  apjilicable  to  built  footing  courses 
only  when  the  pressure  above  the  course  is  less  than  the  safe  strength 
of  the  mortar  (see  §  133  and  §  161a).  The  proper  projection  for 
rubble  masonry  lies  somewhere  between  the  values  given  for  stone 
and  those  given  for  concrete.  If  the  rubble  consists  of  large  stones 
well  bedded  in  good  strong  mortar,  then  the  values  for  this  class  of 
masonry  will  be  but  little  less  than  those  given  in  the  table.  If  the 
rubble  consists  of  small  irregular  stones  laid  with  Portland  or  nat- 


ART.  2.]  DESIGNING   THE   FOOTING.  211 

■nral  cement  mortar,  the  projection  should  not  much  exceed  that 
given  for  concrete.  If  the  rabble  is  laid  in  lime  mortar,  the  pro- 
jection of  the  footing  course  should  not  be  more  than  half  that 
allowed  when  cement  mortar  is  used. 

309.  Timber  Footing.  In  very  soft  earth  it  would  be  inexpe- 
dient to  use  masonry  footings,  since  the  foundation  would  be  very 
deep  or  occupy  the  space  usually  devoted  to  the  cellar.  One  method 
of  overcoming  this  difficulty  consists  in  constructing  a  timber  grat- 
ing, sometimes  called  a  grillage,  by  setting  a  series  of  heavy  timbers 
firmly  into  the  soil,  and  laying  another  series  transversely  on  top  of 
these.  The  timbers  may  be  fastened  at  their  intersections  by  spikes 
or  drift-bolts  (§  381)  if  there  is  any  possibility  of  sliding,  which  is 
unlikely  in  the  class  of  foundations  here  considered.  The  earth 
should  be  packed  in  between  and  around  the  several  beams.  A 
flooring  of  thick  planks,  often  termed  a  ijlatform,  is  laid  on  top  of 
the  grillage  to  receive  the  lowest  course  of  masonry.  In  extreme 
cases,  the  timbers  in  one  or  more  of  the  courses  are  laid  close  to- 
gether. Timber  should  never  be  used  except  where  it  will  be  always 
wet. 

The  amount  that  a  course  of  timber  may  project  beyond  the  one 
next  above  it  can  be  determined  by  equation  (7),  page  208.  Making 
R  in  that  equation  equal  to  1,000 — the  value  ordinarily  used, — and 
solving,  we  obtain  the  following  results  for  the  safe  jirojection:  If 
the  pressure  on  the  foundation  is  0.5  ton  per  square  foot,  the  safe 
projection  is  7. 5  times  the  thickness  of  the  course  ;  if  the  pressure 
is  1  ton  per  square  foot,  the  safe  projection  is  5.3  times  the  thick- 
ness of  the  course  ;  and  if  the  pressure  is  2  tons  per  square  foot,  the 
safe  projection  is  3.7  times  the  thickness  of  the  course.  The  above 
values  give  a  factor  of  safety  of  about  10.  To  use  any  other  factor, 
insert  in  equation  (7),  above,  the  corresponding  fractional  part  of  the 
ultimate  transverse  strength  of  the  particular  timber  to  be  used, 
and  solve. 

The  above  method  of  computation  is  not  applicable  to  two  or 
more  courses  of  timber,  if  one  is  transverse  to  the  other. 

310.  This  method  of  increasing  the  area  of  the  footing  is  much 
used  at  Xew  Orleans.  The  Custom-house  at  that  place  is  founded 
upon  a  3-inch  plank  flooring  laid  7  feet  below  the  street  pavement. 
A  grillage,  consisting  of  timbers  12  inches  square  laid  side  by  side, 
is  laid  upon  the  floor,  over  which  similar  timbers  are  placed  trans- 
versely, 2  feet  apart  in  the  clear. 


212  ORDINARY   FOUNDATIONS.  [CHAP.  X. 


Most  of  the  buildings  of  the  World's  Colnmbiau  Exposition, 
Chicago,  1893,  were  founded  upon  timber  footings. 

311.  Steel  Footing.  Very  recently,  steel,  usually  in  the  form 
of  railroad  rails  or  I-beams,  has  been  used  instead  of  timber  in 
foundations.  The  rails  or  I-beams  are  placed  side  by  side,  and 
concrete  is  rammed  in  between  them. 

Steel  is  superior  to  timber  for  this  purpose,  in  that  the  latter 
can  be  used  only  Avhere  it  is  always  wet,  while  the  former  is  not 
affected  by  variations  of  wetness  and  dryness.  Twenty  years'  ex- 
perience in  this  use  of  steel  at  Chicago  shows  that  after  a  short  time 
the  surface  of  the  metal  becomes  encased  in  a  coating  which  2:)re- 
vents  further  oxidation.  The  most  important  advantage,  however, 
in  this  use  of  steel  is  that  the  off-set  can  be  much  greater  with  steel 
than  with  wood  or  stone;  and  hence  the  foundations  may  be  shal- 
low, and  still  not  occupy  the  cellar  space. 

The  proper  projections  for  the  steel  beams  can  be  computed  by 
a  formula  somewhat  similar  to  that  of  §  306;  but  the  steel  footing 
is  appropriately  a  part  of  the  steel-skeleton  construction,  and  hence 
will  not  be  considered  here.  For  a  presentation  of  the  method 
of  computations  formerly  employed  in  Chicago,  see  Engmeering 
News,  vol.  xxvi.  page  122;  and  for  adverse  criticisms  thereon,  see 
ibid.,  pages  265,  312,  -415,  and  vol.  xxxii.  page  387.  Concerning 
the  effect  of  the  strength  of  the  base  of  the  column,  see  Johnson's 
"  Modem  Framed  Structure "5,"  pages  444-46.  For  a  discussion 
which  consider.5  the  deflection  of  l!ie  several  beams,  see  Btigineering^ 
Record,  vol.  xxxix.  pages  333-34,  354-56,  383,  407-8.  The  last 
is  the  most  exact  method  of  analj^sis,  and  also  secures  the  greatest 
economy  of  material. 

312.  Inverted  Arch.  Inverted  arches  are  frequently  built  under 
and  between  the  bases  of  piers,  as  shown  in  Fig.  55.     Employed  in 

this  Avay,  the  arch  simply  distributes 
the  pressure  over  a  greater  area;  but 
it  is  not  well  adapted  to  this  use,  for 
(1)  it  is  nearly  impossible  to  prevent 
the  end  piers  of  a  series  from  being 
pushed  outward  by  the  thrust  of  the 
^''^■^^"  arch,  and   (2)   it  is  generally  impos- 

sible, with  inverted  arches,  to  make  the  areas  of  the  different  parts 
of  the  foundiition  proportional  to  the  load  to  be  supported  (see  § 


ART.  3.]  PREPAEING   THE    BED.  21? 

297).     The   only  advantage  the  inverted  arch   has  over  masonry 
footings  is  in  the  shallower  foundation  obtained. 

313.  In  a  few  cases  masonry  piers  have  been  sunk  to  a  solid  sub- 
stratum by  excavating  the  material  from  the  inside,  and  then  resting 
arches  on  these  piers.  This  is  an  exj^ensive  method,  and  has  essen- 
tially the  same  objections  as  the  inverted  arch. 

Art.  3.  Prepaeixg  the  Bed. 

314.  On  Rock  To  prepare  a  rock  bed  to  receive  a  foundation 
it  is  generally  only  necessary  to  cut  away  the  loose  and  decayed  por- 
tions of  the  rock,  and  to  dress  it  to  a  plane  surface  as  nearly  perpen- 
dicular to  the  direction  of  the  pressure  as  is  practicable.  If  there 
are  any  fissures,  they  should  be  filled  with  concrete.  A  rock  that 
is  very  much  broken  can  be  made  amply  secure  for  a  foundation  by 
the  liberal  use  of  good  cement  concrete.  The  piers  of  the  Xiagara 
Cantilever  Bridge  are  founded  upon  the  top  of  a  bank  of  bowlders, 
which  were  first  cemented  together  with  concrete. 

Sometimes  it  is  necessary  that  certain  parts  of  a  structure 
start  from  a  lower  level  than  the  others.  In  this  case  care  should 
be  taken  (1)  to  keep  the  mortar-joints  as  thin  as  possible,  (2)  to  lay 
the  lower  j)ortions  in  cement,  and  (3)  to  proceed  slowly  with  the 
work  ;  otherwise  the  greater  quantity  of  mortar  in  the  wall  on  the 
lower  portions  of  the  slope  will  cause  greater  settling  there  and  a 
consequent  breaking  of  the  joints  at  the  stepping-places.  The 
bonding  over  the  off-sets  should  receive  particular  attention. 

315.  On  Firm  Earth.  For  foundations  in  such  earths  as  hard 
clay,  clean  dry  gravel,  or  clean  sharp  sand,  it  is  only  necessary  to 
dig  a  trench  from  3  to  G  feet  deep,  so  that  the  foundation  may  be 
below  the  disintegrating  effect  of  frost.  Provision  should  also  be 
made  for  the  drainage  of  the  bed  of  the  foundation. 

With  this  class  of  foundations  it  often  happens  that  one  part  of 
the  structure  starts  from  a  lower  level  than  another.  When  this  is 
the  case  great  care  is  required.  All  the  precautions  mentioned  in 
the  second  paragraph  of  §  314  should  be  observed,  and  great  care 
should  also  be  taken  so  to  proportion  the  load  per  unit  of  area  that 
the  settlement  of  the  foundation  may  be  uniform.  This  is  difficult 
to  do,  since  a  variation  of  a  few  feet  in  depth  often  makes  a  great 
difference  in  the  supporting  power  of  the  soil. 


214  ORDIXARY    FOUXDATIOXS.  [CHAP.   X. 

316.  In  Wet  Ground.  The  difficulty  in  soils  of  this  class  is  in 
disposing  of  the  water,  or  in  preventing  the  semi-liquid  soil  from 
running  into  the  excavation.  The  difficulties  are  similar  to  those 
met  with  in  constructing  foundations  underwater — see  Chapter  XII. 
Tln-ee  general  methods  of  laying  a  foundation  in  this  kind  of  soil 
will  be  briefly  described. 

317.  Coffer-Dam.  If  the  soil  is  only  moderately  wet — not  satu- 
rated,— it  is  sufficient  to  inclose  the  area  to  be  excavated  with  sheet 
piles  (boards  driven  vertically  into  the  ground  in  contact  with  each 
other).  This  curbing  is  a  simple  form  of  a  coffer-dam  (Art.  1, 
Chap.  XII).  The  boards  should  be  sharpened  wholly  from  one 
side  ;  this  point  being  placed  next  to  the  last  pile  driven  causes 
them  to  crowd  together  and  make  tighter  joints.  The  sheeting  may 
be  driven  by  hand,  by  a  heavy  weight  raised  by  a  tackle  and  then 
dropped,  or  by  an  ordinary  pile-driver  (§§  335-3G).  Unless  the 
amount  of  water  is  quite  small,  it  will  be  necessary  to  drive  a  double 
row  of  sheeting,  breaking  joints.  It  will  not  be  possible  to  entirely 
prevent  leaking.  The  water  that  leaks  in  may  be  bailed  out,  or 
pumped — either  by  hand,  or  by  steam  (see  §  395). 

To  prevent  the  sheeting  from  being  forced  inward,  it  may  be 
braced  by  shores  placed  horizontally  from  side  to  side  and  abutting 
against  wales  (horizontal  timbers  which  rest  against  the  sheet  piles). 
The  bracing  is  put  in  successively  from  the  top  as  the  excavation 
proceeds ;  and  as  the  masonry  is  built  up,  short  braces  between  the 
sheeting  and  the  masonry  are  substituted  for  the  long  braces  which 
previously  extended  from  side  to  side.  Iron  screws,  somewhat 
similar  to  jack-screws,  are  used,  instead  of  timber  shores,  in  exca- 
vating trenches  for  the  foundations  of  buildings,  for  sewers,  etc. 

If  one  length  of  sheeting  will  not  reach  deep  enough,  an  addi- 
tional section  can  be  placed  inside  of  the  one  already  in  position, 
when  the  excavation  has  reached  a  sufficient  depth  to  require  it. 
Ordinary  planks  8  to  12  inches  wide  and  1^  or  2  inches  thick  are 
used. 

For  a  more  extended  account  of  the  use  of  coffer-dams,  see 
Chapter  XII — Foundations  Under  Water,  Art.  1 — Coffer-Dams. 

318.  In  some  cases  the  soil  is  more  easily  excavated  if  it  is  first 
drained.  To  do  this,  dig  a  hole — a  sump — into  which  the  water  will 
drain  and  from  which  it  may  be  pumped.  If  necessary,  several 
sumps  may  be  sunk,  and  deepened  as  the  excavation  proceeds. 


ART.  3.]  PREPARING   THE    BED.  215 

Quicksand  or  soft  alluvium  may  sometimes  be  pumped  out  along 
with  the  water  by  a  centrifugal  or  a  mud  pump  (§  395  and  §  448). 
On  large  jobs,  such  material  is  sometimes  taken  out  with  a  clam- 
shell or  Milroy  dredge  (§  412). 

319.  Concrete.  Concrete  can  frequently  be  used  advantage- 
ously in  foundations  in  wet  soils.  If  the  water  can  be  removed,  the 
concrete  should  be  deposited  in  continuous  layers,  about  6  inches 
thick,  and  gently  rammed  until  the  water  begins  to  ooze  out  on  the 
upper  surface  (see  §  153).  If  the  water  can  not  be  removed,  the 
concrete  may  be  deposited  under  the  water  (see  §  154),  although  it 
is  more  difficult  to  insure  good  results  by  this  method  than  when 
the  concrete  is  deposited  in  the  open  air.* 

320.  Grillage.  If  the  semi-liquid  soil  extends  to  a  considerable 
depth,  or  if  the  soft  soil  which  overlies  a  solid  substratum  can  not  be 
removed  readily,  it  is  customary  to  drive  piles  at  uniform  distances 
over  the  area,  and  construct  a  grillage  (see  §  380)  on  top  of  them. 
This  construction  is  very  common  for  bridge  abutments  (Chapter 
XV).  The  piles  should  be  sawed  off  (§  3TS)  below  low-water,  which 
usually  necessitates  a  coffer-dam  (§  317,  and  Art.  1  of  Chapter  XII), 
and  the  excavation  of  the  soil  a  little  below  the  low- water  line. 

For  a  more  extended  account  of  this  method  of  laying  a  founda- 
tion, see  §§  380-82. 

321.  In  excavating  shallow  pits  in  sand  containing  a  small 
amount  of  water,  dynamite  cartridges  have  been  successfully  used  to 
drive  the  water  out.  A  hole  is  bored  with  an  ordinary  auger  and 
the  cartridge  inserted  and  exploded.  The  explosion  drives  the  water 
back  into  the  soil  so  far  that,  by  working  rapidly,  the  hole  can  be 
excavated  and  a  layer  of  concrete  placed  before  the  water  returns. 

322.  Conclusion.  It  is  hardly  worth  while  here  to  discuss  this 
subject  further.  It  is  one  on  which  general  instruction  can  not  be 
given.  Each  case  must  be  dealt  with  according  to  the  attendant 
circumstances,  and  a  knowledge  of  the  method  best  adapted  to  any 
given  conditions  comes  only  by  experience. 

*  For  tlie  composition,  cost,  etc. ,  of  concrete,  see  Art.  2  of  Chap,  iv,  pp. 
106-13t. 


CHAPTER  XL 
PILE  FOUNDATIONS. 

323.  Definitions.  Pile.  Although  a  pile  is  generally  unde^ 
stood  to  be  a  round  timber  driven  into  the  soil  to  support  a  loadj 
the  term  has  a  variety  of  applications  which  it  will  be  well  to  explain. 

Bearing  Pile.  One  used  to  sustain  a  vertical  load.  This  is  the 
ordinary  pile,  and  usually  is  the  one  referred  to  when  the  word  pile 
is  employed  without  qualification. 

Slteet  Piles.  Thick  boards  or  timbers  driven  in  close  contact 
to  inclose  a  space,  to  prevent  leakage,  etc.  Generally  they  are  con- 
siderably wider  than  thick;  but  are  sometimes  square,  in  which  case- 
they  are  often  called  dose  }) Has. 

False  Pile.  A  timber  added  to  a  pile  after  driving,  to  supple' 
ment  its  length. 

Foundation  Pile.  One  driven  to  increase  the  supporting  power 
of  the  soil  under  a  foundation. 

Screiu  Pile.  An  iron  shaft  to  the  bottom  of  which  is  attached 
a  broad-bladed  screw  having  only  one  or  two  turns. 

Disk  Pile.  A  bearing  pile  near  the  foot  of  which  a  disk  is  keyed 
or  bolted  to  give  additional  bearing  power. 

Pneuniatie  Pile.  A  metal  cylinder  which  is  sunk  by  atmos- 
pheric pressure.  Tliis  form  of  pile  will  be  discussed  in  the  next 
chapter  (see  §  431). 

Art.  1.  Descriptions,  and  Methods  of  Driving. 

3^4.  Iron  Piles.  Both  cast  and  wrought  iron  are  employed  for 
ordinary  bearing  piles,  sheet  piles,  and  for  cylinders.  Iron  cylin- 
ders are  generally  sunk  either  by  dredging  the  soil  from  the  inside 
(§  415),  or  by  the  pneumatic  process  (see  the  next  chapter,  particu- 
larly §§  431-35).  For  another  method  of  employing  iron  cylinders, 
see  §§  384-85. 

216 


AET.    1.]  DESCKIPTIONS,    AND   METHODS    OF   DRIVING.  217 


Cast-iron  piles  are  beginning  to  be  used  as  substitutes  for  com- 
mon wooden  ones.  Lugs  or  flanges  are  usually  cast  on  the  sides  of 
the  piles,  to  which  bracing  may  be  attached  for  securing  them  in 
position.  A  wood  block  is  laid  upon  the  top  of  the  pile  to  receive 
the  hammer  used  in  driving  it;  and,  after  being  driven,  a  cap  with 
a  socket  in  its  lower  side  is  placed  ujion  the  pile  to  receive  the  load. 
The  supporting  power  is  sometimes  increased  by  keying  on  an  iron 
disk.  The  advantages  claimed  for  cast-iron  piles  are:  (1)  thev  are 
not  subject  to  decay;  (2)  they  are  more  readily  driven  than  wooden 
ones,  especially  in  stony  ground  or  stiff  clay;  and  (3)  they  possess 
greater  crushing  strength,  which,  however,  is  an  advantage  only 
when  the  pile  acts  as  a  column  (see  §  355).  The  principal  disadvan- 
tage is  that  they  are  deficient  in  transverse  resistance  to  a  suddenly 
applied  force.  This  objection  applies  only  to  the  handling  of  the 
piles  before  being  driven,  and  to  such  as  are  liable,  after  being  driven, 
to  sudden  lateral  blows,  as  from  floating  ice,  logs,  etc. 

Recently,  rolled  sections  of  wrought-iron  have  been  employed  to  a, 
limited  degree  for  bearing-piles,  but  present  prices  prohibit  an  ex- 
tended use  of  wrought-iron  piles.  It  is  possible  that  iron  may  take 
the  place  of  wood  for  piles  where  they  are  alternately  wet  and  dr}?-, 
or  where  they  are  difficult  to  drive;  but  where  the  piles  are  always, 
wet — as  is  usually  the  case  in  foundation  work, — wood  is  as  durable 
as  iron;  and  hence,  on  account  of  cheapness,  is  likely  to  have  the 
preference. 

325.  Screw  Piles.  These  are  generally  wholly  of  iron,  although 
the  stem  is  sometimes  wood.  The  screw  pile  usually  consists  of  a 
rolled-iron  shaft,  3  to  8  inches  in  diameter,  having  at  its  foot  one  or 
two  turns  of  a  cast-iron  screw,  the  blades  of  which  may  vary  from  1  ^ 
to  5  feet  in  diameter.  The  piles  ordinarily  employed  for  light- 
houses exposed  to  moderate  seas  or  to  heavy  fields  of  ice  have  a 
shaft  3  to  5  inches  in  diameter  and  blades  3  to  4  feet  in  diameter, 
the  screw  weighing  from  600  to  700  pounds.  For  bridge  piei-s, 
the  shafts  are  from  6  to  8  inches  and  the  blades  from  4  to  6  feet  in 
diameter,  the  screw  w^eighing  from  1,500  to  4,000  pounds. 

Screw  piles  were  invented  by  Mitchell  of  Belfast,  and  are  largely 
used  in  Europe,  but  not  to  any  great  extent  in  this  countr}'.  They 
liave  been  used  in  founding  small  light-houses  on  the  sea-shore,  for 
signal  stations  in  marine  surveying,  for  anchorage  for  buoys,  and 
for  various  purposes  inland. 


318  PILE   FOUNDATIONS.  [CHAP.  XI. 

For  founding  beacons,  etc.,  the  screw  pile  has  the  special  advan- 
tage of  not  being  drawn  out  by  the  upward  force  of  the  waves  against 
the  superstructure.  Even  when  all  cohesion  of  the  ground  is  de- 
stroyed in  screwing  down  a  pile,  a  conical  mass,  with  its  apex  at  the 
bottom  of  the  pile  and  its  base  at  the  surface,  would  have  to  be 
lifted  to  draw  the  pile  out.  The  supporting  power  also  is  consider- 
abl '  owing  to  the  increased  bearing  surface  of  the  screw  blade. 
Screw  piles  have,  therefore,  an  advantage  in  soft  soil.  They  could 
also  be  used  advantageously  in  situations  where  the  jar  of  driving 
ordinary  piles  might  disturb  the  equilibrium  of  adjacent  structures. 

326.  These  piles  are  usually  screwed  into  the  soil  by  men  work- 
ing with  capstan  bars.  Sometimes  a  rope  is  wound  around  the 
shaft  and  the  two  ends  pulled  in  opposite  directions  by  two  capstans, 
and  sometimes  the  screw  is  turned  by  attaching  a  large  cog-wheel  to 
the  shaft  by  a  friction-clutch,  which  is  rotated  by  a  worm-screw 
operated  by  a  hand  crank.  Of  course  steam  or  horse-power  could 
be  used  for  this  purpose. 

The  screw  will  penetrate  most  soil  .  It  will  pass  through  loose 
pebbles  and  stones  without  much  difficulty,  and  push  aside  bowlders 
of  moderate  size.  Ordinary  clay  does  not  present  much  obsti'uction; 
clean,  dry  sand  gives  the  most  difficulty.  The  danger  of  twisting 
off  the  shaft  limits  the  depth  to  which  they  may  be  sunk.  Screw 
piles  with  blades  4  feet  in  diameter  have  been  screwed  40  feet  into 
a  mixture  of  clay  and  sand.  The  resistance  to  sinking  increases 
very  rapidly  with  the  diameter  of  the  screw;  but  under  favorable 
circumstances  an  ordinary  screw  pile  can  be  sunk  very  quickly. 
Screws  4  feet  in  diameter  have,  in  less  than  two  hours,  been  sunk 
by  hand-labor  20  feet  in  sand  and  clay,  the  surface  of  which  was 
20  feet  below  the  water.  For  depths  of  15  to  30  feet,  an  average  of 
4  to  8  feet  per  day  is  good  work  for  wholly  hand-labor. 

For  an  illustrated  and  detailed  account  of  the  founding  of  a  rail- 
road bridge  pier  on  screw  piles,  see  Engineering  News,  Vol.  XIII. 
pp.  210-12. 

327.  Disk  Files.  These  differ  but  little  from  screw  piles,  a 
flat  disk,  instead  of  a  screw,  being  keyed  on  at  the  foot  of  the  iron 
stem.  Disk  piles  are  sunk  by  the  water-jet  (§  343).  One  of  the  few 
cases  in  which  they  have  been  used  in  this  country  was  in  founding 
an  ocean  pier  on  Coney  Island,  near  New  York  City.  The  shafts 
were  wrought-iron,  lap-welded  tubes,  8f  inches  outside  diameter,  in 


AKT.   1.]  DESCKIPTIOJSrS,    AND    METHODS    OF   DRIVING.  219 

sections  12  to  20  feet  long ;  the  disks  were  2  feet  in  diameter  and 
9  inches  thick,  and  were  fastened  to  the  shaft  by  set-screws.  Many 
of  the  piles  were  57  feet  long,  of  which  17  feet  was  in  the  sand.* 

328.  Sand  Piles.  For  an  accoiint  of  the  method  of  using  sand 
as  piles,  see  §  2SS. 

329.  Sheet  Piles.  These  are  flat  piles,  which,  being  driven 
successively  edge  to  edge,  form  a  vertical  or  nearly  vertical  sheet 
for  the  purpose  of  preventing  the  materials  of  a  foundation  from 
spreading,  or  of  guarding  them  against  the  undermining  action  of 
water.  They  may  be  made  either  of  timber  or  iron.  Ordinarily 
sheet  piles  are  simply  thick  planks,  sharpened  and  driven  edge  to 
edge.  Sometimes  they  have  a  tongue  on  one  edge  and  a  correspond- 
ing groove  on  the  other,  to  aid  in  guiding  them  into  place  while 
driving.  When  heavy  timbers  are  employed  as  sheet  piling,  wooden 
blocks  or  iron  lugs  are  fastened  on  the  edges  to  assist  in  guiding 
them  into  position.  Sheet  piles  should  be  sharpened  wholly,  or  at 
least  mainly,  from  one  side,  and  the  long  edge  placed  next  to  the 
pile  already  driven.  This  causes  them  to  crowd  together  and 
make  comparatively  close  joints. 

When  a  space  is  to  be  inclosed  with  sheet  piling,  two  roAvs  of 
guide  piles  are  first  driven  at  regular  intervals  of  from  6  to  10  feet, 
and  to  opposite  sides  of  these,  near  the  toj),  are  notched  or  bolted  a 
pair  of  parallel  string-pieces,  or  wales,  from  5  to  10  inches  square, 
so  fastened  to  the  guide  piles  as  to  leave  a  space  between  the  wales 
equal  to  the  thickness  of  the  sheet  jDiles.  If  the  sheeting  is  to  stand 
more  than  8  or  10  feet  above  the  ground,  a  second  pair  of  wales  is 
required  near  the  level  of  the  ground.  The  sheet  piles  are  driven 
(§§  334-45)  between  the  wales,  working  from  both  ends  towards 
the  middle  of  the  space  between  a  pair  of  guide  piles,  so  that  the 
last  or  central  pile  acts  as  a  wedge  to  tighten  the  whole. 

330.  Wooden  Bearing  Piles.  Spruce  and  hemlock  answer 
very  well,  in  soft  or  medium  soils,  for  foundation  piles  or  for  piles 
always  under  water  ;  the  hard  pines,  elm,  and  beech,  for  firmer 
soils  ;  and  the  hard  oaks,  for  still  more  compact  soils.  Where  the 
pile  is  alternately  wet  and  dry,  white  or  post  oak  and  yellow  or 
southern  pine  are  generally  used. 


*  For  a  detailed  and  illustrated  description  of  this  work,  see  an  article  by  Charles 
Macdonald,  C.E.,  in  Trans.  Am.  See.  of  C.  E.,  Vol.  VIII.  pp.  227-37. 


220  PILE   FOUNDATIONS.  [CHAP.  XI 

Piles  should  never  be  less  than  8  inches  in  diameter  at  the  small 
end  and  never  more  than  18  inches  at  the  large  end.  Specifications 
usually  require  that  these  dimensions  shall  not  be  less  than  10  nor 
more  than  14  inches  respectively.  Piles  should  be  straight-gi'ained, 
should  be  trimmed  close,  and  should  have  the  bark  removed. 

331.  Specifications  for  Piles.  The  ordinary  specifications  are 
about  as  follows  :* 

Piles,  whether  used  in  foundations,  trestle-work,  or  pile  bridges,  shall  be 
of  good  qualitj%  sound,  white  oak^or  such  other  timber  as  the  engineer  maj' 
direct,  not  less  than  ten  inches  (10")  in  diameter  at  the  smaller  end  and 
14  inches  (14")  at  the  larger,  and  of  such  lengths  as  the  engineer  may  require. 
They  must  be  straight-grained,  must  be  trimmed  close,  and  must  have  all  the 
bark  taken  off  before  being  driven.  They  must  be  cut  off  square  at  the  butt, 
and  be  properlj^  sharpened.  If  required  by  the  engineer,  the  point  shall  be 
shod  with  iron  shoes  [see  §  332],  and  the  head  hooped  with  iron  bands  of  ap- 
proved size  and  form  [see  §  332],  which  will  be  paid  for  by  the  pound. 

332.  Pile  Caps  and  Shoes.  To  prevent  bruising  and  splitting 
in  driving,  2  or  3  inches  of  the  head  is  usually  chamfered  off.  As 
an  additional  means  of  preventing  splitting,  the  head  is  often 
hooped  with  a  strong  iron  band,  2  to  3  inches  wide  and  -I  to  1  inch 
thick.  The  expense  of  removing  these  bands  and  of  replacing  the 
broken  ones,  and  the  consequent  delays,  led  to  the  introduction, 
recently,  of  a  cap  for  the  protection  of  the  head  of  the  pile.  The 
cap  consists  of  a  cast-iron  block  with  a  tapered  recess  above  and 
below,  the  chamfered  head  of  the  pile  fitting  into  the  lower  recess 
and  a  cushion  piece  of  hard  wood,  upon  which  the  hammer  falls, 
fitting  into  the  upper  one.  The  cap  preserves  the  head  of  the  pile, 
adds  to  the  effectiveness  of  the  blows  (§  361),  and  keeps  the  pile  head 
in  place  to  receive  the  blows  of  the  hammer. 

A  further  advantage  of  the  pile  cap  is  that  it  saves  piles.  In 
hard  driving,  without  the  cap  the  head  is  crushed  or  broomed  to 
such  an  extent  that  the  pile  is  adzed  or  sawed  off  several  times 
before  it  is  completely  driven,  and  often  after  it  is  driven  a  portion 
of  the  head  must  be  sawed  off  to  secure  sound  wood  upon  which  to 
rest  the  grillage  or  platform  (§  380).  In  ordering  piles  for  any 
special  work  where  the  driving  is  hard,  allowance  must  be  made  for 
this  loss. 

Piles  are  generally  sharpened  before  being  driven,  and  some- 

*  See  also  "  Piling"  in  the  general  specifications  for  railway  masonry,  as  given  in 
Appendix  I. 


ART.  1.]         DESCRIPTIONS,    AND   METHODS   OF   DRIVING.  221 

times,  particularly  in  stony  ground,  the  point  is  protected  by  an 
iron  shoe.  The  shoe  may  be  only  two  V  -shaped  loops  of  bar  iron 
placed  over  the  point,  in  planes  at  right  angles  to  each  other,  and 
spiked  to  the  piles  ;  or  it  may  be  a  wrought  or  cast  iron  socket,  of 
which  there  are  a  number  of  forms  on  the  market. 

333.  Splicing  Piles.  It  frequently  happens,  in  driving  piles  in 
swampy  places,  for  false-works,  etc.,  that  a  single  pile  is  not  long 
enough,  in  which  case  two  are  spliced  together.  A  common  method 
of  doing  this  is  as  follows  :*  after  the  first  j)ile  is  driven  its  head  is 
cut  off  square,  a  hole  2  inches  in  diameter  and  12  inches  deep  is 
bored  in  its  head,  and  an  oak  treenail,  or  dowel-pin,  23  inches 
long,  is  driven  into  the  hole  ;  another  pile,  similarly  squared  and 
bored,  is  placed  upon  the  lower  pile,  and  the  driving  continued. 
Spliced  in  this  way  the  pile  is  deficient  in  lateral  stiffness,  and  the 
upper  section  is  liable  to  bounce  off  while  driving.  It  is  better  to 
reinforce  the  splice  by  flatting  the  sides  of  the  piles  and  nailing  on, 
wnth  say  8-inch  spikes,  four  or  more  pieces  2  or  3  inches  thick,  4  or 
5  inches  wide,  and  4  to  6  feet  long.  In  the  erection  of  the  bridge 
over  the  Hudson  at  Poughkeepsie,  K.  Y.,  two  piles  were  thus 
spliced  together  to  form  a  single  one  130  feet  long. 

Piles  may  be  made  of  any  required  length  or  cross-section  by 
bolting  and  fishing  together,  sidewise  and  lengthwise,  a  number  of 
squared  timbers.  Such  piles  are  frequently  used  as  guide  piles  in 
sinking  pneumatic  caissons  (§  436).  Hollow-built  piles,  40  inches 
in  diameter  and  80  feet  long,  were  used  for  this  purpose  in  con- 
structing the  St.  Louis  Bridge  (§  457).  They  were  sunk  by  pump- 
ing the  sand  and  water  from  the  inside  of  them  with  a  sand  pump 
(§  448). 

334.  PILE-DKIVING  Machines.  Pile-driving  machines  may  be 
classified  according  to  the  character  of  the  driving  power,  which 
may  be  (1)  a  falling  weight,  (2)  the  force  of  an  explosive,  or  (3)  the 
erosive  action  of  a  jet  of  water.  Piles  are  sometimes  set  in  holes 
bored  with  a  well-auger,  and  the  earth  rammed  around  them.  This 
is  quite  common  in  the  construction  of  small  highway  bridges  in 
the  prairie  States,  a  10-  or  a  12-inch  auger  being  generally  used. 
The  various  pile-driving  machines  will  now  be  briefly  described  and 
compared. 

*  See  "  Piling"  in  the  General  Specifications  for  Railroad  Masonry,  as  given  in 
Appendix  I. 


222  PILE    FOUXDATIOXS.  [CHAP.   XI. 

335.  Drop-hammer  Pile-driver.  The  usual  method  of  driving 
piles  is  by  a  succession  of  blows  given  with  a  heavy  block  of  wood 
or  iron — called  a  ram,  monkey,  or  hammer — which  is  carried  by  a 
rope  or  chain  passing  over  a  pulley  fixed  at  the  top  of  an  upright 
frame,  and  allowed  to  fall  freely  on  the  head  of  the  pile.  The 
machine  for  doing  this  is  called  a  drop-hammer  pile-driver,  or  a 
monkey  pile-driver — usually  the  former.  The  machine  is  generally 
placed  upon  a  car  or  scow. 

The  frame  consists  of  two  uprights,  called  leaders,  from  10  to  60 
feet  long,  placed  about  2  feet  apart,  which  guide  the  falling  weight 
in  its  descent.  The  leaders  are  either  wooden  beams  or  iron  chan- 
nel-beams, usually  the  former.  The  hammer  is  generally  a  mass  of 
iron  weighing  from  500  to  4,000  jiounds  (usually  about  2,000)  with 
grooves  in  its  sides  to  fit  the  guides  and  a  staple  in  the  top  by  which 
it  is  raised.  The  rope  employed  in  raising  the  hammer  is  usually 
wound  up  by  a  steam-engine  placed  on  the  end  of  the  scow  or  car, 
opposite  the  leaders. 

A  car  pile-driver  is  made  especially  for  railroad  work,  the 
leaders  resting  upon  an  auxiliary  frame,  by  wliich  piles  may  be 
driven  14  to  16  feet  in  advance  of  the  end  of  the  track  ;  and  the 
frame  is  pivoted  so  that  piles  may  be  driven  on  either  side  of  the 
track.  This  method  of  pivoting  the  frame  carrying  the  leaders  is 
also  sometimes  applied  to  a  machine  used  in  driving  piles  for  foun- 
dations. 

In  railroad  construction,  it  is  not  possible  to  use  the  pile-driving 
car  with  its  steam-engine  in  advance  of  the  track  ;  hence,  in  this 
kind  of  work,  the  leaders  are  often  set  on  blocking  and  the  ham- 
mer is  raised  by  horses  hitched  directly  to  the  end  of  the  rope. 
Portable  engines  also  are  sometimes  used  for  this  purpose.  Occa- 
sionally the  weight  is  raised  by  men  with  a  windlass,  or  by  pulling 
directly  on  the  rope. 

A  machine  used  for  driving  sheet  piles  differs  from  that  de- 
scribed above  in  one  particular,  viz. :  it  has  but  one  leader,  in  front 
of  which  the  hammer  moves  up  and  down.  With  this  construction, 
the  machine  can  be  brought  close  up  to  the  wall  of  a  coffer-dam 
(§  317  and  §  390),  and  the  pile  already  driven  does  not  interfere 
with  the  driving  of  the  next  one. 

336.  There  are  two  methods  of  detaching  the  weight,  i.  e.,  of 
letting  the  hammer  fall:  (1)  by  a  nipper,  and  (2)  by  a  friction-clutch. 


ART,   1.]         DESCRIPTIOXS,    AXD    METHODS    OF    DRIVIXG.  223 

1.  The  nipper  consists  of  a  block  wliicli  slides  freely  between 
the  leaders  and  which  carries  a  pair  of  hooks,  or  tongs,  projecting 
from  its  lower  side.  The  tongs  are  so  arranged  that  when  lowered 
on  to  the  top  of  the  hammer  they  automatically  catch  in  the  staple 
in  the  top  of  the  hammer,  and  hold  it  while  it  is  being  lifted,  until 
they  are  disengaged  by  the  upper  ends  of  the  arms  striking  a  pair  of 
inclined  surfaces  in  another  block,  the  trip,  Avhich  may  be  placed 
between  the  leaders  at  any  elevation,  according  to  the  height  of  fall 
desired. 

AVith  this  form  of  machine,  the  method  of  operation  is  as  fol- 
lows :  The  pile  being  in  place,  with  the  hammer  resting  on  the  head 
of  it  and  the  tongs  being  hooked  into  tlie  staple  in  tlie  top  of  the 
hammer,  the  rope  is  wound  up  until  the  upper  ends  of  the  tongs 
strike  the  trip,  which  disengages  the  tongs  and  lets  the  hammer 
fall.  As  the  hoisting  rope  is  unwound  the  nipper  block  follows  the 
hammer,  and,  on  reaching  it,  the  tongs  automatically  catch  in  the 
staple,  and  the  j^receding  operations  may  be  repeated.  This  method 
is  objectionable  owing  to  the  length  of  time  required  {ci)  for  the 
nipper  to  descend  after  the  hammer  has  been  dropped,  and  (^)  to 
move  the  trip  when  the  height  of  fall  is  changed.  Some  manufac- 
turers of  pile-driving  machinery  remove  the  last  objection  by  making 
an  adjustable  trip  which  is  raised  and  lowered  by  a  light  line  pass- 
ing over  the  top  of  the  leaders.     This  is  a  valuable  improvement. 

When  the  rope  is  wound  up  by  steam,  the  maximum  speed  is 
from  6  to  14  blows  per  minute,  dependmg  upon  the  distance  the 
hammer  falls.  The  speed  can  not  be  increased  by  the  skill  of  the 
operator,  although  it  could  be  by  making  the  ni^^per  block  heavier. 

2.  The  method  by  using  vl  friction-clutch,  or  friction-drum,  as  it 
is  often  called,  consists  in  attaching  the  rope  permanently  to  the 
staple  in  the  top  of  the  hammer,  and  droppmg  the  hammer  by  set- 
ting free  the  winding  drum  by  the  use  of  a  fnction-clutch.  The 
advantages  of  this  method  are  {a)  that  the  hammer  can  be  dropped 
from  any  height,  thus  securing  a  light  or  heavy  blow  at  pleasure; 
and  {h)  that  no  time  is  lost  in  waiting  for  the  nipper  to  descend  and 
in  adjusting  the  trip. 

When  the  rope  is  wound  up  by  steam,  the  speed  is  from  20  to 
30  blows  per  minute,  but  is  largely  dependent  upon  the  skill  of  the 
man  Avho  controls  the  friction-clutch.  The  hammer  is  caught  on 
the  rebound,  is  elevated  with  the  speed  of  a  falling  body,  and  hence 


924 


PILE    FOUNDATICXS. 


[chap.  XI. 


the  absolute  maximum  speed  is  attained.  The  rope,  by  which  the 
hammer  is  elevated,  retards  the  falling  weight ;  and  hence,  for  an 
equal  effect,  this  form  requires  a  heavier  hammer  than  when  the 
nipper  is  used.  Although  the  friction-drum  pile-driver  is  much 
more  efficient,  it  is  not  as  generally  used  as  the  nipper  driver.  The 
former  is  a  little  more  ex})eiisive  in  first  cost. 

337.  Steam-hammer  Pile-driver.  As  regards  frequency  of  use, 
the  next  machine  is  probably  the  steam-hammer  pile-driver,  invented 
by  Nasmyth*  in  1839.  It  consists  essentially  of  a  steam  cylinder 
(stroke  about  3  feet),  the  piston-rod  of  which  carries  a  weight  of 
about  3,500  pounds.  The  steam-cylinder  is  fastened  to  and  between 
the  tops  of  two  I-beams  about  8  to  10  feet 
long,  the  beams  being  united  at  the  bottom  by 
a  piece  of  iron  in  the  shape  of  a  frustum  of  a 
cone,  which  has  a  hole  through  it.  The  under 
side  '^f  this  connecting  piece  is  cut  out  so  as  to 
fit  the  top  of  the  pile.  The  striking  weight,wliich 
works  up  and  down  between  the  two  I-beams 
as  guides,  has  a  cylindrical  projection  on  the 
bottom  which  passes  through  the  hole  in  the 
piece  connecting  the  feet  of  the  guides  and 
strikes  the  pile.  The  steam  to  operate  the  ham- 
mer is  conveyed  from  the  boiler  through  a  flex- 
ible tube.  Fig.  56  shows  the  striking  weight  of 
the  latest  form  of  steam-hammer.  It  differs 
from  that  described  above  in  having  four  rods 
for  guides,  instead  of  the  two  I-beams. 

The  whole  mechanism  can  be  raised  and 
lowered  by  a  rope  passing  over  a  pulley  in  the 
top  of  the  leaders.  After  a  pile  has  been  placed 
in  position  for  driving,  the  machine  is  lowered 
upon  the  top  of  it  and  entirely  let  go,  the  pile 
being  its  only  support.  When  steam  is  admitted 
below  the  piston,  it  rises,  carrying  the  striking 
weight  with  it,  until  it  strikes  a  trip,  which 
cuts  off'  the  steam,  and  the  hammer  falls  by  its 
At  the  end  of  the  down  stroke  the  valves  are  again 


own  weight. 

*  It  is  ordinarily  called  Nasmyth's  hammer,  but  Bourdon  should  at  least  share 
the  credit  (see  Engineering  News,  vol.  xiii.  pp.  59.  60). 


ART.   1.]         DESCRIPTIOXS,    AND   METHODS   OF   DRIVING.  225 

automatically  reversed,  and  the  stroke  repeated.  By  altering  the 
adjustment  of  this  trip-piece,  the  length  of  stroke  (and  thus  the 
force  of  the  blows)  can  be  increased  or  diminished.  The  admission 
and  escape  of  steam  to  and  from  the  cylinder  can  also  be  controlled 
directly  by  the  attendant,  and  the  number  of  blows  per  minute 
is  increased  or  diminished  by  regulating  the  supply  of  steam.  The 
machine  can  give  60  to  80  blows  per  minute. 

338.  Drop-hammer  vs.  Steam-hammer.  The  drop-hammer  is 
capable  of  driving  the  pile  against  the  greater  resistance.  The 
maximum  fall  of  the  drop-hammer  is  40  or  50  feet,  while  that  of 
the  steam-hammer  is  about  3  feet.  The  drop-hammer  ordinarily 
weighs  about  1  ton,  while  the  striking  weight  of  the  steam-hammer 
usually  weighs  about  1^  tons.  The  energy  of  the  maximum  blow 
of  the  drop-hammer  is  45  foot-tons  (=  45  ft.  X  1  ton),  and  the 
energy  of  the  maximum  blow  of  the  steam-hammer  is  4.5  foot-tons 
(=3  ft.  X  1|-  tons).  The  energy  of  the  maximum  blow  of  the 
drop-hammer  is,  therefore,  about  10  times  that  of  the  steam- 
hammer. 

However,  the  effectiveness  of  a  blow  does  not  dej)end  alone  upon 
its  energy.  A  considerable  part  of  the  energy  is  invariably  lost  by 
the  compression  of  the  materials  of  the  striking  surfaces,  and  the 
gi'eater  the  velocity  the  greater  this  loss.  An  extreme  illustration 
of  this  would  be  trying  to  drive  piles  by  shooting  rifle-bullets  at 
them.  A  1-ton  hammer  falling  45  ft.  has  10  times  the  energy  of  a 
li-ton  hammer  falling  3  ft.,  but  in  striking,  a  far  larger  part  of  the 
former  than  of  the  latter  is  lost  by  the  compression  of  the  pile  head. 
In  constructing  the  foundation  of  the  Brooklyn  dry  dock,  it  wa& 
practically  demonstrated  that  "there  was  little,  if  any,  gain  in 
having  the  fall  more  than  45  feet."  The  loss  due  to  the  compres- 
sion depends  upon  the  material  of  the  pile,  and  whether  the  head  of 
it  is  bruised  or  not.  The  drop-hammer,  using  the  pile-cap  and  the 
friction-drum,  can  drive  a  pile  against  a  considerably  harder  resist- 
ance than  the  steam-hammer. 

It  is  frequently  claimed  that  the  steam-hammer  can  drive  a  pile 
agamst  a  greater  resistance  than  the  drop-hammer.  As  compared 
with  the  old  style  drop-hammer,  i.  e.,  without  the  friction-drum 
and  the  pile-cap,  this  is  probably  true.  The  striking  of  the  weight 
upon  the  head  of  the  pile  splits  and  brooms  it  very  much,  which 
materially  diminishes  the  effectiveness  of  the  blow.    In  hard  driving 


226  PILE   FOUNDATIONS.  [CHAP.  XI, 

with  the  drop-hammer,  without  the  pile-cap,  the  heads  of  the  piles, 
even  when  hooped,  will  crush,  bulge  out,  and  frequently  split  for 
many  feet  below  the  hoop.  For  this  reason,  it  is  sometimes  speci- 
fied that  piles  shall  not  be  driven  with  a  drop-hammei-. 

The  rapidity  of  the  blows  is  an  important  item  as  affecting  the 
efficiency  of  a  pile-driver.  If  the  blows  are  delivered  rapidly, 
the  soil  does  not  have  sufficient  time  to  recompact  itself  about 
the  pile.  With  the  steam-driver  the  blows  are  delivered  in  such 
quick  succession  that  it  is  probable  that  a  second  blow  is  de- 
livered before  the  pile  has  recovered  from  the  distortion  produced 
by  the  first,  which  materially  increases  the  effectiveness  of  the 
second  blow.  In  this  respect  the  steam-hammer  is  superior  to  the 
drop-hammer,  and  the  friction-clutch  driver  is  superior  to  the 
nipper  driver. 

In  soft  soils,  the  steam-hammer  drives  piles  faster  than  either 
form  of  the  drop-hammer,  since  after  being  placed  in  position  on 
the  head  of  the  pile  it  pounds  away  without  the  loss  of  any  time. 

339.  In  a  rough  way  the  first  cost  of  the  two  drivers — exclusive 
of  scow  or  car,  hoisting  engine,  and  boiler,  which  are  the  same  in 
each — is  about  |>80  for  the  drop-hammer  driver,  and  about  1800  for 
the  steam-driver.  Of  course  these  prices  will  vary  greatly.  The  per 
cent,  for  wear  and  tear  is  greater  for  the  drop-hammer  than  for  the 
steam-hammer.  For  work  at  a  distance  from  a  machine-shop  the 
steam-driver  is  more  liable  to  cause  delays,  owing  to  breakage  of 
some  part  whicli  can  not  be  readily  repaired. 

340.  Gunpowder  Pile-driver.  This  machine  was  invented  by 
Shaw,  of  Philadelphia,  in  1870.  The  expansive  force  of  gunpowder 
is  utilized  both  in  driving  the  pile  and  in  raising  the  ram.  The 
essential  parts  of  the  machine  are  the  ram  and  gun.  The  former 
consists  of  a  mass  of  iron  weighing  generally  about  1,500  pounds, 
which  terminates  below  in  a  sort  of  piston  ;  this  piston  fits  tightly 
into  a  chamber  in  another  mass  of  iron,  the  gun.  The  ram  travels 
between  vei-tical  guides  much  as  in  the  other  machines  ;  and  the 
gun  and  ram  are  hoisted  as  is  the  steam-hammer.  The  ram  having 
been  raised  to  the  top  of  the  guides,  and  the  gun  placed  upon  the 
top  of  the  pile,  a  cartridge  of  from  1  to  3  ounces  of  gunpowder 
is  placed  in  the  cylinder,  or  gun,  and  the  ram  is  allowed  to  descend. 
The  piston  enters  the  cylinder,  compresses  the  air,  and  generates 
heat  enough  to  ignite  the  cartridge,  when  the  expansive  force  of 


ART.   1.]         DESCRIPTIOXS,    AXD    METHODS   OF   DRIVING.  227 

the  powder  forces  the  pile  down  and  the  ram  up.  A  cartridge  is 
thrown  into  tlie  gun  each  time  as  the  ram  ascends.  The  rapidity 
•of  the  blows  is  limited  by  the  skill  of  the  operator  and  by  the  heat- 
ing of  the  gun.  Thirty  to  forty  blows,  of  from  5  to  10  feet  each, 
can  be  made  per  minute. 

341.  The  only  advantage  of  this  machine  is  that  the  hammer 
does  not  come  in  contact  with  the  head  of  the  pile,  and  hence  does 
not  injure  it.  The  disadvantages  are  (1)  that  it  is  of  no  assistance 
in  handling  the  pile  ;  (2)  that  it  is  not  economical ;  (3)  that  the 
gases  soon  destroy  the  gun  ;  (4)  that  a  leakage  of  gas  occurs  as  the 
gun  gets  hot,  which  renders  it  less  efficient  as  the  rapidity  of  firing 
is  increased  ;  and  (5)  that  the  gun  gets  so  hot  as  to  explode  the 
cartridge  before  the  descent  of  the  ram,  which,  of  course,  is  an 
entire  loss  of  the  explosive.  Its  first  cost  is  great.  It  is  not  now 
used. 

342.  Driving  Piles  with  Dynamite.  It  has  been  proposed  to 
drive  piles  by  exploding  dynamite  placed  directly  upon  the  top  of 
the  pile.  It  is  not  known  that  this  method  has  been  used  except 
in  a  few  instances.  It  would  be  a  slow  method,  but  might  prove 
valuable  where  only  a  few  piles  were  to  be  driven  by  saving  the 
transportation  of  a  machine  ;  or  it  might  be  employed  in  locations 
Avhere  a  machine  could  not  be  operated.  The  higher  grades  of 
dynamite  are  most  suitable  for  this  purpose.* 

343.  Driving  Piles  with  Water  Jet.  Although  the  water  jet 
is  not  strictly  a  pile-driving  machine,  the  method  of  sinking  piles 
by  its  use  deserves  careful  attention,  because  it  is  often  the  cheapest 
and  sometimes  the  only  means  by  which  piles  can  be  sunk  in  mud, 
silt,  or  sand. 

The  method  is  very  simple.  A  jet  of  water  is  forced  into  the 
soil  just  below  the  point  of  the  pile,  thus  loosening  the  soil  and 
allowing  the  pile  to  sink,  either  by  its  own  weight  or  with  very  light 
blows.  The  water  may  be  conveyed  to  the  point  of  the  pile  through 
a  flexible  hose  held  in  place  by  staples  driven  into  the  pile  ;  and 
after  the  pile  is  sunk,  the  hose  may  be  withdrawn  for  use  again. 
An  iron  pipe  may  be  substituted  for  the  hose.  It  seems  to  make 
very  little  difference,  either  in  the  rapidity  of  the  sinking  or  in  the 
accuracy  with  which  the  pile  preserves  its  position,  whether  the 
nozzle  is  exactly  under  the  middle  of  the  pile  or  not. 

*  For  a  brief  description  of  explosives,  see  pp.  119-24. 


228  PILE   POUNDATIONS.  [CHAP.  XI. 

The  water  jet  seems  to  have  been  first  used  in  engineering  in 
1852,  at  the  suggestion  of  General  Geo.  B.  McClellan.  It  has  been 
extensively  employed  on  the  sandy  shores  of  the  Gulf  and  South 
Atlantic  States,  where  the  compactness  of  the  sand  makes  it  diffi- 
cult to  obtain  suitable  foundations  for  light-houses,  wharves,  etc. 
Another  reason  for  its  use  in  that  section  is,  that  the  jialmetto  piles 
— the  only  ones  that  will  resist  the  ravages  of  the  teredo — are  too 
soft  to  withstand  the  blows  of  the  drop-hammer  pile-driver.  By 
employing  the  water  jet  the  necessity  for  the  use  of  the  j)ile-hammer 
is  removed,  and  consequently  palmetto  piles  become  available. 
The  jet  has  also  been  employed  in  a  great  variety  of  ways  to  facili- 
tate the  passage  of  common  piles,  screw  and  disk  piles,  cylinders, 
caissons,  etc.,  etc.,  through  earthy  material.* 

344.  The  efficiency  of  the  jet  depends  upon  the  increased  fluidity 
given  to  the  material  into  which  the  piles  are  sunk,  the  actual  dis- 
placement of  material  being  small.  Hence  the  efficiency  of  the  jet  is 
greatest  in  clear  sand,  mud,  or  soft  clay  ;  in  gravel,  or  in  sand  con- 
taining a  large  percentage  of  gravel,  or  in  hard  clay,  the  jet  is  almost 
useless.  For  these  reasons  the  engine,  pump,  hose,  and  nozzle 
should  be  arranged  to  deliver  large  quantities  of  water  with  a  mod- 
erate force,  rather  than  smaller  quantities  with  high  initial  velocity. 
In  gravel,  or  in  sand  containing  considerable  gravel,  some  benefit 
might  result  from  a  velocity  sufficient  to  displace  the  pebbles  and 
drive  them  from  the  vicinity  of  the  pile  ;  but  it  is  evident  that 
any  practicable  velocity  would  be  powerless  in  gravel,  except  for  a 
very  limited  depth,  or  where  circumstances  favored  the  prompt 
removal  of  the  pebbles. 

The  error  most  frequently  made  in  the  application  of  the  water 
jet  is  in  using  pumps  with  insufficient  capacity.  Both  direct-acting 
and  centrifugal  pumps  are  frequently  employed.  The  former 
affords  the  greater  power  ;  but  the  latter  has  the  advantage  of  a  less 
first  cost,  and  of  not  being  damaged  as  greatly  by  sand  in  the  water 
used. 

The  pumping  plant  used  in  sinking  the  disk-piles  for  the  Coney 
Island  pier  (see  §  327),  "  consisted  of  a  Worthington  pump  with  a 
12-inch  steam  cylinder,  8^-inch  stroke,  and  a  water  cylinder  7^ 
inches  in  diameter.     The  suction  hose  was  4  inches  in  diameter, 

*  See  a  pamphlet — "The  Water  Jet  as  an  Aid  to  Engineering  Construction" — 
published  (1881)  by  the  Engineer  Department  of  the  U.  S.  Army. 


ART.  1.]        DESCKIPTIOXS,    AXD    METHODS    OF    DRIVIXG.  229 

and  the  discharge  hose,  which  was  of  four-ply  gum,  was  3  inches. 
The  boiler  was  upright,  42  inches  in  diameter,  8  feet  high,  and 
contained  62  tubes  2  inches  in  diameter.  An  abundance  of  steam 
was  supplied  by  the  boiler,  after  the  exhaust  had  been  turned  into 
the  smoke-stack  and  soft  coal  used  as  fuel.  An  average  of  about 
160  pounds  of  coal  was  consumed  in  sinking  each  j)ile.  With  the 
power  above  described,  it  was  found  that  piles  could  be  driven  in 
clear  sand  at  the  rate  of  3  feet  per  minute  to  a  depth  of  12  feet ; 
after  which  the  rate  of  progress  graduall}'  diminished,  until  at  18 
feet  a  limit  was  reached  beyond  Avhich  it  was  not  practicable  to 
go  without  considerable  loss  of  time.  It  frequently  happened  tliat 
the  pile  would  "  bring  up  '  on  some  tenacious  material  which  was 
assumed  to  be  clay,  and  through  which  the  water  jet,  unaided, 
could  not  be  made  to  force  a  passage.  In  such  cases  it  was  found 
that  by  raising  the  pile  about  6  inches  and  allowing  it  to  drop  sud- 
denly, with  the  jet  still  in  operation,  and  repeating  as  rapidly  as 
possible,  the  obstruction  was  finally  overcome  ;  although  in  some  in- 
stances five  or  six  hours  were  consumed  iu  sinking  as  many  feet."  * 

In  the  shore-protection  work  on  the  Great  Lakes,  under  the 
direction  of  the  United  States  Army  engineers,  the  pumping  plant 
'■'consisted  of  a  vertical  tubular  boiler,  with  an  attached  engine 
having  an  8  X  12-inch  cylinder,  and  giving  about  130  revolutions  per 
minute  to  a  42-inch  driving-wheel.  A  No.  4  Holly  rotary  pump, 
with  18-inch  pulley,  Avas  attached  by  a  belt  to  the  driving-wheel  of 
the  engine,  giving  about  300  revolutions  per  minute  to  the  pump. 
The  pump  was  supplied  with  a  4-inch  suction  pipe,  and  discharged 
through  a  3-inch  hose  about  50  feet  iu  length.  The  hose  was  pro- 
vided Avith  a  nozzle  3  feet  in  length  and  2  inches  in  diameter.  The 
boiler,  engine,  pump,  and  pile-driver  were  mounted  on  a  i^latform 
12  feet  in  width  and  24  feet  in  length."  f 

345.  Jet  vs.  Hammer.  It  is  hardly  possible  to  make  a  compari- 
son between  a  water-jet  and  a  hammer  pile-driver,  as  the  conditions 
most  favorable  for  each  are  directly  opposite.  For  example,  sand 
yields  easily  to  the  jet,  but  offers  great  resistance  to  driving  with 
the  hammer  ;  on  the  other  hand,  in  stiff  clay  the  hammer  is  much 

*  Chas.  McDonald,  in  Trans.  Am.  Soc.  of  C.  E.,  vol.  vlii.  pp.  227-37. 
+  "The  Water-Jet  as  an  Aid  to  Engineering  Construction,"  p.  11 ; — a  pamphlet 
published  (1881)  by  the  Engineer  Department  of  the  U.  S.  Army. 


230  PILE    FOUND  \TIOXS.  [CHAP.   XI. 

more  expeditious.  For  inland  work  the  hammer  is  better,  owing  to 
the  difficulty  of  obtaining  the  large  quantities  of  water  required  for 
ihe  jet ;  but  for  river  and  harbor  work  the  jet  is  the  most  advan- 
tageous. Under  equally  favorable  conditions  there  is  little  or  no 
difference  in  cost  or  speed  of  the  two  methods.* 

The  Jet  and  the  hammer  are  often  advantageously  used  together, 
especially  in  stiff  clay.  The  efficiency  of  the  water-jet  can  be  greatly 
increased  by  bringing  the  weight  of  the  pontoon  upon  which  the 
machinery  is  placed,  to  bear  upon  the  pile  by  means  of  a  block  and 
tackle. 

346.  Cost  of  Piles.  At  Chicago  and  at  points  on  the  Missis- 
sippi above  St.  Louis,  pine  2Ji^e-  cost  from  10  to  15  cents  per  lineal 
foot,  according  to  length  and  location.  Soft-ioood  piles,  including 
rock  elm,  can  be  had  in  almost  any  locality  for  8  to  10  cents  per 
foot.  Oak  piles  20  to  30  feet  long  cost  from  10  to  12  cents  per 
foot ;  30  to  40  feet  long,  from  12  to  14  cents  per  foot ;  40  to  60 
feet  long,  from  20  to  30  cents  per  foot. 

347.  Cost  of  Pile  Driving.  There  are  many  items  that  affect 
the  cost  of  work,  which  can  not  be  included  in  a  brief  summary,  but 
which  must  not  be  forgotten  in  using  such  data  in  making  estimates. 
Below  are  the  details  for  the  several  classes  of  work. 

348.  Railroad  Construction.  The  following  table  is  a  summary 
of  the  cost,  to  the  contractor,  of  labor  in  driving  piles  (exclusive  of 
hauling)  in  the  construction  of  the  Chicago  branch  of  the  Atchison, 
Topeka  and  Santa  Fe  K.  K.  The  piles  were  driven,  ahead  of  the 
track,  with  a  horse-power  drop-hammer  weighing  2,200  i:)Ounds. 
The  average  depth  driven  was  13  feet.  The  table  includes  the 
cost  of  driving  piles  for  abutments  for  Howe  truss  bridges  and 
for  the  false  work  for  the  erection  of  the  same.  These  two  items 
add  considerably  to  the  average  cost.  The  contractor  received 
the  same  price  for  all  classes  of  work.  The  work  was  as  varied  as 
such  jobs  usually  are,  piles  being  driven  in  all  kinds  of  soil.  Owing 
to  the  large  amount  of  railroad  work  in  progress  in  1887,  the  cost 
of  material  and  labor  was  about  10  per  cent,  higher  than  the  aver- 
age of  the  year  before  and  after.  Cost  of  labor  on  pile-driver :  1 
foreman  at  $4  per  day,  6  laborers  at  |2,  2  teams  at  $3.50;  total  cost 
of  labor  =  $23  per  day. 

*  Report  of  Chief  of  Engineers  U.  S.  A.,  1883,  pp.  1264-72. 


ABl.  1.]        DESCRIPTIONS,    AND  METHODS   OF    DRIVING.  231 

Cost  of  Pile  Driving  in  Railroad  Construction. 

Number  of  piles  included  in  this  report 4,409 

'•        "   lineal  feet  included  in  this  report 109,568 

Average  length  of  the  piles,  in  feet 24.8 

Number  of  days  employed  in  driving 494 

"         "   lineal  feet  driven  per  day 221.8 

Cost  of  driving,  per  pile $2.53 

"     "        "          "    foot 10.4    cents. 

349.  Railroad  Eepairs.  The  following  are  the  data  of  pile 
driving  for  repairs  to  bridges  on  the  Indianapolis,  Decatur  and 
Springfield  R.  R.  The  work  was  done  from  December  21,  1885,  to 
January  5,  1886.  The  piles  varied  from  12  to  32  feet  in  length, 
the  average  being  a  little  over  21  feet.  The  average  distance  driven 
was  about  10  feet.  The  hammer  weighed  1,650  pounds;  the  last 
fall  was  37  feet,  and  the  corresponding  penetration  did  not  exceed 
2  inches.  The  hammer  was  raised  by  a  rope  attached  to  the  draw- 
bar of  a  locomotive — comjDaratively  a  very  expensive  way. 

TABLE  26. 
Cost  of  Piles  for  Bridge  Repairs. 


Items  of  Expense. 


Total. 


Per  Pile. 


I^aftor  ;  Loading  and  unloading  piles,  71^  days $16.00 

Bridge  gang,  driving,  12  days     

Engine  crew,  transportation  and  driving,  13  days. 
Train  crew.  "  "'         "      " 

Supplies  :  Engine  supplies 

6  pile  rings  and  2  plates 

Repairs 


Total  expense  for  driving . 


Material :  4,192  feet  oak  piles  at  1Z]4  cts $565.92 


153.75 
45.90 
71.50 
23.49 
13.29 
11.04 


$334.97 


Total  COST  $900.89 


$0.08 
0.78 
0.23 
0.37 
0.13 
0.06 
0.05 


1.70 


Per  Foot. 


0.4  cts. 

3.7 

1.1 

1.6 

0.5 

0.3 

0.3 


.9  cts. 


$2.86  1     13.5  cts. 


56       21.4  cts. 


On  the  same  road,  9  piles,  each  20  feet  long,  were  driven  9  feet, 
for  bumping-posts,  with  a  1,650-pound  hammer  dropping  17  feet. 
The  hammer  was  raised  with  an  ordinary  crab-Avinch  and  single 
line,  with  double  crank  worked  by  four  men.  The  cost  for  labor  was 
8.3  cents  per  foot  of  ])ile,  and  the  total  expense  was  21.8  cents  per  foot. 

350.  Bridge  Construction.  The  following  table  gives  the  cost 
of  labor  in  driving  the  piles  for  the  Northern  Pacific  R.  R.  bridge 
over  the  Red  River,  at  Grand  Forks,  Dakota,  constructed  in  1887. 
The  soil  was  sand  and  clay.  The  penetration  under  a  2,250-pound 
hammer  falling  30  feet  was  from  2  to  4  inches.  The  foreman  re- 
ceived $5  per  day,  the  stationary  engineer  83.50,  and  laborers  $2. 


232 


PILE    FOUNPATIONS. 


[chap.  XI. 


TABLE  27 
Cost  of  Labor  in  Driving  Piles  in  Bridge  Construction. 


Kind  of  Labor. 

o 

Ed 

u 

a 

gs 

Hffl 

0        H 
«        g 

a 

> 

P-( 

si 
> 

$68.95 
432.70 

78.75 

$63.65 
252.92 

$53.50 
430.50 
47.50 

$37.00 
215.45 
179.80* 

$61.60 

Driving..   .  — 

565.80 
131. 90t 

$580.40 

$316.57 

$531.50 

$432.25 

$759.30 

224 

7,238 
32.3 
1.1 

102 
3,710 

104 
7,023 
38.2 
4.1 

121 
4,6.39 
38.4 
6.6 

167 

Total  number  of  feet  remaining  in  the  structure.. 
Average  length  of  piles      "           "    "           " 
Average  length  of  piles  cut  oflf 

7,316 
43.8 
3.7 

Cost  per  foot  of  pile  remaining  in  the  structure. . . 

8.0  cts. 

8. 5 cts. 

7.6  cts. 

9.3  cts. 

10.4  cts. 

Average  cost  for  driving,  per  foot  remaining  in  the  structure  =  8.8  cents. 

*  Sawed  off  under  8  feet  of  water. 

+  Including  $70.25  for  excavating  and  bailing  in  order  to  get  at  the  sawing. 

351.  Foundation  Piles.  The  contract  price  for  the  foundation 
piles — white  oak — for  the  raih-oad  bridge  over  the  Missouri  Eiver,  at 
Sibley,  Mo. ,  was  22  cents  per  foot  for  the  piles  and  28  cents  per  foot 
for  driving  and  sawing  off  below  water.  They  were  50  feet  Icng^ 
and  were  driven  in  sand  and  gravel,  in  a  coffer-dam  16  feet  deep, 
by  a  drop-hammer  weighing  3,203  pounds,  falling  36  feet.  The  ham' 
mer  was  raised  by  steam  power. 

352.  In  the  construction  of  a  railroad  in  southern  Wisconsin 
during  1885-87,  the  contract  price — the  lowest  competitive  bid — for 
the  piles,  in  place,  under  the  piers  of  several  large  bridges  averaged 
as  in  the  following  table.  The  piles  were  driven  in  a  strong  current 
and  sawed  off  under  water,  hence  the  comparatively  great  expense. 

TABLE  28. 
Contract  Price  op  Foundation  Piles. 


Kind  of  Driving. 

Contract  Price  per  Lineal  Foot. 

For  Part  remaining  in 
Structure. 

For  Pile  Heads  Sawed 
oflf. 

Rock  Elm 
Pine 
Oak 
Oak 

Ordinary 
Hard 

40  cents 
40     " 
48     " 
50    " 

15  cents 
20    " 
25    " 
30    " 

AET.  2.]  BEAKIXG    POWER    OF    PILES.  233 

353.  In  1887  the  contract  price  for  piles  in  the  foundations  of 
bridge  piers  in  the  river  at  Chicago  was  35  cents  per  foot  of  pile 
left  in  the  foundation.  This  pricp  corered  cost  of  timber  (10  to  15 
cents),  driving,  and  cutting  off  12  to  14  feet  below  the  surface  of 
the  water, — about  17  feet  being  left  in  the  foundation. 

The  cost  of  driving  and  sawing  off  may  be  estimated  about 
as  follows  :  (17  +  13)  feet  of  pile  at  13  cents  per  foot  =  $3.90  ;  17 
feet  of  pile,  left  in  the  structure,  at  35  cents  per  foot  =  $5.95. 
$5.95  —  $3.90  =  $2.05  =  the  cost  per  pile  of  driving  and  sawing  off, 
which  is  equivalent  to  nearly  7  cents  j)er  foot  of  total  length  of  pile. 
In  this  case  the  waste  or  loss  in  the  pile  heads  cut  off  adds  consider- 
ably to  the  cost  of  the  piles  remaining  in  the  structure.  In  mak- 
ing estimates  this  allowance  should  never  be  overlooked. 

354.  Harbor  and  River  "Work.  In  the  shore-protection  work  at 
Chicago,  done  in  1882  by  the  Illinois  Central  R.  K.,  a  crew  of  9 
men,  at  a  daily  expense,  for  labor,  of  $17.25,  averaged  6o  piles  per  10 
hours  in  water  7  feet  deep,  the  piles  being  24  feet  long  and  being 
driven  14  feet  into  the  sand.  The  cost  for  labor  of  handling,  sharp- 
ening, and  driving,  was  a  little  over  26  cents  per  pile,  or  1.9  cents 
per  foot  of  distance  driven,  or  1.1  cents  per  foot  of  pile.*  Both 
steam-hammers  and  water- jets  were  used,  but  not  together.  N'otice 
that  this  is  very  cheap,  owing  (1)  to  the  use  of  the  jet,  (2)  to  little 
loss  of  time  in  moving  the  driver  and  getting  the  pile  exactly  in  the 
predetermined  place,  (3)  to  the  piles  not  being  sawed  off,  and  (4) 
to  the  skill  gained  by  the  workmen  in  a  long  job. 

On  the  Mississippi  Eiver,  under  the  direction  of  the  U.  S. 
Army  engineers,  the  cost  in  1882  for  labor  for  handling,  sharpen- 
ing, and  driving,  was  $3.11  per  pile,  or  20  cents  per  foot  driven 
The  piles  were  35  feet  long,  the  depth  of  water  15.5  feet,  and  the 
depth  driven  13.6  feet.  The  water- jet  and  drop-hammer  were  used 
together.  Tlie  large  cost  was  due,  in  part  at  least,  to  the  current, 
which  was  from  3  to  6  miles  per  hour.f 

Art.  2.  Bearing  Power  of  Piles. 

355.  Two  cases  must  be  distinguished  ;  that  of  columnar  piles  or 
those  whose  lower  end  rests  upon  a  hard  stratum,  and  that  of  ordi- 
nary bearing  piles  or  those  whose  supporting  power  is  due  to  the 

*  Report  of  the  Chief  of  Engineers,  U.  S.  A.,  for  1883,  pp.  136&-70. 
+  Ibid.,  p.  1260. 


234  PILE    FOUNDATIONS.  [CHAP.   XI. 

friction  of  the  earth  on  the  sides  of  the  pile.  In  the  first  case,  the 
bearing  power  is  limited  by  the  strength  of  the  pile  considered  as  a 
column  ;  and,  since  the  earth  prevents  lateral  deflection,  at  least  to 
a  considerable  degree,  the  strength  of  such  a  pile  will  approximate 
closely  to  the  crushing  strength  of  the  material.  This  class  of  piles 
needs  no  further  consideration  here, 

356.  Methods  of  Determining  Supporting  Power.  There 
are  two  general  methods  of  determining  the  supporting  power  of 
ordinary  bearing  piles:  first,  by  considering  the  relation  between  the 
supporting  power  and  the  length  and  size  of  the  pile,  the  weight  of 
the  hammer,  height  of  fall,  and  the  distance  the  pile  was  moved  by 
the  last  blow  ;  or,  second,  by  applying  a  load  or  direct  pressure  to 
each  of  a  number  of  piles,  observing  the  amount  each  will  support, 
and  expressing  the  result  in  terms  of  the  depth  driven,  size  of  pile, 
and  kind  of  soil.  The  first  method  is  applicable  only  to  piles  driven 
by  the  impact  of  a  hamm.er  ;  the  second  is  applicable  to  any  pile, 
no  matter  how  driven. 

1.  If  the  relation  between  the  supporting  power  and  the  length 
and  size  of  pile,  the  weight  of  the  hammer,  the  height  of  fall, 
and  the  distance  the  pile  was  moved  by  the  last  blow  can  be  stated 
in  a  formula,  the  supporting  power  of  a  pile  can  be  found  by  insert- 
ing these  quantities  in  the  formula  and  solving  it.  The  relation 
between  these  quantities  must  be  determined  from  a  consideration 
of  the  theoretical  conditions  involved,  and  hence  such  a  formula  is 
a  rational  formula. 

2.  By  applying  the  second  method  to  piles  under  all  the  con- 
ditions likely  to  occur  in  practice,  and  noting  the  load  supported, 
the  kind  of  soil,  amount  of  surface  of  pile  in  contact  with  the  soil, 
otc,  etc.,  data  could  be  collected  by  which  to  determine  the  sup- 
porting power  of  any  pile.  A  formula  expressing  the  su^^porting 
power  in  terms  of  these  quantities  is  an  empirical  foj-nmla. 

357.  Rational  Formulas.  The  deduction  of  a  rational  for- 
mula for  the  supporting  power  of  a  pile  is  not,  strictly,  an  appro- 
priate subject  for  mathematical  investigation,  as  the  conditions  can 
not  be  expressed  with  mathematical  precision.  However,  as  there 
is  already  a  great  diversity  of  formulas  in  common  use,  which  give 
widely  divergent  results,  a  careful  investigation  of  the  subject  is 
necessary. 

The  present  practice  in  determining  the  bearing  power  of  piles  is 


ART.  2.]  BEAEIXG   POWER  OF   PILES.  235 

neither  scientific  nor  creditable.  Many  engineers,  instead  of  in- 
quiring into  the  relative  merits  of  the  different  formulas,  take  an 
average  of  all  the  formulas  they  can  find,  and  feel  that  they  have  a 
result  based  on  the  combined  wisdom  of  the  profession.  This  prac- 
tice is  exactly  like  that  of  the  ship's  surgeon  who  poured  all  his 
medicines  into  a  black  jug,  and  whenever  a  sailor  was  ailing  gave 
him  a  spoonful  of  the  mixture.  Other  engineers,  knowing  the  great 
diversity  and  general  unreliability  of  the  formulas,  reject  them 
all  and  trust  to  their  own  experience  and  judgment.  The  self- 
reliant  engineer  usually  chooses  the  latter  course,  while  the  timid 
one  trusts  to  the  former. 

To  correctly  discriminate  between  the  several  formulas,  it  is 
necessary  to  have  a  clear  understanding  of  all  the  conditions  in- 
volved. The  object  of  the  following  discussion  is  to  discover  the 
general  principles  which  govern  the  problem. 

358.  When  the  ram  strikes  the  head  of  the  pile,  the  first  effect 
is  to  compress  both  the  head  of  the  pile  and  the  ram.  The  more 
the  ram  and  pile  are  compressed  the  greater  the  force  required,  until 
finally  the  force  of  compression  is  sufficient  to  drive  the  pile  through 
the  soil.  The  amount  of  the  pressure  on  the  head  of  the  pile  when 
it  begins  to  move,  is  what  we  wish  to  determine. 

To  produce  a  formula  for  the  pressure  exerted  upon  the  pile  by 
the  impact  of  a  descending  weight,  let 

W  =  the  weight  of  the  ram,  in  tons  ; 

w  —    "        "  "       pile      " 

S  =  the  section  of  the  ram,  in  sq.  ft.; 

s  =    "         "         "      pile     "    " 

L  =  the  length  of  the  ram,  in  feet ; 

I  =    "        "         "       pile     " 

E  =  the  co-efficient  of  elasticity  of  the  ram,  in  tons  per  sq.  ft.  ; 

h  =  the  height  of  fall,  in  feet  ; 

d  =  the  penetration  of  the  pile,  i.  e.,  the  distance  the  pile  is 
moved  by  the  last  blow,  in  feet.  The  distance  d  is  the 
amount  the  pile  as  a  whole  moves,  and  not  the  amount 
the  top  of  the  head  moves.  This  can  be  found  accu- 
rately enough  by  measuring  the  movement  of  a  point, 
say,  2  or  3  feet  below  the  head. 

P  —  the  pressure,  in  tons,  which  will  just  move  the  pile  the  very 


236  PILE   FOUNDATIONS.  [CHaP.  XI. 

small  distance  d, — that  is  to  say,  the  pressure  produced 
by  the  last  blow;  or,  briefly,  P  may  be  called  the  sup- 
porting power. 
Then  Wh  is  the  accumulated  energy  of  the  ram  at  the  instant  it 
strikes  the  head  of  the  pile.     This  energy  is  spent  (1)  in  compress- 
ing the  ram,  (2)  in  compressing  the  head  of  the  pile,  (3)  in  moving 
the  pile  as  a  whole  against  the  resistance  of  the  soil,  (4)  in  overcom- 
ing the  inertia  of  the  pile,  (5)  in  overcoming  the  inertia  of  the  soil 
at  the  lower  end  of  the  j^ile,  and   (6)   by  the  friction  of  the  ram 
against  guides  and  air.     These  will  be  considered  in  order. 

1.  The  energy  consumed  in  compressing  the  hammer  is  repre- 
sented by  the  product  of  the  mean  pressure  and  the  compression,  or 
shortening,  of  the  ram.  The  pressure  at  any  point  in  a  striking 
weight  varies  as  the  amount  of  material  above  that  point ;  that  is  to 
say,  the  pressure  at  any  point  of  the  hammer  varies  inversely  as  its 
distance  from  the  lower  surface.  The  pressure  at  the  lower  surface 
is  P,  and  that  at  the  upper  one  is  zero  ;  hence  the  mean  pressure 
is  I  P.    From  the  principles  of  the  resistance  of  materials,  the  com- 

p  L 

pression,  or  the  shortening,  is  ^^-^,  in  which  p  is  the  uniform  pres- 

sure.     From  the  above,  p  =  ^  P.     Consequently  the  shortening  is 

IPL 

2  SB' 

If  the  fibers  of  the  face  of  the  ram  are  not  seriously  crushed,  the 
mean  pressure  will  be  one  half  of  the  maximum  pressure  due  to  im- 
pact ;  or  the  mean  pressure  during  the  time  the  ram  and  pile  are 

1  P^  L 
being  compressed  is  ^P.     Then  the  energy  consumed  is— -^7-=^. 

The  yielding  of  the  material  of  the  ram  is  j^robably  small,  and  might 
be  omitted,  but  as  it  adds  no  complication,  as  will  presently  appear, 
it  is  included. 

3.  The  mean  pressure  on  the  head  of  the  pile  is  ^P,  as  above. 
For  simplicity  assume  that  the  pile  is  of  uniform  section  through- 
out. To  determine  the  shortening,  notice  that  for  the  part  of  the 
pile  above  the  ground  the  maximum  pressure  is  uniform  through- 
out, but  that  for  the  part  under  the  surface  the  maximum  pressure 
varies  as  some  function  of  the  length.  If  the  soil  were  homogeneous, 
the  pressure  would  vary  about  as  the  length  in  the  ground  ;  and 


ART.  2.]  BEARIK&   POWER   OF  PILES.  237 


1  PI 

hence  the  shortening  would  be  -  • — .     But,  remembering  that  the 

resistance  is  generally  greater  at  the  lower  end  than  at  the  upper, 
and  that  any  swaying  or  vibration  of  the  upper  end  will  still  further 
diminish  the  resistance  near  the  top,  it  is  probable  that  the  mean 
pressure  is  below  the  center.  It  will  here  be  assumed  that  the  mean 
pressure  on  the  fibers  of  the  pile  is  two  thirds  of  that  on  the  head, 

2  PI 
which  is  equivalent  to  assuming  that  the  shortening  is when 

3  se 

the  pile  is  wholly  immersed.     If  only  a  part  of  the  pile  is  in  contact 

PI'        2  PI        Pi  2\ 

with  the  soil,  the  shortening  will  be  ■ — ■  +  -  — i  —  — [/'  -|-  _  /j^ 

in  which  /'  is  the  exposed  portion  and  l^  the  part  immersed.  For 
simplicity  in  the  following  discussion   the   shortening  of  the  pile 

2i  P I 

will  be  taken  at  -  — .     If  a  formula  is  desired  for  the  case  when 
6  se 

the  top  projects  above  the  ground,  it  will  only  be  necessary  to  sub- 
stitute (I'  +f  Z,)  for  I  in  equations  (1)  and  (2)  below. 

1  P^  I 

Then  the  energy  lost  in  the  compression  of  the  pile  is . 

3   se 

3.  The  energy  represented  by  the  penetration  of  the  pile  is  P  d. 

4.  In  the  early  stage  of  the  contact  between  the  ram  and  the 
pile,  part  of  the  energy  of  the  ram  is  being  used  up  in  overcoming  the 
inertia  of  the  pile  ;  but  in  the  last  stage  of  the  compression,  this 
energy  is  given  out  by  the  stoppage  of  the  pile.  At  most,  the  effect 
of  the  inertia  of  the  pile  is  small ;  and  hence  it  will  be  neglected. 

5.  The  energy  lost  in  overcoming  the  inertia  of  the  soil  at  the 
lower  end  of  the  pile  will  vary  with  the  stiffness  of  the  soil  and  with 
the  velocity  of  penetration.  It  is  impossible  to  determine  the  amount 
of  this  resistance,  and  hence  it  can  not  be  included  in  a  formula. 
Omitting  this  element  causes  the  formula  to  give  too  great  a  support- 
ing power.  The  error  involved  can  not  be  very  great,  and  is  to  be 
covered  by  the  factor  of  safety  adopted. 

6.  The  friction  of  the  ram  against  the  guides  and  against  the  air 
diminishes  the  effect  of  the  blow,  but  the  amount  of  this  can  not  be 
computed.  Omitting  this  element  will  cause  the  formula  for  the 
supporting  power  to  give  too  great  a  result.  The  friction  against 
the  air  increases  very  rapidly  with  the  height  of  fall,  and  hence  the 


238  PILE    FOUNDATIONS.  [CHAP.  XI* 

smaller  the  fall  the  more  nearly  will  the  formula  give  the  true  sup- 
porting power. 

359.  Equating  the  energy  of  the  falling  weight  with  that  con- 
sumed in  compressing  the  pile  and  ram,  and  in  the  penetration  of 
the  pile,  as  discussed  in  paragraphs  1,  2,  and  3  above,  we  have 

'^"  =  1^  +  1^  +  ^" w 

Solving  equation  (1)  gives 


_  V  _  ,         12  S  Bse  36  cP  S'  E'  s'  e 


3  Ls  e-{-4:l  S  B  '    {3  L  s  e -\- 4  I  S  B)' 

6  d  S  Bse 


3  Lse  +  Al  S  B 


.     .     (2) 


An  examination  of  equation  (3)  shows  that  the  pressure  upon  the 
pile  varies  with  the  height  of  fall,  the  weight,  section,  length,  and 
co-efficient  of  elasticity  of  both  ram  and  pile,  and  with  the  penetra- 
tion. It  is  easy  to  see  that  the  weight  of  the  ram  and  the  height 
of  the  fall  should  be  included.  The  penetration  is  the  only  element 
which  varies  with  the  nature  of  the  soil,  and  so  of  course  it  also 
should  be  included.  It  is  not  so  easy  to  see  that  the  length,  section, 
and  co-efficient  of  elasticity  of  the  material  of  the  pile  and  ram 
should  be  included.  If  any  one  will  try  to  drive  a  large  nail  into 
hard  wood  with  a  piece  of  leather  or  rubber  intervening  between 
the  hammer  and  the  head  of  the  nail,  he  will  be  impressed  with  the 
fact  that  the  yielding  of  the  leather  or  rubber  appreciably  diminishes 
the  effectiveness  of  the  blow.  Essentially  the  same  thing  occurs  in 
trying  to  drive  a  large  nail  with  a  small  hammer,  except  that  in  this 
case  it  is  the  yielding  of  the  material  of  the  hammer  which  dimin- 
ishes the  effect  of  the  blow.  In  driving  piles,  the  materials  of  the 
pile  and  ram  act  as  the  rubber  in  the  first  illustration;  and,  reason- 
ing by  analogy,  those  elements  which  determine  the  yielding  of  the 
materials  of  the  pile  and  ram  should  be  included  in  the  formula. 
Obviously,  then,  the  pressure  due  to  impact  Avill  be  greater  the 
harder  the  material  of  the  pile.  Notice  also  that  if  the  head  of  the 
pile  is  bruised,  or  ''broomed,"  the  yielding  will  be  increased;  and, 
consequently,  the  pressure  due  to  the  blow  will  be  decreased. 


ART.  2.]  BEARING    POWER   OF    PILES.  339 

360.  The  Author's  Formula  for  Practice.  To  simplify  equation 
(2),  put 

6  S  E  se         _ 
d  Lse  +4:1  S  E  ~  ^' 

and  then  equation  (2)  becomes 

P  =  V-Zq  Wh  +  q'  cV  -  q  d.       ....     (3) 

Equation  (3)  can  be  simplified  still  further  by  computing  q  for 
the  conditions  as  they  ordinarily  occur  in  practice.  Of  course,  in 
this  case  it  will  only  be  possible  to  assume  some  average  value  for 
the  various  quantities.  Assume  the  section  of  the  pile  to  be  0.8  sq. 
ft.;  the  section  of  the  ram,  2  sq.  ft.;  the  length  of  the  ram,  2.5  ft.; 
the  length  of  the  pile,*  25  ft. ;  the  co-eflBcient  of  elasticity  of  the 
ram,  1,080,000  tons  per  sq.  ft.;  and  the  co-efficient  of  elasticity  of 
the  pile,  108,000  tons  per  sq.  ft.  (an  average  value  for  oak,  elm, 
pine,  etc.,  but  not  for  palmetto  and  other  soft  woods).  Computing 
the  corresponding  value  of  q,  Ave  find  it  to  be  5,160;  but  to  secure 
round  numbers,  we  may  take  it  at  5,000,  which  also  gives  a  little 
additional  security. 

Equation  (3)  then  becomes 

P  =  100  (  VWh  +  {bO(iy  -  50  d),      ...     (4) 

wliich  is  the  form  to  he  used  in  jiractice. 

Equation  (4)  is  approximate  because  of  the  assumptions  made  in 
deducing  equation  (1),  and  also  because  of  the  average  value  taken 
for  q\  but  probably  the  error  occasioned  by  these  aj)proximations  is 
not  material. 

361.  Notice  that,  since  the  co-efficient  of  elasticity  of  sound 
material  was  used  in  deducing  the  value  of  q,  equation  (4)  is  to  be 
applied  only  on  condition  that  the  last  blow  is  struck  upon  sound 
Avood;  and  therefore  the  head  of  the  test  pile  should  be  sawed  off  so 
as  to  present  a  solid  surface  for  the  last,  or  test,  blow  of  the  hammer. 
{This  limitatio7i  is  exceedingly  important.)  Since  the  penetration 
per  blow  can  be  obtained  more  accurately  by  taking  the  mean  dis- 
tance for  two  or  three  blows  than  by  measuring  the  distance  for  a 
single  one,  it  is  permissible  to  take  the  mean  penetration  of  two  or 

*  The  quantity  to  be  used  here  is  the  length  out  of  the  ground  pliLS  about  two 
thirds  of  the  part  in  the  ground  (see  paragraph  2  of  §  353). 


240 


PILE    F0UXDATI02s"S. 


[chap.  XI. 


tliree  blows;  but  their  number  and  force  should  be  such  as  not  to 
crush  the  head  of  the  pile. 

In  this  connection  the  following  table,  given  by  Don.  J.  Whitte- 
naore,  in  the  Transactions  of  the  American  Society  of  Civil  Engi- 
neers, vol.  xii.  p.  442,  to  show  the  gain  in  efficiency  of  the  driving 
power  by  cutting  off  the  bruised  or  broomed  head  of  the  pile,  is  very 
instructive.  The  pile  was  of  green  Norway  pine;  the  ram  was  of 
the  Nasmyth  type,  and  weighed  2,800  pounds. 

Table   showing  the  Gain  in  Efficiency  of  the  Driving  Power  bi 
Cutting  off  the  Broomed  Head  of  the  Pile. 

3d  ft.  of  penetration  required 5  blows. 

4th     "  "                 "  15 

5th     "  "                 "  20 

6th     "  "                 "  29 

7th     "  "                 "  35 

8th     "  "                 "  ......  46 

9th     "  "  61 

10th     "  "                 '••  73 

11th     "  "                 "  109 

12th     "  "                 "  153 

13th     "  "                 "  257 

14th     "  "                 "  .......  684 

Head  of  the  pile  adzed  off. 

15th  ft.  of  penetration  required 275 

16th     "  "                 "  572 

17th     "  "                 "  833 

18th     "  "                 "  . 825 

Head  of  the  pile  adzed  off. 

19th  ft.  of  penetration  required .  213 

20th     "  "                 "  275 

21st      "  "                  "  371 

22d      "  "                 "  378 


Total  number  of  blows, 5,228 


Notice  that  the  average  penetration  per  blow  was  2^  times  greater 
during  the  15th  foot  than  during  the  14th;  and  nearly  4  times 
greater  in  the  19th  than  in  the  18th.  It  does  not  seem  unreason- 
able to  believe  that  the  first  blows  after  adzing  the  head  off  were 
(Correspondingly  more  effective  than  the  later  ones;  consequently, 
it  is  probable  that  the  first  blows  for  the  15th  foot  of  penetration 
were  more  than  5  times  as  efficient  as  the  last  ones  for  the  14th  foot, 
and  also  that  the  first  blows  for  the  19th  foot  were  8  or  10  times 
more  efficient  than  the  last  ones  for  the  18th  foot.  Notice  also  that 
since  the  head  was  only  "^ adzed  off,"  it  is  highly  probable  that  the 
spongy  wood  was  not  entirely  removed. 


ART.  2. J  BEARIXG    POWER   OF    PILES.  341 

If  the  penetration  for  the  last  blow  before  the  head  Avas  adzed  off 
were  used  in  the  formula,  the  apparent  supporting  power  would  be 
yery  much  greater  than  if  the  penetration  for  the  first  blow  after 
adzing  off  is  employed.  This  shows  how  unscientific  it  is  to  pre- 
scribe a  limit  for  the  penetration  without  specifying  the  accompany- 
ing condition  of  the  head  of  the  pile,  as  is  ordinarily  done. 

362.  Weisbach's  Formula.  Equation  (2),  page  238,  is  essentially 
equivalent  to  Weisbach's  formula  foi-  the  supporting  power  of  a  pile. 
Weisbach  assumes  that  the  pressure  is  uniform  throughout,  and 
obtains  the  formula* 

in  which  H  =  —j—,  and  H^  =  -j. 

363.  Rankine's  Formula.  Equation  (2),  page  238,  is  also  essen- 
tially equivalent  to  Kankine's  formula ;  and  differs  from  it,  only 
because  he  assumes  the  pressure  to  vary  directly  as  the  length  of 
the  pile,  and  neglects  the  compression  of  the  ram.  Rankine's 
formula  is  f 

Equation  (2)  differs  from  "Weisbach's  and  Rankine's  on  the  safe 
side. 

364.  Empirical  Formitlas.  General  Principles.  (1)  An  empiri- 
cal formula  should  be  of  correct  form;  (2)  the  constants  in  it  should 
be  correctly  deduced  ;  and  (3)  the  limits  Avithin  which  it  is  applica- 
ble should  be  stated. 

For  example,  suppose  that  it  were  desired  to  determine  the 
equation  of  the  straight  line  A  B,  Fig.  57. 
Since  the  given  line  is  straight,  we  will  as- 
sume that  the  empirical  formula  is  of  the 
form  y  =  m  x.  We  might  find  m  by  measur- 
ing the  ordinates  1,  2,  3,  and  place  m  equal 
to  their  mean.  If  1,  2,  3,  be  the  numerical 
values  of  the  respective  ordinates,  the  for- 
mula becomes  y  =  2  x,  which  gives  the  line 
0  C.     The  mean  ordinate  to  0  C  is  equal  to  fio.  57. 

the  mean  ordinate  to  A  B,  but  the  two  are  not  by  any  means  the 

*  Mechanics  of  Engineering,  6th  ed.  (Coxa's  Trauslation),  p.  701- 
+  Civil  Engineering,  p.  602. 


24:2  PILE    FOUNDATIONS.  [CHAP.  XI. 

same  line.     It  is  evident  that  this  empirical  formula  is  of  the  wrong 

form. 

For  another  illustration,  assume  that  some  law  is  correctly  repre- 
sented by  the  curve  A  B,  Fig.  58.     The  form 
of  the  empirical  formula  may  be  such  as  to 
give   the  curve  CD.     These   curves   coincide 
a  c  exactly  at  tAvo  points,  and  the  mean  ordinate 

to  the  two  is  the  same.  To  use  a  com- 
mon expression,  we  may  say  that,  "on  the 
average,  the  empirical  formula  agrees  exactly 


Fig.  58.  with  the   facts  ;"  but  it  is,   nevertheless,   not 

even  approximately  true.  The  constants  were  not  correctly  de- 
duced. 

Even  if  of  the  correct  form  and  correctly  deduced,  an  empirical 
formula  can  be  safely  applied  only  within  the 
limits  of  those  values  from  which  it  was  deter- 
mined. For  example,  a  law  may  be  repre- 
sented by  the  curve  A  B,  Fig.  59.  From 
observations  made  in  the  region  C  E,  the  em-  Fig.  59. 

pirical  formula  has  been  determined,  which  gives  the  curve  C  E  D, 
which  between  the  limits  C  and  E  is  all  that  can  be  desired,  but 
which  is  grossly  in  error  between  the  limits  E  and  D.  To  use  an 
empirical  formula  intelligently,  it  is  absolutely  necessary  that  the 
limits  within  which  it  is  applicable  should  be  known. 

Of  course,  the  observations  from  which  the  empirical  formula 
was  deduced  can  not  be  used  to  test  the  correctness  of  the  formula; 
such  a  procedure  can  check  only  the  mathematical  work  of  deriving 
the  constants. 

Elementary  as  the  preceding  principles  are,  many  empirical 
formulas  are  worthless  owing  to  a  disregard  of  these  conditions  in 
deducing  them. 

365.  Comparison  of  Empirical  Formulas.  We  will  now  briefly 
consider  the  empirical  formulas  that  are  most  frequently  employed 
to  determine  the  supporting  power  of  piles.* 

HasweWs  formula  for  the  dynamic  effect  of  a  falling  body  is  f 
P  =  4.426  W  V,  "as  deduced  from  experiments." 

The  experiments  consisted  in  letting  a  weight  of  a  few  ounces 

*  For  explanation  of  the  nomenclature,  see  p.  23.5. 

t  Haswell's  Engineers'  and  Mechanics'  Pocket-Book,  p.  419. 


AKT.   2.]  BEARING    POWER   OF    PILES.  243 

fall  a  few  inches  upon  a  coiled  spring ;  and  hence  the  formula  is 
entirely  inapplicable  to  pile  driving. 

Beaufoy's  formula  is  P=:  0.5003  WV^,  '^as  determined  by 
experiment."  This  formula  was  deduced  under  the  same  conditions 
as  Haswell's^  and  hence  is  useless  for  pile  driving.  The  difference 
between  the  formulas  is  due  to  the  fact  that  Haswell  used  only  one 
"weight  and  one  spring,  and  varied  the  height  of  the  fall,  while  Beau- 
foy  employed  one  weight  and  springs  of  such  relative  stiffness  as 
would  stop  the  weight  in  nearly  the  same  distance  for  different 
heights  of  fall.*  Notice  that  Haswell's;  and  also  Beaufoy's  foi-mula, 
would  give  the  same  bearing  power  for  all  soils,  other  things  being 
the  same. 

Ny  Strom's  formula]  is  P  =  ,  ,„  , — -r^—-,.    In  a  later  book,t  Nvs- 
•^  -^  '  {y['-\-w)  d  ^     - 

^    Wli 
trom gives  the  formula  P  =  —  — —-,  assuming  that  "about  25  per 

cent,  of  the  energy  of  the  ram  is  lost  by  the  crushing  of  the  head  of 
the  pile."  Both  of  these  formulas  are  roughly  approximate,  theo- 
retical formulas,  although  frequently  cited  as  "practical  formulas." 

W  h 

Mason's  formula  §  is  P  =  ,  „,  , r— ,.      As    in    the    preceding 

( P^  +  iv)  a 

cases,  this  is  frequently  referred  to  as  a  "practical  formula  ;"  but  an 
examination  of  the  original  memoir  shows  that  it  is  wholly  a  theo- 
retical formula  with  no  pretensions  of  being  anything  else.  It  is 
also  sometimes  referred  to  as  having  been  "  tested  by  a  series  of 
experiments  ;"  but  apparently  the  only  basis  for  this  is  that  the 
piles  upon  which  Fort  Montgomery  (Eouse's  Point,  N.  Y.)  stood 
from  1846  to  1850  without  any  sign  of  failure,  when  tested  by  this 
formula,  showed  a  co-efficient  of  safety  of  3^^.  The  evidence  is  not 
conclusive:  (1)  the  factor  is  large  enough  to  cover  a  considerable 
error  in  the  formula;  (2)  since  the  formula  assumes  that  all  of  the 
energy  in  the  descending  ram  is  expended  in  overcoming  the  resist- 
ance to  penetration,  the  computed  bearing  power  is  too  small,  and 
consequently  the  co-efficient  of  safety  is  even  greater  than  as  stated; 


*  Van  Nostrand's  Engin'g  Mag.,  vol.  xvii.  p.  325. 
+  Nystrom's  Pocket-Book,  p.  158. 
X  New  Mechanics,  p.  134. 

§  Resistance  of  Piles,  J.  L.  Mason,  p.  8;  No.  5  of  Papers  on  Practical  Engineering, 
published  by  the  Engineering  Department  of  the  U.  S.  Army. 


244  PILE    FOUNDATIONS.  [CHAP.  XL 

and  (3)  it  is  probably  safe  to  say  that  after  a  pile  has  stood  a  short 

time  its  bearing  power  is  greater  than  at  the  moment  the  driving 

ceased,  owing  to  the  settlement  of  the  earth  about  it. 

Wh 
Sander's  formula*  is  P'  =  -T-r,  in  which  P'  is  the  safe  bear- 

o  a 

ing  power.     This  formula  was  deduced  on  the  assumptions  that  the 

energy  of  the  falling  weight  was  wholly  employed  in  forcing  the 

pile  into  the  ground, — i.  e.,  on  the  assumption  that  P  d  ^=  Wit,  or 

W  h 
P  =  — ;— , — and  that  the  safe  load  was  one  eighth  of  the  ultimate 
d  •' 

supporting  power.  It  is  therefore  a  roughly  approximate,  theoreti- 
cal formula. 

Notice  that,  since  some  of  the  energy  is  always  lost,  P  d,  the 
energy  represented  by  the  movement  of  the  pile,  must  always  be 
less  than  W  li,  the  energy  of  the  hammer ;  hence,  P  is  always  less 

than — -j-;  or,  in  mathematical  language,  P< — —.     This  relation  is 

It  tt 

very  useful  for  determining  the  greatest  possible  value  of  the  sup- 

porting  power.     P  will  always  be  considerably  less  than — ^  ;    and 

this  difference  is  greater  the  lighter  the  weight,  the  greater  the 
fall,  the  softer  the  material  of  the  pile,  or  the  more  the  head  Ib 
bruised.  When  d  is  very  small,  say  \  inch  or  less,  the  difference  is 
so  great  as  to  make  this  relation  useless. 

Trautioine'' s  formula,^  in  the  nomenclature   of  page   235,  is 

P  = — ^.     It  was   deduced  from    the   observed  sapporting 

power  of  piles  driven  in  soft  soil.  Strictly  speaking,  it  is  applicable 
only  under  conditions  similar  to  those  from  which  it  was  dednced ; 
and  hence  it  is  inapplicable  for  hard  driving  and  to  piles  whose 
heads  are  not  bruised  about  the  same  amount  as  were  the  experi- 
mental ones.  No  formula  can  be  accurate  which  does  not,  in  some 
way,  take  cognizance  of  the  condition  of  the  head  of  the  pile.  For 
example,  experiments  Nos.  3  and  4  of  the  table  on  page  246  are 
the  same  except  in  the  condition  of  the  heads  of  the  piles,  and  yet 


*  Jour.  Frank.  Inst.,  3d  series,  vol.  xxii.  p.  — . 
t  Engineer's  Pocket-Book,  Ed.  1885,  p.  643. 


AKT.   2.]  BEAEIXG    POAVEK   OF    PILES.  245 

the  load  supported  by  the  former  was  2^  times  that  supported  by  the 

latter.     This  formula  is  not  applicable  to  piles  driven  with  a  steam 

hammer,  since  according  to  it  the  energy  represented  by  the  sinking. 

of  the  pile  is  greater  than  the  total  energy  in  the  descending  weight. 

For  example,  if  W  —  1|  tons,  h  =  2  feet,  and  d  =  1  inch  =  ^j  of  a 

Wh 
foot,   the  formula  P  <  — 7—  becomes  P  <  36   tons.      Trautwine's 

a 

formula  gives  P  =  49  tons;  that  is  to  say,  Trautwine's   formala 

makes  the  supporting  power  one  third  more  than  it  would  be  if  710 

energy  were  lost. 

Engineering  News  formula*  the  most  recent  and   the  most 

popnlar,  is  P '  =  -^ ,  in  which  P'  is  the  safe  load  in  tons;  and 

«  -)-  1 

d'    is   the   penetration,   in   inches,   under  the  last   blow,   which   is 

assumed  to  be  appreciable  and  at  an  approximately  uniform  rate. 

366.   The  Authors  Enqjirical  Formula.     Certain   assumptions 

and  approximations  were  made  in  deducing  equation  (3),  page  239. 

If  it  is  thought  not  desirable  to  trust  entirely  to  theory,  then  the 

formula 


P  =   V2q  Wh  -{-(fd'  -  qd      .     .     .     .        (7) 

may  be  considered  as  giving  only  the  form  which  the  empirical 
formula  should  have.  Under  this  condition  q  becomes  a  numerical 
co-efficient  to  be  determined  by  experiment,  which  must  be  mad«» 
by  driving  a  pile  and  measuring  d,  after  which  the  sustaining  power 
must  be  determined  by  applying  a  direct  pressure.  The  last,  or 
test,  blow  should  be  struck  on  sound  wood. 

367.  Table  29  gives  all  the  experiments  on  the  supporting- 
power  of  piles  for  which  the  record  is  complete.  Unfortunately 
these  experiments  do  not  fulfill  the  conditions  necessary  for  a  proper 
determination  of  q  in  equation  (7).  It  is  known  that  in  some  of  the 
cases  the  head  of  the  pile  was  coni^iderably  broomed,  and  there  is 
internal  evidence  that  this  was  so  in  the  others. 

The  data  of  the  following  table  substituted  in  equation  (7)  give 
values  of  q  from  1.5  to  337,  with  an  average  of  130.  The  range  of 
these  results  shows  the  inconsistency  of  the  experiments,  and  the 
smallness  of  the  average  shows  that  the  last  blow  was  not  struc>  ^o 
sound  wood.     This  value  of  q  is  of  no  practical  use 

*  Engineering  News.  vol.  xx.  pp.  511,  512  (Dee.  29,  1888;. 


246 


PILE    FOLXDATIOXS. 


[chap.  XI. 


TABLE  29. 
Data  of  Experiments  on  the  Supporting  Power  of  Piles. 


P  !d 

at  a 

s  o  S 

Id 

o  S 

?  H 

Q  OEh 

m  b 

W  a  5 

5  a 

>  r  « 

Big" 

AUTHORITT, 

a  u 

2  ■<  o 

M  J 

sc  2; 

H  o  as 

l« 

n  a,  H 
O 

1 

0.455 

5 

0.031 

30.2 

Circular  of  the  Office  of  Chief  of  Engineers 
U.  S.  A.,  Nov.  12,  '81,  pp.  2,  3. 

2 

0.8 

86 

15 

7.3 

Trautwine's  Pocket-Book,  ed.  1885,  p.  643. 

3 

1.12 

80 

0.042 

112.0 

Jour.  Frank.  Inst.,  vol.  55,  p.  101. 

4 

1.1 

30 

0.042 

45.9 

Delafield's  "Foundations  in  Compicssible  Soils," 
pp.  17,  18; — a  pamphlet  published  by  En- 
gineers' Department  of  U.  S.  A. 

5 

0.95 

29 

0.125 

50.0 

Trautwine  in  Railroad  Gazette,  July  8,  1887,  p. 
453. 

368.  As  confirming  the  reliability  of  \X\q  form  of  equations  (3), 
(4),  and  (7),  it  is  interesting  to  notice  that  A.  C.  Hertiz*  found, 
from  the  records  of  the  driving  and  afterwards  pulling  up  of  nearly 
400  piles,  the  following  relation  : 


d  = 


Wh 


P 

500' 


which  may  be  put  in  the  form 


P  =  i/500  Wh  +  (250  (7)'  -  250  cl 


(8) 


Equation  (8)  has  exactly  the  form  of  equation  (3),  page  239, 
although  deduced  in  an  entirely  different  way.  The  value  (250)  of 
the  constant  q  in  equation  (8)  is  less  than  that  in  equation  (4), 
page  239,  which  shows  that  the  heads  of  the  piles  were  broomed. 
The  value  of  q  in  equation  (8)  is  greater  than  that  deduced  from  the 
data  of  Table  29,  which  shows  that  the  piles  from  which  equation 
(8)  was  determined  were  not  bruised  as  much  as  those  in  the  above 
table. 

369.  Supporting  Power  Determined  by  Experiment.  It  is 
not  certain  that  the  bearing  power  of  a  pile  when  loaded  with  a  con- 
tinued quiescent  load  will  be  the  same  as  that  during  the  very  short 


*  Proc.  Inst,  of  C.  E.,  vol.  Ixiv.  pp.  311-15  ;  republished  in  Van  Nostrand's  Maga- 
zine, vol.  XXV.  pp.  373-76. 


ART.   2.]  BEARING   POWER   OF   PILES.  247 

period  of  the  blow.  The  friction  on  the  sides  of  the  pile  will  have 
a  greater  effect  in  the  former  case,  while  the  resistance  to  penetra- 
tion of  the  point  will  be  greater  in  the  latter.  This,  and  the  fact 
that  the  supporting  power  of  piles  sunk  by  the  Avater-jet  can  be 
determined  in  no  other  way,  shows  the  necessity  of  experiments  to 
determine  the  bearing  power  under  a  steady  load. 

Unfortunately  no  extended  experiments  have  been  made  in  this 
direction.  We  can  give  only  a  collection  of  as  many  details  as  pos- 
sible concerning  the  piles  under  actual  structures  and  the  loads 
which  they  sustain.  In  this  way,  we  may  derive  some  idea  of  the 
sustaining  power  of  piles  under  various  conditions  of  actual  practice. 

370.  Ultimate  Load.  In  constructing  a  light-house  at  Proctors- 
ville.  La.,  in  1856-57,  a  test  pile,  12  inches  square,  driven  29.5  feet, 
bore  29.9  tons  without  settlement,  but  with  31.2  tons  it  "settled 
slowly."  The  soil,  as  determined  by  borings,  had  the  following 
character  :  "  For  a  depth  of  9  feet  there  was  mud  mixed  with 
sand  ;  then  followed  a  layer  of  sand  about  5  feet  thick,  next  a  layer 
of  sand  mixed  with  clay  from  4  to  G  feet  thick,  and  then  followed 
fine  clay.  By  draining  the  site  the  surface  was  lowered  about  6 
inches.  The  pile,  by  its  own  weight,  sank  5  feet  4  inches."  The 
above  load  is  equivalent  to  a  frictional  resistance  of  600  lbs.  per 
sq.  ft.  of  surface  of  pile  in  contact  with  the  soil.  This  pile  is  No. 
1  of  the  table  on  page  246. 

At  Philadelphia  in  1873,  a  pile  was  driven  15  ft.  into  "soft  river 
mud,  and  5  hours  after  7.3  tons  caused  a  sinking  of  a  very  small 
fraction  of  an  inch ;  under  9  tons  it  sank  f  of  an  inch,  and  under 
15  tons  it  sank  5  ft."  The  above  load  is  equivalent  to  320  lbs. 
per  sq.  ft.  of  surface  of  contact.  This  pile  is  No.  2  of  the  table  on 
page  246. 

In  the  construction  of  the  dock  at  the  Pensacola  navy  yard,  a  pile 
driven  16  feet  into  clean  white  sand  sustained  a  direct  pressure  of 
43  tons  without  settlement,  while  45  tons  caused  it  to  rise  slowly; 
and  it  required  46  tons  to  draw  a  pile  that  had  been  driven  16  feet 
into  the  sand.  This  is  equivalent  to  a  frictional  resistance  of  1,900 
lbs.  per  sq.  ft.     This  pile  is  No.  4  of  the  table  on  page  246. 

"  In  the  construction  of  a  foundation  for  an  elevator  at  Buffalo, 
N.  Y.,  a  pile  15  inches  in  diameter  at  the  large  end,  driven  18  ft., 
bore  25  tons  for  27  hours  without  any  ascertainable  effect.  The 
weight  was  then  gradually  increased   until  the  total  load  on  the 


248  PILE    FOUJv^DATIOXS.  [CHAP.   XI. 

pile  was  37^  tons.  Up  to  this  weight  there  had  been  no  depression 
of  the  pile,  but  with  37^  tons  there  was  a  gradual  depression  which 
aggregated  |  of  an  inch,  beyond  whicli  there  was  no  depression 
until  tlie  weight  was  increased  to  50  tons.  With  50  tons  there  was 
a  further  depression  of  ^  of  an  inch,  making  the  total  depression 
1^  inches.  Then  the  load  was  increased  to  75  tons,  under  which 
the  total  depression  reached  3|  inches.  The  experiment  was  not 
carried  beyond  this  point.  The  soil,  in  order  from  the  top,  was 
as  follows  :  2  ft.  of  blue  clay,  3  ft.  of  gravel,  5  ft.  of  stiff  red  clay, 

2  ft.  of  quicksand,  3  ft.  of  red  clay,  2  ft.  of  gravel  and  sand,  and 

3  ft.  of  very  stiff  blue  clay.  All  the  time  during  this  experiment 
there  were  three  pile-drivers  at  work  on  the  foundation,  thus  keep- 
ing up  a  tremor  in  the  ground.  The  water  from  Lake  Erie  had 
free  access  to  the  pile  through  the  gravel."*  This  is  equivalent 
to  a  frictional  resistance  of  1,850  lbs.  per  sq.  ft.  This  is  pile  No.  5 
of  the  table  on  page  246. 

371.  In  making  some  repairs  at  the  Hull  docks,  England, 
several  hundred  sheet-piles  were  drawn  out.  They  were  12  X  10 
inches,  driven  an  average  depth  of  18  feet  in  stiff  blue  clay,  and 
the  average  force  required  to  pull  them  was  not  less  than  35.8 
tons  each.  The  frictional  resistance  was  at  least  1,875  lbs.  per  sq. 
ft.  of  surface  in  contact  with  the  soil,  f 

372.  Safe  Load.  The  piles  under  the  bridge  over  the  Missouri 
at  Bismarck,  Dakota,  were  driven  32  ft.  into  the  sand,  and  sustain 
20  tons  each — equivalent  to  a  frictional  resistance  of  GOO  lbs.  per  sq. 
ft.  The  piles  at  the  Plattsmouth  bridge,  driven  28  ft.  into  the 
sand,  sustain  less  than  13^  tons,  of  which  about  one  fifth  is  live 
load, — equivalent  to  a  frictional  resis'tance  of  300  lbs.  per  sq.  ft. 

At  the  Hull  docks,  England,  piles  driven  16  ft.  into  "  alluvial 
mud  "  sustain  at  least  20  tons,  and  according  to  some  25  tons  ;  for 
the  former,  the  friction  is  about  800  lbs.  per  sq.  ft.  The  piles 
under  the  Eoyal  Border  bridge  "were  driven  30  to  40  ft.  into  sand 
and  gravel,  and  sustain  70  tons  each," — the  friction  being  about 
1,400  lbs.  per  sq.  ft. 

373.  "The  South  Street  bridge  approach,  Philadelphia,  fell  by 
the  sinking  of  the  foundation  piles  under  a  load  of  24  tons  each. 

*  By  courtesy  of  John  C.  Trautwine,  Jr. ,  from  private  correspondence  of  John  E. 
Payne  and  W.  A.  Haven,  engineers  in  charge. 
+  Proc.  Inst,  of  C.  E.,  vol.  Ixiv.  pp.  311-15. 


AeT.  2.]  BEARING    POWER   OF    PILES.  249 

They  were  driven  to  an  absolute  stoppage  by  a  1-ton  hammer  fall- 
ing 32  feet.  Their  length  was  from  24  to  41  feet.  The  piles  were 
driven  through  mud,  then  tough  clay,  and  into  hard  gravel."* 
According  to  Trau twine's  formula  their  ultimate  supporting  power 
was  164  tons,  and  according  to  the  Engineering  News  formula  the 
safe  load  was  64  tons.  It  is  probable  that  the  last  blow  was  struck 
on  a  broomed  head,  which  would  greatly  reduce  the  penetration, 
and  that  consequently  their  supporting  power  was  greatly  over- 
estimated.. If  the  penetration  when  the  last  blow  was  struck  on 
sound  wood  were  2  inches,  then  according  to  Trau  twine's  formula 
the  tdtimate  supporting  power  was  54.7  tons,  and  according  to  the 
Engineering  Neivs  formula  the  safe  load  was  21.3  tons. 

374.  Supporting  Power  of  Screw  and  Disk  Piles.  The  sup- 
porting power  depends  upon  the  nature  of  the  soil  and  the  dejothto 
which  the  pile  is  sunk.  A  screw  pile  "  in  soft  mud  above  clay  and 
sand "  supported  1.8  tons  per  sq.  ft.  of  blade,  f  A  disk  pile  in 
"  quicksand  "  stood  5  tons  per  sq.  ft.  under  vibrations.  %  Charles 
McDonald,  in  constructing  the  iron  ocean-pier  at  Coney  Island,  as- 
sumed that  the  safe  load  upon  the  flanges  of  the  iron  disks  sunk  into 
the  sand,  was  5  tons  per  sq.  ft. ;  but  "  many  of  them  really  support 
as  much  as  6.3  tons  per  sq.  ft.  continually  and  are  subject  to  occa- 
sional loads  of  8  tons  per  sq.  ft.,  without  causing  any  settlement 
that  can  be  detected  by  the  eye."§ 

375.  Factor  of  Safety.  On  account  of  the  many  uncertainties 
in  connection  with  piles,  a  wide  margin  of  safety  is  recommended  by 
all  authorities.  The  factor  of  safety  ranges  from  2  to  12  according 
to  the  importance  of  the  structure  and  according  to  the  faith  in  the 
formula  employed  or  the  experiment  taken  as  a  guide.  At  best, 
the  formulas  can  give  only  the  siipporting  power  at  the  time  when 
the  driving  ceases.  If  the  resistance  is  derived  mainly  from  fric- 
tion, it  is  probable  that  the  supporting  power  increases  for  a  time 
after  the  driving  ceases,  since  the  co-efficient  of  friction  is  usually 
greater  after  a  period  of  rest.  If  the  supporting  power  is  derived 
mainly  from  the  resistance  to  penetration  of  a  stiff  substratum,  the 
bearing  power  for  a  steady  load  will  probably  be  smaller  than  the 

*  Trans.  Am.  Soc.  of  C.  E.,  vol.  vii.  p.  264. 
tProc.  Inst,  of  C.  E.,  vol.  xvii.  p.  451. 
Xlbid.,  p.  443. 
§  Trans.  Am.  Soc.  C.  E.,  vol.  \iii.  p.  236. 


250  PILE   FOUNDATIONS.  [OHAP.  XL 

force  required  to  drive  it,  as  most  materials  require  a  less  force  to 
change  their  form  slowly  than  rapidly.  If  the  soil  adjoining  the 
piles  becomes  wet,  the  supporting  power  will  be  decreased;  and 
vibrations  of  the  structure  will  have  a  like  effect. 

The  formulas  in  use  for  determining  the  supporting  power  of 
piles  are  so  unreliable,  that  it  is  quite  impossible  to  determine  the 
factor  of  safety  for  any  existing  structure  with  anything  like  accu- 
racy. 

The  factor  to  be  employed  should  vary  with  the  nature  of  the 
etructure.  For  example,  the  abutments  of  a  stone  arch  should  bo 
constructed  so  that  they  will  not  settle  at  all ;  but  if  a  railroad  pile 
trestle  settles  no  serious  damage  is  done,  since  the  track  can  be 
shimmed  up  occasionally.  In  a  few  cases,  a  small  settlement  has 
taken  place  in  a  rail-road  trestle  when  the  factor  of  safety  was  3  or 
4,  as  computed  by  equation  (4),  page  239. 

Art.  3.  Areangement  of  the  Foundation. 

376.  Disposition  of  the  Piles.  The  length  of  the  piles  to  be 
used  is  determined  by  the  nature  of  the  soil,  or  the  conveniences 
for  driving,  or  the  lengths  most  easily  obtained.  The  safe  bearing 
power  may  be  determined  from  the  data  presented  in  §§  370-73,  or, 
better,  by  driving  a  test  pile  and  applying  equation  (4),  page  239. 
Then,  knowing  the  weight  to  be  supported,  and  having  decided 
upon  the  length  of  piles  to  be  used,  and  having  ascertained  their 
safe  bearing  power,  it  is  an  easy  matter  to  determine  how  many  piles 
are  required.  Of  course,  the  number  of  piles  under  the  different 
parts  of  a  structure  should  be  proportional  to  the  weights  of  those 
parts. 

If  the  attempt  is  made  to  drive  piles  too  close  together,  they  are 
liable  to  force  each  other  up.  To  avoid  this,  the  centers  of  the 
piles  should  be,  at  least,  2^  or  3  feet  apart.  Of  course,  they  may 
be  farther  apart,  if  a  less  number  will  give  sufficient  supporting 
power,  or  if  a  greater  area  of  foundation  is  necessary  to  prevent 
overturning. 

When  a  grillage  (§  380)  is  to  be  placed  on  the  head  of  the  piles, 
great  care  must  be  taken  to  get  the  latter  in  line  so  that  the  lowest 
course  of  grillage  timber,  in  this  case  called  capping,  may  rest 
squarely  upon  all  the  piles  of  a  row.     In  driving  under  water,  a 


Ar.T.  3.]  AKEAXGEMEXT   OF   THE    FOUNDATION.  251 

convenient  way  of  marking  the  positions  of  the  piles  is  to  construct 
a  light  frame  of  narrow  boards,  called  a  spider,  in  which  the  posi- 
tion of  the  piles  is  indicated  by  a  small  square  opening.  This  frame 
may  be  held  in  place  by  fastening  it  to  the  sides  of  the  coffer-dam, 
or  to  the  piles  already  driven,  or  to  temporary  supports.  Under 
ordinary  circumstances,  it  is  reasonably  good  work  if  the  center 
of  the  pile  is  under  the  cap.  Piles  frequently  get  considerably  out 
of  place  in  driving,  in  which  case  they  may  sometimes  be  forced 
back  with  a  block  and  tackle  or  a  jack-screw.  When  the  heads  of 
the  piles  are  to  be  covered  with  concrete,  the  exact  position  of  the 
piles  is  comparatively  an  unimportant  matter. 

In  close  driving,  it  is  necessary  to  commence  at  the  center 
of  the  area  and  work  towards  the  sides ;  for  if  the  central  ones  are 
left  until  the  last,  the  soil  may  become  so  consolidated  that  they 
can  scarcely  be  driven  at  all. 

377.  Butt  vs.  Top  Down.  According  to  Eankine*  all  piles 
should  be  driven  large  end  down,  having  first  been  sharpened  to  a 
point  1|-  to  2  times  as  long  as  the  diameter  of  the  pile.  This  is  at 
least  of  doubtful  utility.  If  the  pile  is  supported  wholly  by  fric- 
tion, then  the  supporting  power  will  be  greater  when  the  small  end 
is  down.  If  the  soil  is  semi-liquid,  the  buoyancy  would  be  slightly 
greater  when  the  large  end  is  down  ;  but  the  buoyancy  constitutes 
but  a  very  small  jmrt  of  the  supporting  power,  and  the  difference 
in  buoyancy  between  top  and  bottom  down  is  still  less.  If  the  pile 
derives  its  support  mainly  from  a  solid  substratum,  then  its  bearing 
power  would  be  greater  with  the  large  end  down  ;  but,  in  this  case, 
it  should  not  be  sharpened.  For  close  driving,  it  is  frequently 
recommended  that,  to  prevent  the  piles  from  forcing  each  other  up, 
they  should  be  driven  butt  end  down.  Notice,  however,  that  if 
the  soil  is  non-compressible,  as  pure  sand,  or  if  the  piles  are  driven 
so  close  as  to  compress  the  soil  considerably,  it  will  rise  and  carry 
the  piles  with  it,  whether  they  were  driven  with  the  big  or  the  little 
end  down.  Piles  are  generally  driven  small  end  down,  but  never- 
theless practical  experience  shows  that  there  are  conditions  in  which 
it  is  apparently  impossible  to  drive  them  in  this  way,  even  in 
comparatively  isolated  positions.  These  conditions  appear  to  occur 
most  frequently  in  swamps,  and  in  connection  with  quicksand. 

*  "  Civil  Engineering,"  p.  602. 


352  PILE    FOUXDATIONS.  [CHAP.   XI. 

378.  Sawing-OFF  the  Piles.  When  piles  are  driven,  it  is 
generally  necessary  to  saw  them  off  either  to  bring  them  to  the 
same  height,  or  to  get  the  tops  lower  than  they  can  be  driven,  or  to 
secure  sound  wood  upon  Avhich  to  rest  the  timber  platform  that 
carries  the  masonry.  When  above  water,  piles  are  usually  sawed  off 
by  hand  ;  and  when  below,  by  machinery — usually  a  circular  saw  on 
a  vertical  shaft  held  between  the  leaders  of  the  pile  driver  or  mounted 
upon  a  special  frame,  and  driven  by  the  engine  used  in  driving  the 
piles.  The  saw-shaft  is  sometimes  attached  to  a  vertical  shaft  held 
between  the  leaders  by  parallel  bars,  by  which  arrangement  the  saw 
can  be  swung  in  the  arc  of  a  circle  and  several  piles  be  cut  off  with- 
out moving  the  machine.  The  piles  are  sometimes  sawed  off  with 
what  is  called  a  pendulum  saw,  i.e.,  a  saw-blade  fastened  between 
two  arms  of  a  rigid  frame  which  extends  into  the  water  and  is  free 
to  swing  about  an  axis  above.  The  saw  is  swung  by  men  pushing 
on  the  frame.  The  first  method  is  the  better,  particularly  when 
the  piles  are  to  be  sawed  off  under  mud  or  silt. 

Considerable  care  is  required  to  get  the  tops  cut  off  in  a  hori- 
zontal plane-.  It  is  not  necessary  that  this  shall  be  done  with  mathe- 
matical accuracy,  since  if  one  pile  does  stand  up  too  far  the  excess 
load  upon  it  will  either  force  it  down  or  crush  the  cap  until  the 
other  piles  take  part  of  the  weight.  Under  ordinary  conditions,  it 
is  a  reasonably  good  job  if  piles  on  land  are  sawed  within  half  an 
inch  of  the  same  height ;  and  under  water,  within  one  inch.  When 
a  machine  is  used  on  land,  it  is  usually  mounted  upon  a  track  and 
drawn  along  from  pile  to  pile,  by  which  device,  after  having  leveled 
up  the  track,  a  whole  row  can  be  sawed  off  Avith  no  further  atten- 
tion. When  sawing  under  water,  the  depth  below  the  surface  is 
indicated  by  a  mark  on  the  saAV-shaft,  or  a  target  on  the  saw- 
shaft  is  observed  upon  with  a  leveling  instrument,  or  a  leveling  rod 
is  read  upon  some  part  of  the  saw-frame,  etc.  In  sawing  piles  off 
under  water,  from  a  boat,  a  great  deal  of  time  is  consumed  (par- 
ticularly if  there  is  a  current)  in  getting  the  boat  into  position 
ready  to  begin  work. 

Piles  are  frequently  sawed  off  under  10  to  15  feet  of  water,  and 
occasionally  under  20  to  25  feet. 

379.  Finishing  the  Foundation.  There  are  two  cases :  (^) 
when  the  heads  of  the  piles  are  not  under  water  ;  and  (2)  when  they 
are  under  water. 


ART.  3.]  ARRANGEMENT   OF   THE    FOUND ATIOJST.  253 

1.  When  the  piles  are  not  under  water  there  are  again  two  case:'  •. 
(a)  when  a  timber  grillage  is  used  ;  and  (b)  when  concrete  alone  ":o 
used. 

2.  "When  the  piles  are  sawed  off  under  water,  the  timber  struct- 
ure (in  this  case  called  a  crib)  which  intervenes  between  the  piles 
and  the  masonry  is  put  together  first,  and  then  sunk  into  place.  The 
construction  is  essentially  the  same  as  when  the  jjiles  are  not  under 
water,  but  differs  from  that  case  in  the  manner  of  getting  the  tim- 
ber into  iU  final  resting  place.  The  methods  of  constructing  foun- 
dations under  water,  including  that  by  the  use  of  timber  cribs,  will 
be  discussed  in  Art.  2  of  the  next  chapter. 

380.  Piles  and  Grillage.  This  is  a  stout  frame  of  one  or  more 
courses  of  timber  drift-bolted  or  pinned  to  the  tops  of  the  piles 
and  to  each  other,  upon  which  a  floor  of  thick  boards  is  placed  to 
receive  the  bottom  courses  of  masonry.  For  illustrated  examples, 
see  Fig.  84,  page  3G3,  Fig.  86,  page  380,  and  Fig.  90,  page  386. 

The  timbers  which  rest  upon  the  heads  of  the  piles,  called  cajjs, 
are  usually  about  1  foot  square,  and  are  fastened  by  boring  a  hole 
through  each  and  into  the  head  of  the  pile  and  driving  into  the 
hole  a  plain  rod  or  bar  of  iron  having  about  25  per  cent,  larger  cross 
section  than  the  hole. 

381.  These  rods  are  called  .drift-bolts,  and  are  usually  either 
a  rod  1  inch  in  diameter  (driven  into  a  f-inch  auger  hole),  or  a 
bar  1  inch  square  (driven  into  a  |-iuch  hole).  Formerly  jag-bolts, 
or  rag-bolts,  /.  e.,  bolts  whose  sides  were  jagged,  or  barbed,  were 
used  for  this  and  similar  purposes ;  but  universal  experience  shows 
that  smooth  rods  hold  much  the  better.  In  some  experiments 
made  at  the  Poughkeepsie  bridge  (§  414),  it  was  found  that  a  1-inch 
rod  driven  into  a  |f-inch  hole  in  hemlock  required  on  the  average 
a  force  of  2^  tons  per  linear  foot  of  rod  to  withdraw  it;  and  a  1-incli 
rod  driven  into  a  f-inch  hole  in  white  or  Norway  pine  required 
5  tons  per  linear  foot  of  rod  to  withdraw  it.*  The  old-style  jag- 
bolt  was  square  because  it  was  more  easily  barbed  ;  and  probably 
this  is  the  reason  why  square  drift-bolts  are  now  more  common. 
Another  advantage  of  the  round  drift-bolt,  over  the  square  one,  is 
that  the  latter  does  not  cut  or  tear  the  wood  as  much  as  the  former. 
The  ends  of  the  rods  should  be  slightly  rounded  with  a  hammer. 

Transverse  timbers  are  put  on  top  of  the  caps  and  drift-bolteu 
to  them.     Old  bridge-timbers,  timbers  from  false  works,  etc.,  are 

*  For  additinnal  data,  see  Note  8,  page  547. 


254  PILE    FOUNDATIONS.  [CHAP.   XI. 

frequently  used,  and  are  ordinarily  as  good  for  tliis  purpose  as  new. 
As  many  courses  may  be  added  as  is  necessary,  each  perpendicular 
to  the  one  beVow  it.  The  timbers  of  the  top  course  are  laid  close 
together,  or,  as  before  stated,  a  floor  of  thick  boards  is  added  on  top 
to  receive  the  masonry.  , 

This  form  of  construction  is  very  common  in  the  foundations  of 
bridge  abutments.  Of  course  no  timber  should  be  used  in  a  foun- 
dation, except  where  it  will  always  be  wet. 

382.  Piles  and  Concrete.  A  thick  layer  of  concrete,  resting 
partly  on  the  heads  of  the  piles  and  partly  on  the  soil  between 
them,  is  frequently  employed  instead  of  the  timber  grillage  as  above. 
Objection  is  sometimes  made  to  the  j^latform  (§  380)  as  a  bed  for  a 
foundation  that,  owing  to  the  want  of  adhesion  between  wood  and 
mortar,  the  masonry  might  slide  off  from  the  platform  if  any  un- 
equal settling  should  take  place.  To  obviate  this,  the  concrete  is 
frequently  substituted  for  the  grillage  and  platform. 

However,  there  is  but  slight  probability  that  a  foundation  will 
ever  fail  on  account  of  the  masonry's  sliding  on  timber,  since,  ordi- 
narily, this  could  take  place  only  when  the  horizontal  force  is 
nearly  half  of  the  downward  pressure.*  This  could  occur  only 
with  dams,  retaining  walls,  or  bridge  abutments,  and  rarely,  if 
ever,  with  these.  One  of  the  fundamental  principles  of  all  masonry 
construction  is  to  build  the  courses  perpendicular  to  the  line  of 
pressure,  which  condition  alone  would  prevent  slipping.  Any  pos- 
sibility of  slipping  can  be  prevented  also  by  omitting  one  or  more 
of  the  timbers  in  the  top  course — the  omitted  timbers  being  per- 
pendicular to  the  direction  of  the  forces  tending  to  produce  sliding, 
— or  by  building  the  top  of  the  grillage  in  the  form  of  steps,  or  by 
driving  drift-bolts  into  the  platform  and  leaving  their  upper  ends 
projecting. 

Although  the  use  of  concrete,  as  above,  may  not  be  necessary  to 
prevent  sliding,  it  adds  materially  to  the  supporting  power  of  the 
foundation  ;  it  utilizes  the  bearing  power  of  the  soil  between  the 
piles  as  well  as  the  supporting  power  of  the  piles  themselves, 
which  is  a  very  important  consideration  in  soft  soils.  Another  ad- 
vantage of  this  form  of  construction  is  that  the  concrete  can  be  laid 
without  exhausting  the  water  or  sawing  off  the  piles.     Frequently 

*  See  Table  36,  page  315. 


ART.  3.]  AREANGEMENT   OF   THE    FOUIf CATIONS'.  255 

concrete  can  also  be  used  advantageously  in  connection  with  timbei* 
grillage  to  pack  in  around  the  timbers. 

383.  Lateral  Yielding.  Notice  that,  although  the  masonry 
may  not  slide  off  from  tlie  timber  platform  (§  382),  the  foundation 
may  yield  laterally  by  the  piles  themselves  being  pushed  over.  If 
the  piles  reach  a  firm  subsoil,  it  will  help  matters  a  little  to  remove 
the  upper  and  more  yielding  soil  from  around  the  tops  of  the  piles 
and  fill  in  with  broken  stone  ;  or  a  wall  of  piles  may  be  driven 
around  the  foundation — at  some  distance  from  it, — and  timber 
braces  be  placed  between  the  wall  of  piles  and  the  foundation. 
When  the  foundation  can  not  be  buttressed  in  front,  the  structure 
may  be  prevented  from  moving  forward  by  rods  which  bear  on  the 
face  of  the  wall  and  are  connected  with  plates  of  iron  or  blocks  of 
stone  imbedded  in  the  earth  at  a  distance  behind  the  wall  (see 
§  551),  or  the  thrust  of  the  earth  against  the  back  of  the  wall  may 
be  decreased  by  supporting  the  earth  immediately  behind  the 
foundation  proper  upon  a  grillage  and  platform  resting  on  piles,  or 
the  same  result  may  be  attained  by  constructing  relieving  arches 
against  the  back  of  the  Avail  (see  §  552). 

384.  CusHiNG's  Pile  Foundation.  The  desire  to  utilize  the 
cheapness  and  efficiency  of  ordinary  piles  as  a  foundation  for  bridge 
piers  and  at  the  same  time  secure  greater  durability  than  is  pos- 
sible with  piles  alone,  led  to  the  introduction  of  what  is  known  as 
Cushing^s  pile  foundation,  first  used  in  1868,  at  India  Point,  Rhode 
Island,  It  consists  of  square  timber  piles  in  intimate  contact  with 
each  other,  forming  a  solid  mass  of  bearing  timber.  Surrounding 
the  pile  cluster  is  an  envelope  of  cast  or  wrought  iron,  sunk  in  the 
mud  or  silt  only  enough  to  protect  the  piles,  all  voids  between  piles 
and  cylinders  being  filled  with  hydraulic  concrete. 

Several  such  foundations  have  been  used,  and  have  proved 
satisfactory  in  every  respect.  The  only  objection  that  has  ever 
been  urged  against  them  is  that  the  piles  may  rot  above  the  water 
line.  If  they  do  rot  at  all,  it  will  be  very  slowly  ;  and  time  alone 
can  tell  whether  this  is  an  important  objection. 

In  making  a  foundation  according  to  the  Gushing  system,  the 
piles  may  be  driven  first  and  the  cylinder  sunk  over  them,  or  the 
piles  can  be  driven  inside  the  cylinder  after  a  few  sections  are 
in  place.  In  the  latter  case,  however,  the  cylinders  may  be  sub- 
jected to  undue  strains  and  to  subsequent  damage  from  shock  and. 


256  PILE   FOrNDATIONS.  [CHAP.  XI. 

Tibration;  and  besides,  the  sawing  off  of  the  piles  would  be  very 
difficult  and  inconvenient,  and  they  would  have  to  be  left  at  irreg- 
ular heights  and  with  battered  tops.  On  the  other  hand,  if  the 
■piles  are  driven  first,  there  is  danger  of  their  spreading  and  there- 
\>y  interfering  with  the  sinking  of  the  cylinder. 

The  special  advantages  of  the  Gushing  piers  are  :  (1)  cheajjness, 
(2)  ability  to  resist  scour,  (3)  small  contraction  of  the  water  way, 
and  (4)  rapidity  of  construction. 

385.  Example.  The  railroad  bridge  over  the  Tenas  River,  near 
Mobile,  rests  on  Gushing  piers.  There  are  thirteen,  one  being  a 
pivot  pier.  Each,  excepting  the  pivot  pier,  is  made  of  two  cast- 
n-on  cylinders,  6  feet  in  exterior  diameter,  located  16  feet  between 
centers.  The  cylinders  were  cast  in  sections  10  feet  long,  of  metal 
1|  inches  thick,  and  united  by  interior  flanges  2  inches  thick  and 
3  inches  wide.  The  sections  are  held  together  by  40  bolts,  each 
1^  inches  in  diameter.  The  lower  section  in  each  pier  was  pro- 
vided with  a  cutting-edge,  and  the  top  section  was  cast  of  a  length 
Bufficient  to  bring  the  pier  to  its  proper  elevation. 

The  pivot  pier  is  composed  of  one  central  cylinder  6  feet  in 
diameter,  and  six  cylinders  4  feet  in  diameter  arranged  hexagonally. 
The  radius  of  the  pivot  circle,  measuring  from  the  centers  of  cylin- 
ders, is  12^  feet.  Each  cylinder  is  capped  with  a  cast-iron  plate 
2i  inches  thick,  secured  to  the  cylinder  with  twenty  1-inch  bolts. 

The  piles  are  sawed  pine,  not  less  than  10  inches  square  at  the 
•small  end.  They  were  driven  first,  and  the  cylinder  sunk  over 
them.  In  each  of  the  large  cylinders,  12  piles,  and  in  each  of  the 
smaller  cylinders,  5  piles,  were  driven  to  a  depth  not  less  than  20 
feet  below  the  bed  of  the  river.  The  piles  had  to  be  in  almost  per- 
fect contact  for  their  whole  length,  which  was  secured  by  driving 
their  points  in  contact  as  near  as  possible,  and  then  pulling  their 
tops  together  and  holding  them  by  8  bolts  1|-  inches  in  diameter. 
In  this  particular  bridge  the  iron  cylinders  were  sunk  to  a  depth 
not  less  than  10  feet  below  the  river  bed  ;  but  usually  they  are  not 
sunk  more  than  3  to  7  feet.  The  piles  were  cut  off  at  low  water, 
the  water  pumped  out  of  the  cylinder,  and  the  latter  then  filled  to 
the  top  with  concrete. 


CHAPTER  XII, 
FOUNDATIONS  UNDER  WATER. 

386.  The  class  of  foundations  to  be  discussed  in  this  chapter 
could  appropriately  be  called  Foundations  for  Bridge  Piers,  since 
the  latter  are  about  the  only  ones  that  are  laid  under  water.  In  this 
class  of  work  the  chief  difficulty  is  in  excluding  the  water  prelim- 
inary to  the  preparation  of  the  bed  of  the  foundation  and  the  con- 
struction of  the  artificial  structure.  This  usually  requires  great 
resources  and  care  on  the  part  of  the  engineer.  Sometimes  the 
preservation  of  the  foundation  from  the  scouring  action  of  the  cur- 
rent is  an  important  matter. 

Preventing  the  undermining  of  the  foundation  is  generally  not  a 
matter  of  much  difficulty.  In  quiet  water  or  in  a  sluggish  stream 
but  little  protection  is  required  ;  in  which  case  it  is  sufficient  to  de- 
posit a  mass  of  loose  stone^  or  riprap,  around  the  base  of  the  pier. 
If  there  is  danger  of  the  riprap's  being  undermined,  the  layer  must 
be  extended  farther  from  the  base,  or  be  made  so  thick  that,  if 
undermined,  the  stone  Avill  fall  into  the  cavity  and  prevent  further 
damage.  A  willow  mattress  sunk  by  placing  stones  npon  it  is  an 
economical  and  efficient  means  of  protecting  a  structure  against 
scour.  A  pier  may  be  protected  also  by  inclosing  it  with  a  row  of 
piles  and  depositing  loose  rock  between  the  pier  and  the  piles.  In 
minor  structures  the  foundation  may  be  protected  by  driving  sheet 
piles  around  it. 

If  a  large  quantity  of  stone  be  deposited  around  the  base  of  the 
pier,  the  velocity  of  the  current,  and  consequently  its  scouring 
action,  will  be  increased.  Such  a  deposit  is  also  an  obstruction  to 
navigation,  and  therefore  is  seldom  permitted.  In  many  cases  the 
only  absolute  security  is  in  sinking  the  foundation  below  the  scour- 
ing action  of  the  water.  The  depth  necessary  to  secure  this  adds  to 
the  difficulty  of  preparing  the  bed  of  the  foundation. 

387.  The  principal  difficulty  in  laying  a  foundation  under  water 
consists  in  excluding  the  water.  If  necessary,  masonry  can  be  laid 
under  water  by  divers  ;  but  this  is  very  expensive  and  is  rarely  re- 
sorted to. 

257 


358  FOUKDATIOIfS   UNDER   WATER.  [CHAP.  XII. 

Tliere  are  five  methods  in  use  for  laying  foundations  under  water: 
(1)  the  method  of  excluding  the  water  from  the  bed  of  the  founda- 
tion by  the  use  of  a  coffer-dam;  (2)  the  method  of  founding  the 
pier,  without  excluding  the  water,  by  means  of  a  timber  crib  sur- 
mounted by  a  water-tight  box  in  which  the  masonry  is  laid;  (3)  the 
method  of  sinking  iron  tubes  or  masonry  wells  to  a  solid  substratum 
by  excavating  inside  of  them;  (4)  the  method  in  which  the  water  is 
excluded  by  the  presence  of  atmospheric  air;  and  (5)  the  method  of 
freezing  a  wall  of  earth  around  the  site,  inside  of  which  the  excava- 
tion can  be  made  and  the  masonry  laid.  These  several  methods  will 
be  discussed  separately  in  the  order  named. 


Art.  1.  The  Coffer-Dam  Process. 

388.  A  coffer-dam  is  an  inclosure  from  which  the  water  is  pumped 
and  in  which  the  masonry  is  laid  in  the  open  air.  This  method  con- 
sists in  constructing  a  coffer-dam  around  the  site  of  the  proposed 
foundation,  pumping  out  the  water,  preparing  the  bed  of  the  foun- 
dation by  driving  piles  or  otherwise,  and  laying  the  masonry  on  the 
inside  of  the  coffer-dam.  After  the  masonry  is  above  the  water  the 
coffer-dam  can  be  removed. 

389.  Construction  of  the  Dam.*  The  construction  of  coffer- 
dams varies  greatly.  In  still,  shallow  water,  a  well-built  bank  of 
clay  and  gravel  is  sufficient.  If  there  is  a  slow  current,  a  wall  of 
bags  partly  filled  with  clay  and  gravel  does  fairly  well;  a  row  of 
cement  barrels  filled  with  gravel  and  banked  up  on  the  outside  has 
also  been  used.  If  the  water  is  too  deep  for-  any  of  the  above 
methods,  a  single  or  double  row  of  sheet  piles  may  be  driven  and 
banked  up  on  the  outside  with  a  deposit  of  impervious  soil  sufficient 
to  prevent  leaking.  If  there  is  much  of  a  current,  the  puddle  on 
the  outside  will  be  washed  away;  or,  if  the  water  is  deep,  a  large 
quantity  of  material  will  be  required  to  form  the  puddle-waH;  and 
hence  the  preceding  methods  are  of  limited  application. 

390.  The  ordinary  method  of  constructing  a  coffer-dam  in  deep 
water  or  in  a  strong  current  is  shown  in  Fig.  60.  The  area  to  be 
inclosed  is  first  surrounded  by  two  rows  of  ordinary  piles,  m,  m.  On 
the  outside  of  the  main  piles,  a  little  below  the  top,  are  bolted  two 

*  See  also  §  317,  page  214. 


AET.  1.] 


THE    COFFER-DAM    PROCESS. 


259 


longitudinal  pieces,  w,  w,  called  wales;  and  on  the  inside  are  fastened 
two  similar  pieces,  g,  g,  which  serve  as  guides  for  the  sheet  piles,  s,  s, 
■while  being  driven.  A  rod,  r,  connects  the  top  of  the  opposite 
main  piles  to  prevent  spreading  when  the  puddle  is  put  in.  The 
timber,  t,  is  put  on  primarily  to  carry  the  footway,  /,  and  is  some- 
times notched  over,  or  otherwise  fastened  to,  the  pieces  iv,  lo  to  pre- 
vent the  puddle  space  from  spreading,  h  and  b  are  braces  extend- 
ing from  one  side  of  the  coffer-dam  to  the  other.  These  braces  are 
put  in  position  successively  from  the  top  as  the  water  is  pumped 


Fig.  60. 

out;  and  as  the  masonry  is  built  up,  they  are  removed  and  the  sides 
of  the  dam  braced  by  short  struts  resting  against  the  pier. 

The  resistance  to  overturning  is  derived  principally  from  the 
main  piles,  m,  m.  The  distance  apart  and  also  the  depth  to  which 
they  should  be  driven  depends  upon  the  kind  of  bottom,  the  depth 
of  water,  and  the  danger  from  floating  ice,  logs,  etc.  Rules  and 
formulas  are  here  of  but  little  use,  judgment  and  experience  being 
the  only  guides.  The  distance  between  the  piles  in  a  row  is  usually 
from  4  to  6  feet. 

The  dimensions  of  the  sheet  piles  (§  329)  employed  will  depend 
upon  the  depth  and  the  number  of  longitudinal  waling  pieces  used. 
Two  thicknesses  of  ordinary  2-incli  plank  are  generally  employed. 
Sometimes  for  the  deeper  dams,  the  sheet  piles  are  timbers  10  or  12 
inches  square. 

The  thickness  of  the  dam  will  depend  upon  (1)  the  width  of  gang- 
way required  for  the  workmen  and  machinery,  (2)  the  thickness  re^ 


260  rOUXDATIOXS    under   water.  [chap.  XII. 

quired  to  prevent  ovc^rturning,  and  (3)  tlie  thickness  of  puddle 
necessary  to  prevent  leakage  througli  the  wall.  The  thickness  of 
shallow  dams  will  usually  be  determined  by  the  first  consideration  ; 
but  for  deep  dams  the  thickness  Avill  be  governed  by  the  second  or 
third  requirement.  If  thb  braces,  b,  I,  are  omitted,  as  is  sometimes 
done  for  greater  convenience  in  working  in  the  coffer-dam,  then  the 
main  ^^iles,  m,  m,  must  be  stronger  and  the  dam  wider  in  order  to 
resist  the  lateral  pressure  of  the  water.  A  rule  of  thumb  frequently 
used  for  this  case  is:  "For  depths  of  less  than  10  feet  make  the 
width  10  feet,  and  for  depths  over  10  feet  give  an  additional  thick- 
ness of  1  foot  for  each  additional  3  feet  of  wall."  Trautwine's  rule 
is  to  make  the  thickness  of  the  puddle-wall  three  fourths  of  its 
height;  but  in  no  case  is  the  wall  to  be  less  than  4  feet  thick.  If 
the  coffer-dam  is  well  braced  across  the  inclosed  area,  the  puddle- 
wall  may  vary  from  3  feet  for  shallow  depths  to  10  feet  for  great 
depths;  the  former  width  has  been  successfully  employed  for  depths 
of  18  to  20  feet,  although  it  is  considerably  less  than  is  customary. 

The  puddle-wall  should  be  constructed  of  impervious  soil,  of 
which  gravelly  clay  is  best.  It  is  a  common  idea  that  clay  alone,  or 
clay  and  fine  sand,  is  best.  With  pure  claj',  if  a  thread  of  water  ever 
so  small  finds  a  passage  under  or  through  the  puddle,  it  will  steadily 
wear  a  larger  opening.  On  the  other  hand,  with  gravelly  clay,  if 
the  water  should  wash  out  the  clay  or  fine  sand,  the  larger  particles 
will  fall  into  the  space  and  intercept  fii'st  the  coarser  sand,  and 
next  the  particles  of  loam  which  are  drifting  in  the  current  of  water; 
and  thus  the  whole  mass  puddles  itself  better  than  the  engineer 
could  do  it  with  his  own  hands.  An  embankment  of  gravel  is  com- 
paratively safe,  and  becomes  tighter  every  day.  While  a  clay  em- 
bankment may  be  tighter  at  first  than  a  gravelly  one,  it  is  always 
liable  to  breakage.  Before  putting  in  the  puddling,  all  soft  mud 
and  loose  soil  should  be  removed  from  between  the  rows  of  sheet 
piles.  The  puddling  should  be  deposited  in  layers,  and  compacted 
as  much  as  is  possible  without  causing  the  sheet  piles  to  bulge  so 
much  as  to  open  the  joints. 

391.  Coffer-dams  are  sometimes  constructed  by  building  a  strong 
crib,  and  sinking  it.  The  crib  may  be  composed  either  of  uprights 
framed  into  caps  and  sills  and  covered  on  the  outside  with  tongued 
and  grooved  planks,  or  of  squared  timbers  laid  one  on  top  of  the 
other,  log-house  fashion,  and  well  calked.     The  outer  uprights  are 


AET.  1.]  THE    COFFER-DAM    PEOCESS.  'Z6\. 

braced  against  the  inside  uprights  and  sills  to  prevent  crushing 
inwards.  This  crib  may  be  built  on  land,  launched,  towed  to  its 
final  place,  and  sunk  by  piling  stones  on  top  or  by  throwing  them 
into  cells  of  the  crib-work  which  are  boarded  up  for  that  purpose. 
The  bottom  of  the  stream  may  be  leveled  off  to  receive  the  crib  by 
dredging,  or  the  dam  may  be  made  tight  at  the  bottom  by  driving 
sheet  piles  around  it.  The  crib  must  be  securely  bolted  together 
(see  §  381)  vertically,  or  the  buoyancy  of  the  water  will  lift  off  the 
upper  courses. 

A  movable  coffer-dam  is  sometimes  constructed  in  the  same 
general  way,  except  that  it  is  made  in  halves  to  allow  of  removal 
from  around  the  finished  pier.  The  two  halves  are  joined  together 
by  fitting  timbers  between  the  projecting  courses  of  the  crib,  and 
then  passing  long  bolts  vertically  through  the  several  courses.  Some 
of  the  compartments  are  made  water-tight  to  facilitate  the  move- 
ment of  the  crib  from  place  to  place.* 

Coffer-dams  are  also  built  by  sinking  an  open  crib,  similar  to  the 
above,  and  then  sheeting  it  on  the  outside  by  driving  piles  around 
it  after  it  is  sunk.    For  shallow  depths,  this  method  is  very  efficient. 

392.  Sometimes  two  coffer-dams  are  employed,  one  inside  of  the 
other,  the  outer  one  being  used  to  keep  out  the  water,  and  the  inner 
one  to  keep  the  soft  material  from  flowing  into  the  excavation.  The 
outer  one  may  be  constructed  in  any  of  the  ways  described  above. 
The  inner  one  is  usually  a  frame- work  sheeted  with  boards,  or  a  crib 
of  squared  timbers  built  log-house  fashion  with  tight  joints.  The 
inner  crib  is  sunk  (by  weighting  it  with  stone)  as  the  excavation 
proceeds.  The  advantages  of  the  use  of  the  inner  crib  are  (1)  that 
the  coffer-dam  is  smaller  than  if  the  saturated  soil  were  allowed  to 
take  its  natural  slope  from  the  inside  of  the  dam  to  the  bottom  of 
the  excavation  ;  (2)  the  space  between  the  crib  and  the  dam  can  be 
kept  full  of  impervious  material  in  case  of  any  trouble  with  the  out- 
side dam  ;  (3)  the  feet  of  the  sheet  piling  are  always  covered,  which 
lessens  the  danger  of  undermining  or  of  an  inflow  of  water  and  mud 
under  the  dam  ;  and  (4)  it  also  reduces  to  a  minimum  the  material 
to  be  excavated. 

393.  Iron  has  been  used  in  a  few  instances  as  a  sheeting  for  cof- 
fer-dams.    Plates  are  riveted  together  to  form  the  walls,  and  stayed 

*  For  an  illustrated  example,  see  Proc.  Engineer's  Club  of  Philadelphia,  vol.  iv. 
No.  4. 


262  FOUNDATIONS   UNDEK    WATER,  [CHAP.   XII. 

on  the  inside  by  horizontal  rings  made  of  angle  iron.  Wood  is 
cheaper  and  more  easily  wrought,  and  therefore  generally  preferred. 

394.  Leakage.  A  serious  objection  to  the  use  of  coffer-dams  is 
the  difficulty  of  preventing  leakage  under  the  dam.  One  of  the 
simplest  devices  to  prevent  this  is  to  deposit  a  bank  of  gravel  around 
the  outside  of  the  dam  ;  then  if  a  vein  of  water  escapes  below  the 
sheet  piling,  the  weight  of  the  gravel  will  crush  down  and  fill  the 
hole  before  it  can  enlarge  itself  enough  to  do  serious  damage.  If  the 
coffer-dam  is  made  of  crib-work,  short  sheet  piles  may  be  driven 
around  the  bottom  of  it ;  or  hay,  willows,  etc.,  may  be  laid  around 
the  bottom  edge,  upon  which  puddle  and  stones  are  deposited  ;  or 
a  broad  flap  of  tarpaulin  may  be  nailed  to  the  lower  edge  of  the 
crib  and  spread  out  loosely  on  the  bottom,  upon  which  stones  and 
puddle  are  placed.  A  tarpaulin  is  frequently  used  when  the 
bottom  is  very  irregular, — in  which  case  it  would  cost  too  much  to 
level  off  the  site  of  the  dam  ;  and  it  is  particularly  useful  where  the 
bottom  is  rocky  and  the  sheet  piles  can  not  be  driven. 

When  the  bed  of  the  river  is  rock,  or  rock  covered  with  but  a 
few  feet  of  mud  or  loose  soil,  a  coffer-dam  only  sufficiently  tight  to 
keep  out  the  mud  is  constructed.  The  mud  at  the  bottom  of  the 
inclosed  area  is  then  dredged  out,  and  a  bed  of  concrete  deposited 
under  the  water  (§  154),  Before  the  concrete  has  set,  another  coffer- 
dam is  constructed,  inside  of  the  first  one,  the  latter  being  made  water- 
tight at  the  bottom  by  settling  it  into  the  concrete  or  by  driving 
sheet  piles  into  it.  However,  the  better  and  moi-e  usual  method  is 
to  sink  the  masonry  upon  the  bed  of  concrete  by  the  method  de- 
scribed in  Art,  2  (pages  2G'J-7l). 

It  is  nearly  impossible  to  prevent  considerable  leakage,  unless  the 
bottom  of  the  crib  rests  upon  an  impervious  stratum  or  the  sheet 
piles  are  driven  into  it.  Water  will  find  its  way  through  nearly  any 
depth  or  distance  of  gravelly  or  sandy  bottom.  Trying  to  [)ump  a 
river  dry  through  the  sand  at  the  bottom  of  a  coffer-dam  is  expen- 
sive. However,  the  object  is  not  to  prevent  all  infiltration,  but  only 
to  so  reduce  it  that  a  moderate  amount  of  bailing  or  pumping  will 
keep  the  water  oat  of  the  way.  Probably  a  coffer-dam  was  never 
built  that  did  not  require  considerable  pumping ;  and  not  infre- 
quently the  amount  is  very  great, — so  great,  in  fact,  as  to  make  it 
clear  that  some  other  method  of  constructing  the  foundation  should 
have  been  chosen. 


AET.  1.]  THE   COFFER-DAM    PROCESS.  263 

Seams  of  sand  are  very  troublesome.  Logs  or  stones  under  the 
edge  of  the  dam  are  also  a  cause  of  considerable  annoyance.  It  is 
sometimes  best  to  dredge  away  the  mud  and  loose  soil  from  the  site 
of  the  proposed  coffer-dam  ;  but,  when  this  is  necessary,  it  is  usu- 
ually  better  to  construct  the  foundation  without  the  use  of  a  coffer- 
dam,— see  Art.  3  of  this  chapter  (page  266).  Coffer-dams  should 
be  used  only  in  very  shallow  water,  or  Avhen  the  bottom  is  clay  or 
some  material  impervious  to  water. 

395.  Pumps.  In  constructing  foundations,  it  is  frequently  neces- 
sary to  do  considerable  bailing  or  pumping.  The  method  to  be  em- 
ployed in  any  jDarticular  case  will  vary  greatly  with  the  amount  of 
water  present,  the  depth  of  the  excavation,  the  appliances  at  hand, 
etc.  The  pumps  generally  used  for  this  kind  of  work  are  (1)  the  ordi- 
nary wooden  hand-pump,  (2)  the  steam  siphon,  (3)  the  pulsometer, 
and  (4)  the  centrifugal  pump.  Rotary  and  direct-acting  steam 
pumps  are  not  suitable  for  use  in  foundation  work,  owing  to  the 
deleterious  effect  of  sand,  etc.,  in  the  water  to  be  pumped. 

1.  Hand  Poioer.  When  the  lift  is  small,  water  can  be  bailed 
•out  faster  than  it  can  be  pumped  by  hand  ;  but  the  labor  is  propor- 
tionally more  fatiguing.  The  ordinary  hand  foundation-pump  con- 
sists of  a  straight  tube  at  the  bottom  of  which  is  fixed  a  common 
flap  valve,  and  in  which  works  a  piston  carrying  another  valve.  The 
tube  is  either  a  square  wooden  box  or  a  sheet-iron  cylinder, — usually 
the  latter,  since  it  is  lighter  and  more  durable.  The  pump  is  oper- 
ated by  applying  the  power  directly  to  the  upper  end  of  the  piston- 
rod,  the  pump  being  held  n  position  by  stays  or  ropes.  There  are 
more  elaborate  foundation-pumps  on  the  market. 

2.  The  steam  siplion  is  the  simplest  of  all  pumps,  since  it  has 
no  movable  parts  whatever.  It  consists  essentially  of  a  discharge 
pipe — open  at  both  ends — through  the  side  of  which  enters  a  smaller 
pipe  having  its  end  bent  up.  The  lower  end  of  the  discharge  pipe 
dips  into  the  water  ;  and  the  small  pipe  connects  with  a  steam  boiler. 
The  steam,  in  rushing  out  of  the  small  pipe,  carries  with  it  the  air 
in  the  upper  end  of  the  discharge  pipe,  thus  tending  to  form  a 
vacuum  in  the  lower  end  of  that  pipe  ;  the  water  then  rises  in  the 
discharge  pipe  and  is  carried  out  with  the  steam.  Although  it  is 
possible  by  the  use  of  large  quantities  of  steam  to  raise  small  quan- 
tities of  water  to  a  great  height,  the  steam  siphon  is  limited  prac- 
tically to  lifting  water  only  a  few  feet.     Its  cheapness  and  simplicity 


264  FOIJNDATIOXS   UNDEK   WATER.  [CHAP.  XII. 

are  recommendations  in  its  favor,  and  its  eflBciency  is  not  much  less 
than  that  of  other  forms  of  pumps.  A  common  form  of  the  steam 
siphon  resembles,  in  external  appearance,  the  Eads  sand-pumjj 
represented  in  Fig.  66  (page  293). 

3.  The  pulsometer  is  an  improved  form  of  the  steam  siphon.  It 
may  properly  be  called  a  steam  pump  which  dispenses  with  all  mov- 
able parts  except  the  valves.  The  height  to  which  it  may  lift  water 
is  practically  unlimited. 

4.  The  centrifugal  ])umif'  consists  of  a  set  of  blades  revolving  in 
a  short  cylindrical  case  which  connects  at  its  center  with  a  suction 
(or  inlet)  pipe,  and  at  its  circumference  with  a  discharge  pipe.  The 
blades  being  made  to  revolve  rapidly,  the  air  in  the  case  is  carried 
outward  by  the  centrifugal  force,  tending  to  produce  a  vacuum  in 
the  suction  pipe  ;  the  water  then  enters  the  case  and  is  discharged 
likewise.  The  distance  from  the  water  to  the  pump  is  limited  by 
the  height  to  which  the  ordinary  pressure  of  the  air  will  raise  the 
water  ;  f  but  the  height  to  which  a  centrifugal  pump  can  lift  the 
water  is  limited  only  by  the  velocity  of  the  outer  ends  of  the  revolv- 
ing blades.  When  a  quick  application  with  a  discharge  of  large 
quantities  of  water  is  the  most  imjDortant  consideration,  the  cen- 
trifugal pump  is  of  great  value.  Since  there  are  no  valves  in  action 
while  the  pump  is  at  work,  the  centrifugal  pump  will  allow  sand 
and  large  gravel — in  fact  almost  anything  that  can  enter  between 
the  arms — to  pass.  Pumps  having  a  G-inch  to  10-inch  discharge 
pipe  are  the  sizes  most  frequently  used  in  foundation  work. 

396.  Preparing  the  Foundation.  After  the  water  is  pumped 
out,  the  bed  of  the  foundation  may  be  prepared  to  receive  the 
masonry  by  any  of  the  processes  described  in  §§  283-91,  which  see. 
Ordinarily  the  only  preparation  is  to  throw  out,  usually  with  hand 
shovels,  the  soft  material.  The  masonry  may  be  started  directly 
upon  the  hard  substratum,  or  upon  a  timber  grillage  i-esting  on 
the  soil  (§§  309-10)  or  on  piles  (§  380). 

397.  Cost.  It  is  universally  admitted  that  estimates  for  the 
cost  of  foundations  under  water  are  very  unreliable,  and  none  are 
more  so  than  those  contemplating  the  use  of  a  coffer-dam.  The 
estimates  of  the  most  experienced  engineers  frequently  differ  greatly 


*  Frequently,  but  improperly,  called  a  rotary  pump. 

t  Some  forms  of  centrifugal  pumps  must  be  immersed  in  the  liquid  to  be  raised. 


ART.   1.]  THE    COFFER-DAil    PROCESS.  265 

from  the  actual  cost.  The  difficulties  of  the  case  have  already  been 
discussed  (§  394). 

For  the  cost  of  piles  and  driving,  see  §§  346-54.  The  timber 
will  cost,  according  to  locality,  anywhere  from  815  to  825  per 
thousand  feet,  board  measure.  The  cost  of  labor  in  placing  the 
timber  can  not  be  given,  since  it  varies  greatly  with  the  design,  size, 
depth,  etc.  The  iron  in  drift-bolts>  screw-bolts,  and  spikes,  is 
usually  estimated  at  3 1  to  5  cents  per  pound  in  place.  Excavation 
in  coffer-dams  frequently  costs  as  high  as  81  to  $1.50  per  cubic 
yard,  including  the  necessary  pumping. 

398.  Example.  The  following  example  is  interesting  as  show- 
ing the  cost  under  the  most  favorable  conditions.  The  data  are  for 
a  railroad  bridge  across  the  Ohio  River  at  Point  Pleasant,  W.  Va.* 
There  were  three  250-foot  spans,  one  400-foot,  and  one  200-foot. 
There  were  two  piers  on  land  and  four  in  the  water ;  and  all  ex- 
tended about  90  feet  above  low  water.  The  shore  piers  were 
founded  on  piles — driven  in  the  bottom  of  a  pit — and  a  grillage,  con- 
crete being  rammed  in  around  the  timber.  The  foundations  under 
water  were  laid  by  the  use  of  a  double  coffer-dam  '(§  392).  The 
water  was  10  feet  deep  ;  and  the  soil  was  3  to  6  feet  of  sand  and 
gravel  resting  on  dry,  compact  clay.  The  foundations  consisted  of 
a  layer  of  concrete  1  foot  thick  on  the  clay,  and  two  courses  of 
timbers.  The  quantities  of  materials  in  the  six  foundations,  and 
the  total  cost,  are  as  follows  : 

Pine  timber  in  cribs  inside  of  coffer-dams,  and  in  foundations,  273,210  ft.  B.M. 

Oak  timber  in  coffer  dams,  main  and  sheet  piling 344,412  "     " 

Poplar  timber  in  coffer-dams 3,597  "      " 

Round  piles  in  foundation  and  coffer-dams 13,571  lin.  ft. 

Excavation  in  foundations 4,342  cu.  yds. 

Concrete       "  "  649   "      " 

Riprap 997   " 

The  total  cost  of  foundations,  including  labor  of  all  kinds,  derricks,  barges, 
engines,  pumps,  iron,  tools,  ropes,  and  everything  necessary  for  the  rapid  com- 
pletion of  the  work,  was  f  64,652.62. 

In  the  construction  of  the  bridge  over  the  Missouri  River,  near 
Plattsmouth,  Neb.,  a  concrete  foundation  49  feet  long,  21  feet 
wide,  and  32  feet  deep,  laid  on  shore,  the  excavation  being  through 
clay,  bowlders,  shale,  and  soapstone,  to  bed-rock  (32  feet  below 

*  Engineering  News,  vol.  xiii.  p.  338. 


266  FOUNDATIONS    UNDER   WATER.  [CHAP.   XII. 

surface  of  the  water),  cost   $39,607.2  3,  or  $42.81  per  yard  for  the 
concrete  laid.* 

399.  For  the  relative  cost  of  foundations,  see  Art.  G,  page   309. 

400.  Conclusion.  Uncertainty  as  to  what  trouble  and  expense  a 
coffer-dam  will  develop  usually  causes  engineers  to  choose  some  other 
method  of  laying  the  foundations  for  bridge  piers.  Coffer-dams 
are  applicable  in  shallow  depths  only  ;  hence  one  objection  to  found- 
ing bridge  piers  by  this  process,  particularly  in  rivers  subject  to 
scour  or  liable  to  ice  gorges,  is  the  danger  of  their  being  either  un- 
dermined or  pushed  off  the  foundation.  When  founded  in  mud  or 
sand,  the  first  mode  of  failure  is  most  to  be  feared.  This  danger  is 
diminished  by  the  use  of  piles  or  large  quantities  of  riprap  ;  but 
such  a  foundation  needs  constant  attention.  When  founded  on 
rock,  there  is  a  possibility  of  the  piers  being  pushed  off  the  founda- 
tion ;  for,  since  it  is  not  probable  that  the  coffer-dam  can  be  pumped 
perfectly  dry  and  the  bottom  cleaned  before  laying  the  masonry  or 
depositing  the  concrete,'  there  is  no  certainty  that  there  is  good 
union  between  the  base  of  the  pier  and  the  bed-rock. 

Coffer-dams  are  frequently  and  advantageously  employed  in 
laying  foundations  in  soft  soils  not  under  water,  as  described  in 
§§  316-21  (pages  214-15). 

Art.  2.  The  Crib  and  Open-Caisson  Process. 

401.  Definitions.  Unfortunately  there  is  an  ambiguity  in  the 
use  of  the  word  caisson.  Formerly  it  always  meant  a  strong,  water- 
tight box  having  vertical  sides  and  a  bottom  of  heavy  timbers,  in 
which  the  pier  is  built  and  which  sinks,  as  the  masonry  is  added, 
until  its  bottom  rests  upon  the  bed  prepared  for  it.  With  the  in- 
troduction of  the  compressed-air  process,  the  term  caisson  was  ap- 
plied to  a  strong,  water-tight  box — open  at  the  bottom  and  closed 
at  the  top — upon  Avhich  the  pier  is  built,  and  which  sinks  to  the 
bottom  as  the  masonry  is  added.  At  present,  the  word  caisson  gen- 
erally has  the  latter  meaning.  In  the  pneumatic  process,  a  water- 
tight box— open  at  the  top — is  usually  constructed  on  the  roof  of 
the  working  chamber  ('^*' pneumatic  chamber''),  inside  of  which  the 
masonry  is  built ;  this  box  also  is  called  a  caisson.     The  caisson 

*  Exclusive  of  cost  of  building.s,  tools,  and  engineering  expenses.  These  items 
amounted  to  6  per  cent,  of  the  total  cost  of  the  entire  bridge. 


ART.  2.]  THE   CRIB   AND   OPEN-CAISSOK   PROCESS.  267 

open  at  the  bottom  is  sometimes  called  an  inverted  caisson,  and  the 
one  open  at  the  top  an  erect  caisson.  The  latter  when  built  over 
an  inverted,  or  pneumatic,  caisson,  is  sometimes  called  a  coffer-dam. 
For  greater  clearness  the  term  caisson  will  be  used  for  the  inverted, 
or  pneumatic,  caisson  ;  and  the  erect  caisson,  which  is  built  over  a 
pneumatic  caisson,  will  be  called  a  coffer-clam.  A  caisson  employed 
in  otlier  than  pneumatic  work  will  be  called  an  oi^en  caisson. 

402.  Principle.  This  method  of  constructing  the  foundation 
consists  in  building  the  pier  in  the  interior  of  an  open  caisson, 
which  sinks  as  the  masonry  is  added  and  finally  rests  upon  the  bed 
prepared  for  it.  The  masonry  usually  extends  only  a  foot  or  two 
below  extreme  low  Avater,  the  lower  part  of  the  structure  being  com- 
posed of  timber  crib-work,  called  simply  a  crih.  The  open  caisson  is 
built  on  the  top  of  the  crib,  which  is  practically  only  a  thick  bottom 
for  the  box.  The  timber  is  employed  because  of  the  greater  facil- 
ity with  which  it  may  be  put  into  place,  as  will  appear  presently. 
Timber,  when  always  wet,  is  as  durable  as  masonry  ;  and  ordinarily 
there  is  not  much  difference  in  cost  between  timb^-  and  stone. 

If  the  soil  at  the  bottom  is  soft  and  unreliable,  or  if  there  is 
danger  of  scour  in  case  the  crib  Avere  to  rest  directly  upon  the  bot- 
tom, the  bed  is  prepared  by  dredging  away  the  mud  (§  407)  to  a 
sufficient  depth  or  by  driving  piles  which  are  afterwards  sawed  off 
(§  3TS)  to  a  horizontal  plane. 

403.  Construction  of  the  Caisson.  The  construction  of  the 
caisson  differs  materially  with  its  depth.  The  simplest  form  is 
made  by  erecting  studding  by  toe- nailing  or  tenoning  them  mto 
the  top  course  of  the  crib  and  spiking  planks  on  the  outside.  For 
a  caisson  6  or  8  feet  deep,  Avhich  is  about  as  deep  as  it  is  wise  to 
try  with  this  simple  construction,  it  is  sufficient  to  use  studding  6 
inches  wide,  3  inches  thick,  and  6  to  8  feet  long,  spaced  3  feet  ajDart, 
mortised  and  tenoned  into  the  deck  course  of  the  crib.  The  sides 
and  floor  (the  upper  course  of  the  crib)  should  be  thoroughly  calked 
with  oakum.  The  sides  may  be  braced  from  the  masonry  as  the 
sinking  proceeds.  When  the  crib  is  grounded  and  the  masonry  is 
above  the  water,  the  sides  of  the  box  or  caisson  are  knocked  off. 

When  the  depth  of  water  is  more  than  8  to  10  feet,  the  caisson 
is  constructed  somewhat  after  the  general  method  shown  in  Fig.  61. 
The  sides  are  formed  of  timbers  framed  together  and  a  covering  of 
thick  planks  on  the  outside.     The  joints  are  carefully  calked  to 


268 


FOUNDATIONS   UNDER  WATER. 


[chap.  XII. 


make  the  caisson  water-tight.  In  deep  caissons,  the  sides  can  be 
built  up  as  the  masonry  progresses,  and  thus  not  be  in  the  way  of 
the  masons.  The  sides  and  bottom  are  held  together  only  by  the 
heavy  vertical  rods  ;  and  after  the  caisson  has  come  to  a  bearing 
upon  the  soil  and  after  the  masonry  is  above  the  water,  the  rods  are 
detached  and  the  sides  removed,  the  bottom  only  remaining  as  a 
part  of  the  permanent  structure. 

For  an  illustration  of  the  form  of  caisson  employed  in  sinking  a 
foundation  by  the  compressed-air  process,  see  Plate  I. 

404.  The   caisson    should    be    so    contrived    that    it    can   be 


Fig.  61. 

grounded,  and  afterwards  raised  in  case  the  bed  is  found  not  to 
be  accurately  leveled.  To  effect  this,  a  small  sliding  gate  is  some- 
times  placed  in  the  side  of  the  caisson  for  the  purpose  of  filling  it 
with  water  at  pleasure.  By  means  of  this  gate,  the  caisson  can  be 
filled  and  grounded;  and  by  closing  the  gate  and  pumping  out  the 
water,  it  can  be  set  afloat.  The  same  result  can  be  accomplished  by 
putting  on  and  taking  off  stone. 

Since  the  caisson  is  a  heavy,  unwieldy  mass,  it  is  not  possible  to 
control  the  exact  position  in  which  it  is  sunk  ;  and  hence  it  should 
be  larger  than  the  base  of  the  proposed  pier,  to  allow  for  a  little  ad- 
justment to  bring  the  pier  to  the  desired  location.     The  margin  to 


4RT.   2.]  THE    CRIB    AXD    OPEN-CAISSON   PROCESS.  269 


be  allowed  will  depend  uiion  the  depth  of  water,  size  of  caisson, 
facilities,  etc.  A  foot  all  round  is  probably  none  too  much  under 
favorable  conditions,  and  generally  a  greater  margin  should  be 
allowed. 

405.  Construction  of  the  Crib.  The  crib  is  a  timber  struct- 
ure below  the  caisson,  which  transmits  the  pressure  to  the  bed  of 
the  foundation.  A  crib  is  essentially  a  grillage  (see  §  309  and  §  380) 
which,  instead  of  being  built  in  place,  is  first  constructed  and  then 
sunk  to  its  final  resting  place  in  a  single  mass.  A  crib  is  usually 
thicker,  /.  e.,  deeper,  than  the  grillage.  If  the  pressure  is  great,  the 
crib  is  built  of  successive  courses  of  squared  timbers  in  contact;  but 
if  the  pressure  is  small,  it  is  built  more  or  less  open.  In  either 
case,  if  the  crib  is  to  rest  upon  a  soft  bottom,  a  few  of  the  lower 
■courses  are  built  open  so  that  the  higher  portions  of  the  bed  may 
be  squeezed  into  these  cells,  and  thus  allow  the  crib  to  come  to  an 
even  bearing.  If  the  crib  is  to  rest  upon  an  uneven  rock  bottom, 
the  site  is  first  leveled  up  by  throwing  in  broken  stone.  If  the  bot- 
tom is  rough  or  sloping,  the  lower  courses  of  the  crib  are  sometimes 
made  to  conform  to  the  bottom  as  nearly  as  possible,  as  determined 
from  soundings.  This  method  requires  care  and  judgment  to  pre- 
vent the  crib  from  sliding  off  from  the  inclined  bed,  and  should  be 
used  with  great  caution,  if  at  all. 

The  crib  is  usually  built  afloat.  Owing  to  the  buoyancy  of  the 
water,  about  one  third  of  a  crib  made  wholly  of  timber  would  pro- 
ject above  the  water,  and  would  require  an  inconveniently  large 
weight  to  sink  it ;  therefore,  it  is  best  to  incorporate  considerable 
stone  in  the  crib-work.  If  the  crib  is  more  or  less  open,  this  is 
done  by  putting  a  floor  into  some  of  the  open  spaces  or  pockets, 
which  are  then  filled  with  stone.  If  the  crib  is  to  be  solid,  about 
every  third  timber  is  omitted  and  the  space  filled  with  broken  stone. 

The  timbers  of  each  course  should  be  securely  drift-bolted  (§  381) 
to  those  of  the  course  below  to  prevent  the  buoyancy  of  the  upper 
portion  from  pulling  the  crib  a})art,  and  also  to  prevent  any  possi- 
bility of  the  upper  part's  sliding  on  the  lower. 

406.  Timber  in  Foundations.  The  free  use  of  timber  in 
foundations  is  the  chief  difl'erence  between  American  and  European 
methods  of  founding  masonry  in  deep  water.  The  consideration 
that  led  to  its  introduction  in  foundations  was  its  cheapness.  Many 
of  the  more  important  bridges  built  some  years  ago  rest  upon  crib- 


270  FOUND ATIOKS   UNDER  WATER.  [CHAP.  XII. 

work  of  round  logs  notched  at  tlieir  intersection  and  secured  hj 
drift-bolts.  At  present,  cribs  are  always  built  of  squared  timber. 
As  a  rule,  there  is  now  but  very  little  difference  between  the  cost 
of  timber  and  masonry  in  foundations.  The  principal  advantage 
in  the  use  of  the  timber  in  foundations  under  water  is  the  facility 
with  which  it  is  put  into  position.  Soft  wood  or  timber  which 
in  the  air  has  comparatively  little  durability,  is  equally  as  good 
for  this  purpose  as  the  hard  woods.  It  has  been  conclusively  proved 
that  any  kind  of  timber  will  last  practically  forever,  if  completely 
immersed  in  water. 

407.  Excavating  the  Site.  When  a  pier  is  to  be  founded  in 
a  sluggish  stream,  it  is  only  necessary  to  excavate  a  hole  m  the 
bed  of  the  stream,  in  which  the  crib  (or  the  bottom  of  the  caisson) 
may  rest.  The  excavation  is  usually  made  with  a  dredge,  any  form 
of  which  can  be  employed.  The  -dipper  dredge  is  the  best,  but  the 
clam-shell  or  the  endless  chain  and  bucket  dredge  are  sometimes 
used.  If  the  bottom  is  sand,  mud,  or  silt,  the  soil  maybe  removed 
(1)  by  pumping  it  with  the  water  through  an  ordinary  centrifugal 
pump  (§  395), — the  suction  hose  of  which  is  kept  in  contact  with, 
or  even  a  little  below,  the  bottom, — or  (2)  by  the  Eads  sand-pump 
(§  -±48).  With  either  of  these  methods  of  excavating,  a  simple  frame 
or  light  coflfer-dam  may  be  sunk  to  keep  part  of  the  loose  soil  from 
running  into  the  excavation. 

408.  If  the  stream  is  shallow,  the  current  swift,  and  the  bottom. 
soft,  the  site  may  be  excavated  or  scoured  out  by  the  river  itself. 
To  make  the  current  scour,  construct  two  temporary  wing-dams, 
which  diverge  up  stream  from  the  site  of  the  proposed  pier.  The 
wings  can  be  made  by  driving  stout  stakes  or  small  piles  into  the 
bed  of  the  stream,  and  placing  solid  panels — made  by  nailing  ordi- 
nary boards  to  light  uprights — against  the  piles  with  their  lower  edge 
on  the  bottom.  The  wings  concentrate  the  current  at  the  location 
of  the  pier,  increase  its  velocity,  and  cause  it  to  scour  out  the  bed  of 
the  stream.  This  process  requires  a  little  time,  usually  one  to  three 
days,  but  the  cost  of  construction  and  operation  is  comparatively 
slight. 

When  the  water  is  too  deep  for  the  last  method,  it  is  sometimes 
possible  to  suspend  the  caisson  a  little  above  the  bed  of  the  stream, 
in  which  case  the  current  will  remove  the  sand  and  silt  from  under 
it.     At  the  bridge  over  the  Mississippi  at  Quincy,  111.,  a  hole  10  feet 


ART.  3.]  DREDGING   THROUGH   WELLS.  271 

deep  was  thus  scoured  out.  If  the  water  is  already  heavily  charged 
with  sedimeut.  it  may  drop  the  sediment  on  striking  the  crib  and 
thus  fill  up  instead  of  scour  out.  Notwithstanding  the  hole  is 
liable  to  be  filled  up  by  the  gradual  action  of  the  current  or  by  a 
sudden  flood,  before  the  crib  has  been  placed  in  its  final  position, 
this  method  is  frequently  more  expeditious  and  less  expensive  than 
using  a  coffer-dam. 

409.  If  the  crib  should  not  rest  squarely  upon  the  bottom,  it 
can  sometimes  be  brought  down  with  a  water-jet  (§  34  ^^  in  the 
hands  of  a  diver.  However,  the  engineer  should  not  employ  r. 
diver  unless  absolutely  necessary,  as  it  is  very  expensive. 

410.  If  the  soft  soil  extends  to  a  considerable  depth,  or  if  the 
necessary  spread  of  foundation  can  not  be  obtained  without  an  un- 
desirable obstruction  of  the  channel,  or  if  the  bottom  is  liable  to 
scour,  then  piles  may  be  driven,  upon  which  the  crib  or  caisson  may 
finally  rest.  Before  the  introduction  of  the  compressed-air  process, 
this  was  a  very  common  method  of  founding  bridge  piers  in  our 
western  rivers  ;  and  it  is  still  frequently  employed  for  small  piers. 
The  method  of  driving  and  sawing  off  the  piles  has  already  been 
described — see  Chapter  XI. 

The  mud  over  and  around  the  heads  of  the  piles  may  be  sucked 
off  with  a  pump,  or  it  may  be  scoured  out  by  the  current  (§  408). 
The  attempt  is  sometimes  made  to  increase  the  bearing  power  of  the 
foundation  by  filling  in  between  the  heads  of  the  piles  with  broken 
stone  or  concrete  ;  but  this  is  not  good  practice,  as  the  stone  does 
but  little  good,  is  difficult  to  place,  and  is  liable  to  get  on  top  of  the 
piles  and  prevent  the  crib  from  coming  to  a  proper  bearing. 

Art.  3.  Dredging  Through  Wells. 

411.  A  timber  crib  is  frequently  sunk  by  excavating  the  material 
through  apartments  left  for  that  purpose,  thus  undermining  the 
crib  and  causing  it  to  sink.  Hollow  iron  cylinders,  or  wells  of 
masonry  with  a  strong  curb,  or  ring,  of  timber  or  iron  beneath  them, 
are  sunk  in  the  same  way. 

This  method  is  applicable  to  foundations  both  on  dry  land  and 
under  water.  It  is  also  sometimes  employed  in  sinking  shafts  in 
tunneling  and  mining. 

412.  Excavators.     The  soil  is  removed  from  under  the  crib 


272  FOUNDATIOJS^S    UNDER   WATER.  [CHAP.  XII. 

with  a  clam-shell  dredge,  or  with  an  endless  chain  and  bucket 
dredge,  or  with  the  Eads  sand-pump,  or,  for  small  jobs,  with  the 
sand-pump  employed  in  driving  artesian  wells. 

The  clam-shell  dredge  consists  of  the  two  halves  of  a  hemi- 
spherical shell,  Avhich  rotate  about  a  horizontal  diameter  ;  the  edges 
of  the  shell  are  forced  into  the  soil  by  the  weiglit  of  the  machine 
itself,  and  the  pull  upon  the  chain  to  raise  the  excavator  draws  the 
two  halves  together,  thus  forming  a  hemispherical  bucket  which 
incloses  the  material  to  be  excavated.  The  Morris  and  Cumming 
dredge  consists  of  two  quadrants  of  a  short  cylinder,  hinged  and 
operated  similarly  to  the  above.  The  orange-peel  dredge  (shown  at 
A  in  Fig.  62,  page  274)  appears  to  have  the  jireference  for  this  kind 
of  work.  It  consists  of  a  frame  from  which  are  suspended  a  num- 
ber of  spherical  triangular  spades  which  are  forced  vertically  into 
the  ground  by  their  own  weight  ;  the  pull  upon  the  excavator  to 
lift  it  out  of  the  mud  draws  these  triangles  together  and  encloses 
the  earth  to  be  excavated.  There  are  several  forms  of  dredges 
similar  to  the  above,  but  differing  from  them  in  details. 

For  a  description  of  the  Eads  sand -pump,  see  §  448. 

413.  In  one  case  in  France,  the  soil  was  excavated  by  the  aid  of 
compressed  air.  An  8-inch  iron  tube  rested  on  the  bottom,  with  its 
top  projecting  horizontally  above  the  water  ;  and  compressed  air  was 
discharged  through  a  small  pipe  into  the  lower  end  of  the  8-inch 
tube.  The  weight  of  the  air  and  water  in  the  tube  was  less  than 
an  equal  height  of  the  water  outside  ;  and  hence  the  water  in  the 
tube  was  projected  from  the  top,  and  carried  with  it  a  portion  of  the 
mud,  sand,  etc.  Pebbles  and  stones  of  considerable  size  were  thus 
thrown  out.     See  §  447. 

414.  Noted  Examples.— Poughkeepsie  Bridge.  The  Pough- 
keepsie  bridge,  which  crosses  the  Hudson  at  a  point  about  75  miles 
above  New  York  City,  is  founded  upon  cribs,  and  is  the  boldest  ex- 
ample of  timber  foundation  on  record.  It  is  remarkable  both  for 
ihe  size  of  the  cribs  and  for  the  depth  of  the  foundation. 

There  are  four  river  piers.  The  crib  for  the  largest  is  100  feet 
long,  60  feet  wide  at  the  bottom  and  40  feet  at  the  top,  and  104 
■feet  high.  It  is  divided,  by  one  longitudinal  and  six  transverse 
walls,  into  fourteen  compartments  through  which  the  dredge  worked. 
The  side  and  division  walls  terminate  at  the  bottom  with  a  12"  X 
12"  oak  stick,  which  served  as  a  cutting  edge.     The  exterior  walls 


ART.  3. J  DREDGIXG   THROUGH    WELLS.  ^1^ 

and  the  longitudinal  division  wall  were  built  solid,  of  triangular 
cross  section,  for  20  feet  above  the  cutting  edge,  and  above  that 
they  were  hollow.  The  gravel  used  to  sink  the  crib  was  deposited 
in  these  hollow  walls.  The  longitudinal  walls  were  securely  tied  to 
each  other  by  the  end  and  cross  division  walls,  and  each  course  of 
timber  was  fastened  to  the  one  below  by  450  1-inch  drift-bolts  30 
inches  long.  The  timber  was  hemlock,  12  inches  square.  The 
fourteen  compartments  in  which  the  clam-shell  dredges  worked 
were  10  X  13  feet  in  the  clear.  The  cribs  were  kept  level  while 
sinking  by  excavating  from  first  one  and  then  the  other  of  the  com- 
partments. Gravel  was  added  to  the  pockets  as  the  crib  sunk. 
\Yhe7i  hard  bottom  was  reached,  the  dredging  pockets  were  filled 
with  concrete  deposited  under  water  from  boxes  holding  one  cubic 
yard  each  and  opened  at  the  bottom  by  a  latch  and  trip-line. 

After  the  crib  was  in  position,  the  masonry  was  started  in  a 
floating  caisson  which  finally  rested  upon  the  top  of  the  crib. 
Sinking  the  crib  and  caisson  separately  is  a  departure  from  the 
ordinary  method.  Instead  of  using  a  floating  caisson,  it  is  generally 
considered  better  to  construct  a  coffer-dam  on  top  of  the  crib,  in 
.which  to  start  the  masonry.  If  the  crib  is  sunk  first,  the  stones 
which  are  thrown  into  the  pockets  to  sink  it  are  liable  to  be  left 
projecting  above  the  top  of  the  crib  and  thus  prevent  the  caisson 
from  coming  to  a  full  and  fair  bearing. 

The  largest  crib  was  sunk  through  about  53  feet  of  water,  20 
feet  of  mud,  45  feet  of  clay  and  sand,  and  17  feet  of  sand  and 
gravel.  It  rests,  at  134  feet  below  high  water,  upon  a  bed  of  gravel 
16  feet  thick  overlying  bed-rock.  The  timber  work  is  110  feet  high, 
including  the  floor  of  the  caisson,  and  extends  to  14  feet  below  high 
water  (T  feet  below  low  water),  at  which  point  the  masonry  com- 
mences and  rises  39  feet.  On  top  of  the  masonry  a  steel  tower  100 
feet  high  is  erected.  The  masonry  in  plan  is  25  X  87 feet,  and  has 
nearly  vertical  faces.  The  lower  chord  of  the  channel  span  is  130 
feet  and  the  rail  is  212  feet  above  high  water. 

The  other  piers  are  nearly  as  large  as  the  one  here  described. 
Tiie  cribs  each  contain  an  average  of  2,500,000  feet,  board  measure, 
of  timber  and  350  tons  of  wrought  iron. 

415.  Atchafalaya  Bridge.  This  bridge  is  over  the  Atchafalaya 
bayou  or  river,  at  Morgan  City,  La.,  about  80  miles  west  of  New 
Orleans.  "  The  soil  is  alluvial  to  an  unknown  depth,  and  is  subject 


274 


FOUNDATIONS   UNDER  -WATER. 


[chap.  XII. 


to  rapid  and  extensive  scour  ;  and  no  stone  suitable  for  piers  could 
be  found  within  reasonable  distance.  Hence  iron  cylinders  were 
idopted.  They  are  foundation  and  pier  combined.  The  cylinders 
were  sunk  120  feet  below  high  water— from  70  to  115  feet  below  the 
mud  line— by  dredging  the  material  from  the  inside  with-a  Milroy 
excavator.  Fig.  62  shows  the  excavator  and  the  appliances  for 
handling  the  cylinders. 


Fig.  62.— Sinking  Iron  Pile  by  Dredging. 


The  cylinders  are  8  feet  in  outside  diameter.  Below  the  level 
of  the  river  bed,  they  are  made  of  cast  iron  1^  inches  thick,  in 
lengths  of  10|-  feet ;  the  sections  were  bolted  together  through  in- 
side flanges  with  1-inch  bolts  spaced  5  inches  apart.  Above  the 
river  bottom,  the  cylinders  are  made  of  wrought-iron  plates  |  inches 
thick,  riveted  together  to  form  short  cylindrical  sections  with  angle- 
iron  flanges.  The  bolts  and  spacing  to  unite  the  sections  are  the 
same  as  in  the  cast-iron  portions. 

The  cylinders  were  filled  with  concrete  and  capped  with  a  heavy 


AET.  3.]  DREDGIXG    THROUGH   WELLS.  275 

cast-irou  plate.     Two  such  cylinders,  braced  together,  form  the  pier 
between  two  250-feet  spans  of  a  railroad  bridge. 

The  only  objection  to  such  piers  relates  to  their  stability.  These 
have  stood  satisfactorily  since  1SS3, 

416.  Hawkesbury  Bridge.  The  bridge  over  the  Hawkesbury 
Eiver  in  south-eastern  Australia  is  remarkable  for  the  depth  of  the 
foundation.  It  is  founded  upon  elliptical  iron  caissons  48  X  20  feet 
at  tlie  cutting  edge,  which  rest  upon  a  bed  of  hard  gravel  126  feet 
below  the  river  bed,  185  feet  below  high  water,  and  227  feet  below 
the  track  on  the  bridge.  The  soil  penetrated  was  mud  and  sand. 
The  caissons  were  sunk  by  dredging  through  three  tubes,  8  feet  in 
diameter,  terminating  in  bell-mouthed  extensions,  which  met  the 
cutting  edge.  The  spaces  between  the  dredging  tubes  and  the 
outer  shell  were  filled  with  gravel  as  the  sinking  progressed.  The 
caissons  were  filled  to  low  water  with  concrete,  and  above,  with  cut- 
stone  masonry. 

417.  Brick  Cylinders.  In  Germany  a  brick  cylinder  was  sunk 
256  feet  for  a  coal  shaft.  A  cylinder  25|-  feet  in  diameter  was  sunk 
76  feet  through  sand  and  gravel,  when  the  frictional  resistance 
became  so  great  that  it  could  be  sunk  no  further.  An  interior 
cylinder,  15  feet  in  diameter,  was  then  started  in  the  bottom  of  the 
larger  one,  and  sunk  180  feet  further  through  running  quicksand. 
The  soil  was  removed  without  exhausting  the  water. 

A  brick  cylinder — outer  diameter  46  feet,  thickness  of  wall  3 
feet — was  sunk  40  feet  in  dry  sand  and  gravel  without  any  difiiculty. 
It  was  built  18  feet  high  (on  a  wooden  curb  21  inches  thick),  and 
weighed  300  tons  before  the  sinking  was  begun.  The  interior  earth 
was  excavated  slowly,  so  that  the  sinking  was  about  1  foot  per  day, 
— the  walls  being  built  up  as  it  sank. 

In  Europe  and  India  masonry  bridge  piers  are  sometimes  sunk 
by  this  process,  a  sufficient  number  of  vertical  openings  being  left 
through  which  the  material  is  brought  up.  It  is  generally  a  tedious 
and  slow  operation.  To  lessen  the  friction  a  ring  of  masonry  is  some- 
times built  inside  of  a  thin  iron  shell.  The  last  was  the  method  em- 
ployed in  putting  down  the  foundations  for  the  new  Tay  bridge.* 

418.  Frictional  Resistance.  The  friction  between  cylinders 
and  the  soil  depends  upon  the  nature  of  the  soil,  the  depth  sunk, 
and  the  method  used  in  sinking.     If  the  cylinder  is  sunk  by  either 

*  For  an  illustrated  account,  see  Engineering  News,  vol.  xiv.  pp.  66-68. 


276 


FOUNDATIONS    UNDER   WATEE. 


[chap.   XII. 


of  the  pneumatic  processes  (§§  425  and  426),  the  flow  of  the  water 
or  the  air  along  the  sides  of  the  tube  greatly  diminishes  the  fric- 
tion.    It  is  impossible  to  give  any  very  definite  data. 

The  following  table*  gives  the  values  of  the  co-efficient  of  fric- 
tion f  for  materials  and  surfaces  which  occur  in  sinking  foundations 
for  bridge  piers.  Each  result  is  the  average  of  at  least  ten  experi- 
ments. "All  materials  were  rounded  off  at  their  face  to  sledge 
shape  and  drawn  lengthwise  and  horizontally  over  the  gravel  or 
sand,  the  latter  being  leveled  and  bedded  as  solid  as  it  is  likely 
to  be  in  its  natural  position.  The  riveted  sheet  iron  contained 
twenty-five  rivets  on  a  surface  of  2.53  X  1.67  =  4.22  square  feet; 
the  rivet-heads  were  half-round  and  |f  inch  in  diameter."  Notice 
that  for  dry  materials  and  also  for  wet  gravel  and  sand,  the  frictional 
resistance  at  starting  is  smaller  than  during  motion,  which  is  con- 
trary to  the  ordinary  statement  of  the  laws  of  friction. 

TABLE  30. 
Co-efficient  op  Friction  of  Materials  and  Surfaces  used  in  Foun- 
dations. 


Kind  op  Materials. 


Sheet  iron  witbout  rivets  on  gravel  and  sand. 

"        "     with  "      "        "        "        "   . 

Cast  iron  (unplaned)  on  gravel  and  sand. . . . . 
Granite  (rouglil}-  worked)  on  gravel  and  sand 

Pine  (sawed)  on  gravel  and  sand 

Sheet  iron  without  rivets  on  sand 

"        "     with  "      "      "    

Cast  iron  (unplaned)  on  sand 

Granite  (roughly  worked)  on  sand 

Pine  (sawed)  on  sand 


For  Dry 
BIatkrials. 


.3:  !>t-5 
ffi  =S 


0.40 
0.40 
0.37 
0.43 
0.41 
0.54 
0.73 
O.oO 
0.65 
0.66 


OS 


0.46 
0.49 
0.47 
0.54 
0.51 
0.63 
0.84 
0.61 
0.70 
0.73 


For  Wet 
Materials. 


cS 


0.33 
0.47 
0.36 
0.41 
0.41 
0.37 
0.52 
0.47 
0.47 
0.58 


e| 


0.44 
0.55 
0.50 
0.48 
0.50 
0.32 
0.50 
0.38 
0.53 
0.48 


419.  Values  from   Actual  Practice.      Cast   Iron.     During    the 
construction  of   the  bridge  over  the  Seine  at  Orival,  a  cast-iron 


*  Bj'  A.  Schmoll  in  "  Zeitschrift  des  Vereines  Deutscher  Ingenieure,"  as  repub- 
lished in  Selected  Abstracts  of  Inst,  of  C.  E.,  vol.  lii.  pp.  298-302. 

t  The  co-efficient  of  friction  is  equal  to  the  total  friction  divided  by  the  total 
normal  pressure;  that  is  to  say,  it  is  the  friction  per  unit  of  pressure  perpendicular 
to  the  surfaces  in  contact. 


AET.  3.]  DEEDGIXG   THROUGH   WELLS.  27? 

cylinder,  standing  in  an  extensive  and  rather  uniform  bed  of  gravel, 
and  having  ceased  to  move  for  thirty-two  hours,  gave  a  frictional  re- 
sistance of  nearly  200  lbs,  per  sq,  ft.  *  At  a  bridge  over  the  Danube 
near  Stadlau,  a  cylinder  sunk  18,75  feet  into  the  soil  (the  lower  3,75 
feet  being  "solid  clay")  gave  a  frictional  resistance  of  100  lbs.  per 
sq.  ft.*  According  to  some  European  experiments,  the  friction  of 
cast-iron  cylinders  in  sand  and  river  mud  was  from  400  to  600  lbs. 
per  sq.  ft.  for  small  depths,  and  800  to  1,000  for  depths  from  20  to 
30  feet.f  At  the  first  Harlem  River  bridge,  Xew  York  City,  the 
frictional  resistance  of  a  cast-iron  pile,  while  the  soil  around  it  was 
still  loose,  was  528  lbs.  per.  sq.  ft.  of  surface  ;  and  later  716  lbs.  per  sq. 
ft.  did  not  move  it.  From  these  two  experiments,  McAlpine,  the  en- 
gmeer  in  charge,  concluded  that  'H.OOO  lbs.  per  sq.  ft.  is  a  safe  value 
for  moderately  fine  material."  X  At  the  Omaha  bridge,  a  cast-iron 
pile  sunk  27  feet  in  sand,  with  15  feet  of  sand  on  the  inside,  could  not 
be  withdrawn  with  a  pressure  equivalent  to  25-4  lbs.  per  sq.  ft.  of 
surface  in  contact  with  the  soil  ;  and  after  removal  of  the  sand  from 
the  inside,  it  moved  with  200  lbs.  per  sq.  ft.§ 

Wrought  Iron.  A  wrought-iron  pile,  penetrating  19  feet  into 
coarse  sand  at  the  bottom  of  a  river,  gave  280  lbs.  per  sq.  ft. :  an- 
other, in  gravel,  gave  300  to  335  lbs.  per  sq.  ft.|| 

Masonry.  In  the  silt  on  the  Clyde,  the  friction  on  brick  and 
concrete  cylinders  was  about  3^  tons  per  sq.  ft.°[  The  friction  on 
the  brick  piers  of  the  DufPerin  (India)  Bridge,  through  clay,  was 
900  lbs,  per  sq,  ft.** 

Fneumatic  Caissons.  For  data  on  the  frictional  resistance  of 
pneumatic  caissons,  see  §  455. 

Piles.  For  data  on  the  frictional  resistance  of  ordinary  piles, 
see  §§  370-71. 

420.  Cost.  It  is  difl&cult  to  obtain  data  under  this  head, 
since  but  comparatively  few  foundations  have  been  put  down 
by  this  process.     Furthermore,  since  the  cost  varies  so  much  with 

*  Van  Nostrand's  Engin'g  Mag.,  voL  xx.  pp.  121-23. 
t  Proc.  Inst,  of  C.  E.,  vol.  1.  p.  131. 

X  McAlpine  in  Jour.  Frank.  Inst.,  vol.  Iv.  p.  105 ;  also  Proc.  Inst,  of  C.  E.,  vol 
xxvii.  p.  286. 

§  Vdn  Nostrand's  Engin'g  Mag.,  vol.  viii.  p.  471. 
1  Proc.  Inst,  of  C.  E.,  vol.  xv.  p.  290. 
H  Ibid.,  vol.  xxxiv.  p.  35. 
**  JLngineerhiy  Xews,  vol.  xix.  p.  160. 


278  FOUNDATIONS    UNDEX   WATER.  [CHAP.  XII. 

the  depth  of  water,  strength  of  current,  kind  of  bottom,  danger  of 
floods,  requirements  of  navigation,  etc.,  etc.,  no  such  data  are  valu- 
able unless  accompanied  by  endless  details. 

Cribs.  The  materials  in  the  cribs  will  cost,  in  place,  about  as 
follows  :  timber  from  $30  to  $40  per  thousand  feet,  boi^rd  measure ; 
drift  and  screw  bolts  from  3|-  to  5  ceyts  per  pound  ;  concrete  from 
'H  to  $6  per  cubic  yard.  Under  ordinarily  favorable  conditions,  the 
sinking  by  dredging  will  cost  about  $1  per  cubic  yard. 

Iron  Tubes.  Wrought-iron  plate  work  will  cost,  exclusive  of 
freight,  from  3  to  4^  cents  per  pound  ;  cast-iron  tubes,  exclusive  of 
freight,  1|  to  2  cents  per  pound. 

421.  For  the  relative  cost  of  different  methods,  see  Art.  6 
of  this  chapter. 

422.  Conclusion.  A  serious  objection  to  this  method  of  sink- 
ing foundations  is  the  possibility  oi  meeting  wrecks,  logs,  or  other 
obstructions,  in  the  underlying  materials  ;  but  unless  the  freezing 
process  (see  Art.  5  of  this  chapter)  shall  pi'ove  all  that  is  claimed 
for  it,  the  method  by  dredging  through  tubes  or  wells  is  the  only 
one  that  can  be  applied  to  depths  which  much  exceed  100  feet — the 
limit  of  the  pneumatic  process. 

Art.  4.     Pneumatic  Process. 

424.  The  principle  involved  is  the  utilization  of  the  difference 
between  the  pressure  of  the  air  inside  and  outside  of  an  air-tight 
chamber.  The  air-tight  chamber  may  be  either  an  iron  cylinder, 
which  becomes  at  once  foundation  and  pier,  or  a  box — open  below 
and  a'r-tight  elsewhere — upon  the  top  of  which  the  masonry  pier 
rests.  The  former  is  called  a  pnetnnaficpilej  the  latter  a  jjneu- 
■matic  caisson.  The  pneumatic  pile  is  seldom  used  now.  There 
are  two  processes  of  utilizing  this  difference  of  pressure, — the 
vac2tum  and  the  2)Ie)mm. 

425.  Vacuum  Process.  The  vacuum  process  consists  in  ex- 
hausting the  air  from  a  cylinder,  and  using  the  pressure  of  the  at- 
mosphere upon  the  top  of  the  cylinder  to  force  it  down.  Exhausting 
the  air  allows  the  water  to  flow  past  the  lower  edge  into  the  air- 
chamber,  thus  loosening  the  soil  and  causing  the  cjdinder  to  sink. 
By  letting  the  air  in,  the  water  subsides,  after  which  the  exhaustion 
may  be  repeated  and   the  pile   sunk   still  farther.     The  vacuum 


AKT.  4.]  PXEUMATIC    PROCESS.  279 


should  be  obtained  suddenly,  so  that  the  pressure  of  the  atmosphere 
shall  have  the  effect  of  a  blow ;  hence,  the  pile  is  connected  by  a 
large  flexible  tube  with  a  large  air-chamber — usually  mounted  upon 
a  boat, — from  which  the  air  is  exhausted.  When  communication  is 
0})ened  between  the  pile  and  the  receiver,  the  air  rushes  from  the 
former  into  the  latter  to  establish  equilibrium,  and  the  external 
pressure  causes  the  pile  to  sink. 

To  increase  the  rapidity  of  sinking,  the  cylinders  may  be  forced 
down  by  a  lever  or  by  an  extra  load  applied  for  that  purpose.  In 
c;ise  the  resistance  to  sinking  is  very  great,  the  material  may  be  re- 
moved from  the  inside  by  a  sand-pump  (§  448),  or  a  Milroy  or  clam- 
shell dredge  (§  412)  ;  but  ordinarily  no  earth  is  removed  from  the 
inside.  Cylinders  have  been  sunk  by  this  method  5  or  6  feet  by  a 
single  exhaustion,  and  34  feet  in  6  hours. 

The  vacuum  process  has  been  superseded  by  the  plenum  process. 

426.  Plenum,  or  Compressed-air,  Process.  The  plenum,  or 
compressed-air,  process  consists  in  pumping  air  into  the  air-chamber, 
so  as  to  exclude  the  water,  and  forcing  the  pile  or  caisson  down  by 
a  load  placed  upon  it.  An  air-lock  (§  4.31)  is  so  arranged  that  the 
workmen  can  pass  into  the  caisson  to  remove  the  soil,  logs,  and 
bowlders,  and  to  watch  tlie  j^rogress  of  the  sinking,  without  re- 
leasing the  pressure.  The  vacuum  process  is  applicable  only  in  mud 
or  sand;  but  the  compressed-air  process  can  be  applied  in  all  kinds 
of  soil. 

427.  History  of  Pneumatic  Processes.  It  is  said  that  Papin, 
the  eminent  physicist — born  at  Blois  in  1647, — conceived  the  idea 
of  employing  a  continued  supply  of  compressed  air  to  enable  work- 
men to  build  under  a  large  diving-bell.  In  1779,  Coulomb  pre- 
sented to  the  Paris  Academy  of  Science  a  paper  detailing  a  plan  for 
executing  all  sorts  of  operations  under  water  by  the  use  of  com- 
pressed air.  His  proposed  apparatus  was  somewhat  like  that  now 
in  general  use. 

In  England  in  1831,  Earl  Dundonald,  then  Lord  Cochrane,  took 
out  a  patent  for  a  device  for  sinking  tubular  shafts  through  earth 
and  water,  by  means  of  compressed  air.  His  air-lock  was  much  like 
modern  ones,  and  was  to  be  placed  at  the  top  of  the  main  shaft. 
His  invention  was  made  with  a  view  to  its  use  in  tunneling  under 
the  Thames,  and  in  similar  enterprises.  In  1841,  Bush  also  took 
out  a  patent  in  England  for  a  plan  of  sinking  foundations  by  the 


280  FOUXDATIOXS    UNDER   WATER  [CHAP.   XII. 

aid  of  compressed  air.  A  German,  by  name  G.  Pfaun  Muller,  made 
a  somewhat  similar  design  for  a  bridge  at  Mayence,  in  1850  ;  but  as 
his  plan  was  not  executed,  it  was,  like  the  patents  of  Cochrane  and 
Bush,  little  known  till  legal  controversies  in  regard  to  patent-rights 
dragged  them  from  obscurity. 

428.  The  first  practical  application  of  the  plenum  process  was 
made  in  France  in  1841  by  M.  Triger.  In  order  to  reach  a  vein  of 
coal  on  a  sandy  island  in  the  Loire,  opposite  to  Chalons,  he  sunk 
an  iron  tube  about  40  inches  in  diameter,  some  60  feet,  by  the 
blows  of  heavy  weights.  The  fine  sand  was  removed  from  the  in- 
terior by  means  of  a  scoop  bucket.  On  reaching  a  layer  of  coarse 
gravel,  he  could  not  force  the  tube  through.  He  therefore  capped 
his  tube  with  an  air-lock,  and  by  compressed  air  forced  out  the 
water  which  had  all  the  while  filled  the  tube,  and  sent  workmen  to 
the  bottom.  The  pressure  he  used  was  never  greater  than  two  at- 
mospheres. The  water  was  discharged  through  a  small  tube,  into 
which,  several  feet  from  the  bottom,  a  jet  of  air  was  allowed  to 
enter,  thus  diminishing  the  specific  gravity  of  the  column  till  it 
was  rapidly  blown  out.  In  1845,  Triger  read  a  paper  on  the  sinking 
of  a  tube  about  6  feet  in  diameter  to  a  depth  of  82  feet  by  the  same 
method,  and  suggested  the  use  of  it  for  the  construction  of  deep 
foundations  for  bridges. 

Dr.  Potts,  of  England,  generally  has  the  credit  of  inventing  the 
vacuum  process,  for  which  he  took  out  a  patent  in  1848.  Many 
times  in  sinking  foundations  by  the  vacuum  process,  the  com- 
pressed-air process  was  resorted  to  so  that  men  could  enter  the  pile 
to  remove  obstructions  ;  and  finally  the  many  advantages  of  the 
compressed-air  process  caused  it  to  entirely  supersede  the  vacuum 
process.  At  present  the  term  "  pneumatic  process "  is  practically 
synonymous  with  compressed-air  process. 

429.  The  first  foundations  sunk  entirely  by  the  compressed-air 
process  were  the  pneumatic  piles  for  the  bridge  at  Eochester,  Eng- 
land, put  down  in  1851.     The  depth  reached  was  61  feet. 

The  first  pneumatic  caisson  W' as  employed  at  Kehl,  on  the  east- 
ern border  of  France,  for  the  foundations  of  a  railroad  bridge  across 
the  Rhine. 

430.  The  first  three  pneumatic  pile  foundations  in  America 
were  constructed  in  South  Carolina  between  1856  and  1860.  Im- 
mediately after  the  civil  war,  a  number  of  pneumatic  piles  were 


ART.  4.]  PNEUMATIC    PROCESS.  281 

sunk  in  western  rivers  for  bridge  piers.  The  first  pneumatic  cais- 
sons in  America  were  those  for  the  St.  Louis  bridge  (§  457),  put 
down  in  1870.  At  that  time  these  were  the  largest  caissons  ever 
constructed,  and  the  depth  reached — 109  feet  8^  inches — has  not 
yet  been  exceeded. 

Of  late  years,  the  pneumatic  caisson  has  almost  entirely  super- 
seded the  pneumatic  pile  ;  in  fact  the  plenum-pneumatic  caisson 
has  almost  entirely  superseded,  except  in  very  shallow  water  or  in 
water  over  about  80  or  100  ft.  deep,  all  other  methods  of  founding 
bridge  piers. 

431.  Pneumatic  Piles.  Although  pneumatic  cylinders  are  now 
rarely  employed,  they  will  be  briefly  described  because  of  their 
historic  interest. 

The  cylinders  are  made  of  either  wrought  or  cast  iron.  The 
wrought-iron  cylinders  are  composed  of  plates,  about  half  an  inch 
thick,  riveted  together  and  strengthened  by  angle  iroos  on  the  m- 
side,  and  re-inforced  at  the  cuttting  edge  by  j)lates  on  the  outsida 
both  to  increase  the  stiffness  and  to  make  the  hole  a  little  larger  so 
as  to  diminish  friction.  The  cast-iron  cylinders  are  composed  of 
sections,  from  6  to  10  feet  long  and  3  to  8  feet  in  diameter,  bolted 
together  by  inside  flanges,  the  lower  section  being  cast  with  a  sharp 
edge  to  facilitate  penetration.  Two  of  these  tubes,  braced  together, 
are  employed  for  ordinary  bridge  piers  ;  and  six  small  ones  around 
a  large  one  for  a  pivot  pier.  They  are  filled  with  concrete,  with  a 
few  courses  of  masonry  or  a  heavy  iron  cap  at  the  toja. 

Fig.  63  shows  the  arrangement  of  the  essential  parts  of  a  pneu- 
matic pile.  The  apparatus  as  shown  is  arranged  for  sinking  by  the 
plenum  process  ;  for  the  vacuum  process  the  arrangement  differs 
only  in  a  few  obvious  particulars.  The  upper  section  constitutes 
the  air-lock.  The  doors  a  and  h  both  open  downwards.  To  enter 
the  cylinder,  the  workmen  pass  into  the  air-lock,  and  close  the 
door  a.  Opening  the  cock  d  allows  the  comjiressed  air  to  enter  the 
lock  ;  and  when  the  pressure  is  equal  on  both  sides,  the  door  h  is 
opened  and  the  workmen  pass  down  the  cylinder  by  means  of  a  ladder. 
To  save  loss  of  air,  the  air-lock  should  be  opened  very  seldom,  or 
made  very  small  if  required  to  be  opened  often. 

The  air-supply  pipe  connects  with  a  reservoir  of  compressed  air 
on  a  barge.  If  the  air  were  pumped  directly  into  the  pile  without, 
the  intervention  of  a  storage  reservoir,  as  was  done  in  the  early  ap- 


382 


FOUNDATION'S   UNDER  WATEE. 


[chap.  XII. 


plications  of  the  plenum  process,  even  a  momentary  stoppage  of  the 
<!ngine  would  endanger  the  lives  of  the  workmen. 

432.  The  soil  may  be  excavated  by  ordinary  hand  tools,  elevated 
to  the  air-lock  by  a  windlass  and  bucket,  and  passed  out  through 
the  main  air-lock.  Sometimes  a  double  air-lock  with  one  large  and 
one  small  compartment  is  used,  the  former  being  opened  only  to  let 
gangs  of  workmen  pass  and  the  latter  to  allow  the  passage  of  the 


Fig.  63.— Pneumatic  Pile. 


skip,  or  bucket,  containing  the  excavated  material.  Sometimes  an 
auxiliary  lock,  g  f,  is  employed.  The  doors  /  and  g  are  so  con- 
nected by  parallel  bars  (not  shown)  that  only  one  can  be.  opened  at 
a  time.  The  excavated  material  is  thrown  into  the  chute,  the 
door/  is  closed,  which  opens  g,  and  the  material  discharges  itself 
on  the  outside. 

Mud  and  sand  are  blown  out  with  the  sand-lift  (§  447)  or  sand- 
pump  (§  448)  without  the  use  of  any  air-lock. 

433.  The  cylinders  are  guided  in  their  descent  by  a  frame-work 
resting  upon  piles  or  upon  two  barges.  One  of  the  chief  difficulties  in 


ART.  4.]  PXEUMATIC    PKOCESS.  2SS 

sinking  pneumatic  piles  is  to  keep  them  vei^ical.  If  the  cylinder 
becomes  inclined,  it  can  generalh^  be  righted  ( L)  by  placing  wooden 
wedges  under  the  lower  side  of  the  cutting  edge,  or  (2)  by  excavat- 
ing under  the  upper  side  so  that  the  air  may  escape  and  loosen  the 
material  on  that  side,  or  (3)  by  drilling  holes  through  the  upper- 
most side  of  the  cylinder  through  which  air  may  escape  and  loosen 
the  soil,  or  (4)  by  straining  the  top  over  with  props  or  tackle.  If 
several  pneumatic  piles  are  to  form  a  pier,  they  should  bo  sunk  one 
at  a  time,  for  when  sunk  at  the  same  time  they  are  liable  to  run 
together. 

434.  Bearing  Power.  The  frictional  resistance  of  iron  cylinders 
has  been  discussed  in  §§  418-19,  page  275-77,  which  see. 

Mc Alpine,  in  sinking  the  piers  of  the  Harlem  bridge,  New  York 
City,  devised  a  very  valuable  but  simple  p— - 

and  cheap  method  of  increasing  the  bear- 
ing power  of  a  pneumatic  cylinder  (see 
Fig.  64).     He  attached  to  the  lower  end 
of  the  cylindrical  column  a  hollow  conical 
iron  section,  the  large  end  of  which  is 
much   larger   than    the   main   cylinder. 
The  base  of  the  pier  was  still  further  in-    V:--':'[':::^'^^:^y        /    '•'^■••^:. 
creased    by    driving    short    sheet    piles  ■A'---'-:-.;    ' ""'  .■.■•'■ 

obliquely  under  the  lower  edge  of  the     •.•••••••■••.•■  v         ••....  • 

conical  base  and  removing  the  soil  from  Fm.  64. 

under  them,  after  which  the  whole  was  filled  in  with  concrete.* 

In  cold  climates  the  contraction  of  the  iron  cylinder  upon  the 
masonry  filling  might  rupture  the  former;  hence,  it  is  sometimes 
recommended  to  fill  the  pile  below  the  frost  line  with  asphaltic  con- 
crete. It  has  also  been  proposed  to  line  the  cylinders  with  thick, 
soft  wood  staves,  which  will  compress  under  the  contraction  of  the 
iron  cylinder.  However,  the  danger  from  this  cause  is  not  very 
serious;  for,  after  the  concrete  has  set,  it  is  strong  enough  to 
support  the  load  if  the  iron  case  were  removed. 

435.  After  the  cylinder  has  reached  the  required  depth,  concrete 
enough  to  seal  it  is  laid  in  compressed  air;  and  when  this  has 
set,  the  remainder  can  be  laid  in  the  open  air.  A  short  distance 
at  the  top  is  usually  filled  with  good  masonry,  and  a  heavy  iron  cap 
put  over  all. 

*  Jour.  Frank.  Inst.,  vol.  Iv.  pp.  98  and  177. 


284  FOUXDATIOXS    UXDER   WATEK.  [CHAP.   XII. 

436.  Pneumatic  Caissons.  A  imcumatic  caisson  is  an  immense 
box — open  below,  bnt  air-tight  and  water-tight  elsewhere, — upon  the 
top  of  wliich  the  masonry  pier  is  built.  The  essential  difference 
between  the  pneumatic  pile  and  the  pneumatic  caisson  is  one  of  de- 
gree rather  than  one  of  quality.  Sometimes  the  caisson  envelops 
the  entire  masonry  of  the  pier  ;  but  in  the  usual  form  the  masonry 
envelo^DS  the  iron  cj'linder  and  rests  upon  an  enlargement  of  the 
lower  end  of  it.  The  pneumatic  pile  is  sunk  to  the  final  depth  be- 
fore being  filled  with  concrete  or  masonry;  but  with  the  caisson 
the  masonry  is  built  upward  while  the  whole  pier  is  being  sunk 
downward,  the  masonry  thus  forming  the  load  which  forces  the 
caisson  into  the  soil.  A  pneumatic  caisson  is,  practically,  a  gigantic 
diving  bell  upon  the  top  of  which  the  masonry  of  the  pier  rests. 

Fig.  65  is  a  section  of  a  pier  of  the  bridge  across  the  Missouri 
Kiver  near  Blair,  Neb.,*  and  shows  the  general  arrangement  of  the 
pier  and  pneumatic  caisson.  The  tube  extending  through  the  mid- 
dle of  the  caisson  and  pier,  known  as  the  air-shaft,  is  for  the  ascent 
and  descent  of  the  men.  The  air-lock — situated  at  the  Junction  of 
the  two  cylinders  which  form  the  air-shaft — consists  of  a  short  sec- 
tion of  a  large  cylinder  which  enveloi^s  the  ends  of  the  two  sections. 
of  the  air-shaft,  both  of  wliich  communicate  with  the  air-lock  by 
doors  as  shown  in  Fig.  65.  The  apartment  in  which  the  men  are 
at  work  is  known  as  the  working  diamhcr  or  air-cliamher.  The 
small  cylinders  shown  on  each  side  of  the  air-shaft  are  employed  in 
supplying  concrete  for  filling  the  working  chamber  when  the  sinking 
is  completed.  The  pipes  seen  in  the  air-chamber  and  projecting 
above  the  masonry  are  employed  in  discharging  the  mud  and  sand, 
as  will  be  described  presently.  The  timbers  which  appear  in  the 
lower  central  portion  of  the  working  chamber  are  parts  of  the  trusses 
which  support  the  central  portions  of  the  roof  of  the  caisson. 

The  masonry  is  usually  begun  about  2  feet  below  low  water,  the 
space  intermediate  between  the  masonry  and  the  roof  of  the  working 
chamber  being  occupied  by  timber  crib-work,  either  built  solid  or 
filled  with  concrete.  In  Fig.  65  the  masonry  rests  directly  upon 
the  roof  of  the  air-chamber,  which  construction  was  adopted  for  the 
channel  piers  of  this  bridge  to  reduce  to  a  minimum  the  obstruction 
to  the  flow  of  the  water. 

Frequently  a  coffer-dam  is  built  upon  the  top  of  the  crib  (see 

*  From  the  report  of  Geo.  S.  Morison,  chief  engineer  of  the  bridge. 


AKT.   4.  J 


PXEUMATIC    PROCESS. 


•ZSo 


Plate  I);  but  in  this  particular  case  the  masonry  was  kept  above  the 
surface  of  the  water,  hence  no  coffer-dam  was  employed.     When 


Fia.  65.— Pneumatic  Caisson.— Blair  Bridge. 

the  coffer-dam  is  not  used,  it  Is  necessary  to  regulate  the  rate  of 
sinking  by  the  speed  with  whic'.:  the  masonry  can  be  built,  which  is 
liable  to  cause  inconvenience  and  delay.     When  the  coffer-dam  is 


'^86  FOUNDATIONS   UNDER   WATER.  [CHAP.   XII. 


dispensed  witli,  it  is  necessary  to  go  on  with  the  constrnctiou  of  the 
masonry  whether  or  not  the  additional  weight  is  needed  in  sinking 
tne  caisson. 

437.  The  details  of  the  construction  of  pneumatic  caissons  can 
be  explained  best  by  the  description  of  a  particular  case. 

438.  Foundation  of  the  Havre  de  Grace  Bridge.  Foldin£ 
Plate  I  *  shows  the  details  of  the  construction  of  the  caisson,  crib, 
and  cofier-dam  employed  in  1884  in  sinking  pier  Xo.  8  of  the 
Baltimore  and  Ohio  R.  R.  bridge  across  the  Susquehanna  River  at 
Havre  de  Grace,  Md.  The  timber  work  of  Fig.  06  (page  293)  also 
shows  some  of  the  details  of  the  construction  of  the  walls  of  the 
working  chamber. 

439.  The  Caisson.  The  details  of  the  construction  of  the  caisson 
areas  follows:  Six  courses  of  timber,  12  X  12  inch,  one  lying  on  top 
of  the  other,  formed  the  skeleton  of  the  walls  of  the  working  cham- 
ber. These  timbers  were  first  put  up  with  a  batter  of  f  of  an  inch 
horizontal  to  1  foot  vertical;  they  were  not  halved  at  the  corners, 
but  every  alternate  piece  was  carried  through  with  a  full  section, 
''  log-house"  fashion.  These  timbers  were  fastened  at  the  corners, 
intersections,  and  several  intermediate  points,  with  drift-bolts  (§  381) 
1  inch  square  and  22  inches  long.  Inside  of  this  timber  shell,  threa 
courses  of  3-inch  plank,  placed  diagonally,  were  spiked  to  the  hori- 
zontal timbers  and  to  each  other  by  G-inch  and  7-inch  boat-spikes. 
Inside  of  the  diagonal  planking  was  another  course  of  3-inch  plank 
placed  vertically  and  Avell  spiked,  the  head  of  each  spike  being 
wrapped  with  oakum  to  prevent  leakage.  The  vertical  seams  were 
thoroughly  calked. 

A  strong  and  thoroughly  braced  truss  (see  also  Fig.  66,  page  293) 
was  nest  erected  longitudinally  through  the  center  of  the  working 
chamber.  The  first  course  in  the  deck  of  the  working  chamber  was 
then  placed  in  position  on  the  central  truss  and  side  walls.  The  work- 
ing chamber  was  9  feet  3  inches  high  from  bottom  of  shoe  to  the 
underside  of  deck,  which  was  higher  than  required  for  working,  but 
was  adopted  so  as  to  permit  greater  depth  of  the  central  truss.  Out- 
side of  the  horizontal  timbers,  after  they  had  been  adzed  to  a  true 
surface,  were  then  placed  the  12-  by  14-inch  sticks  (shown  at  the  ex- 

*  Compiled  from  the  original  working  drawings.  The  accompanying  description 
is  from  personal  inspection  aided  by  an  article  in  Engineering  JVews  by  Col.  Wm.  M 
Patton,  engineer  in  charge. 


AKT.   -i.]  PNEUMATIC    PKOCESS.  28^ 

treme  left  of  Fig.  66)  15  feet  long,  extending  2  feet  below  the  bottom 
horizontal  timber  and  having  their  lower  ends  beveled  as  shown. 
These  timbers  extended  6  feet  above  the  horizontal  members, 
and  were  shouldered  at  the  upper  end  so  that  three  of  the  deck 
courses  rested  upon  them.  "Four  screw-bolts  were  passed  through 
each  outside  post  and  through  the  entire  wall;  and,  in  addition  tc 
these,  two  drift-bolts,  1  inch  square  and  30  inches  long,  in  each  ver- 
tical served  to  more  thoroughly  bind  the  wall  together.  This  com- 
pound of  timber  and  planking  formed  the  walls  of  the  working 
chamber.  After  the  first  deck  course  was  in  place,  a  few  jiieces  of 
the  second  course  were  laid  diagonally  to  give  it  stiffness;  the  under- 
side of  this  deck  or  roof  was  then  lined  with  planks  and  thoroughly 
calked,  and  a  false  bottom  put  into  the  w^orking  chamber  prepara- 
tory to  launching  it. 

After  the  caisson  was  launched  the  deck  courses,  eight  in  all, 
were  put  on.  The  first  course  was  made  of  single-length  timbers, 
reaching  from  inside  to  inside  of  the  vertical  wall  posts,  and  resting 
on  top  of  the  horizontal  timbers  and  inside  planking  and  also  on  the 
top  chord  of  the  central  truss,  and  being  fastened  to  these  members 
by  22-inch  drift-bolts.  The  second  course  was  laid  diagonally  and 
was  made  of  varying  lengths  of  timbers.  The  third  course  was  laid 
from  side  to  side  across  the  caisson,  and  the  fourth  course  longi- 
tudinally and  resting  on  the  shoulders  of  the  12  X  14  inch  verticals. 
The  fifth  course  was  laid  across,  the  sixth  diagonally — crossing  the 
second  course, — and  the  seventh  and  eighth  courses  extended  to  the 
extreme  ovitside  limits  of  the  caisson  and  rested  on  the  heads  of  the 
vertical  posts.  This  general  arrangement  of  the  top  courses,  resting 
as  they  did  on  the  heads  and  shoulders  of  the  outside  verticals,  gave 
a  direct  bearing  on  the  posts  and  relieved  the  wall  bolts  of  the  great 
shearing  strain  to  which  they  would  otherwise  have  been  subjected. 

The  outside  posts  were  bolted  to  the  deck  courses  by  one  3-foot 
screw-bolt  and  two  30-inch  drift-bolts,  fastening  them  to  the  longi- 
tudinal and  diagonal  courses  respectively.  The  several  deck  courses 
were  bolted  to  each  other  by  22-inch  drift-bolts  (not  shown  in  the 
illustrations),  spaced  5  feet  apart  along  each  stick.  All  the  timbers 
in  the  deck  were  bedded  in  cement  mortar  and  the  vertical  Joints 
were  grouted,  so  as  to  give  a  full  and  uniform  bearing  for  each  stick 
and  also  decrease  the  leakage  and  danger  from  fire. 

The  center  truss  (see  also  Fig.  66)  was  constructed  to  bear  a  uni- 


§88  FOUNDATIONS   UNDER   -WATER.  [CHAP.  XII. 


formly  distributed  load,  or  to  act  as  a  cantilever.  It  was  composed 
of  a  top  and  bottom  chord,  each  made  of  two  12  X  12  inch  sticks, 
with  posts  and  diagonals  of  wood,  and  vertical  and  diagonal  tie-rods 
1.J  inch  m  diameter;  the  iron  vertical  rods  extended  through  the 
first  deck  courses,  and  the  top  chord  was  also  bolted  to  the  deck 
with  drift-bolts.  The  object  of  this  was  to  enable  the  truss  to  act 
as  a  stiffening  rib  to  the  deck,  independently  of  its  action  as  a 
girder.  The  bottom  chord  was  also  extended  to  the  ends,  and  by 
means  of  straps  and  bolts  acted  both  as  a  strut  and  tie-brace  for  the 
ends  of  the  caisson,  and  constituted  the  only  end  bracing. 

The  sides  of  the  caissons  were  braced  against  outside  pressures  by 
IG  X  10  inch  timbers  abutting  against  the  walls  and  bottom  chord  of 
the  center  truss,  and  against  pressure  from  the  inside  by  2-inch  iron 
tie-rods  extending  from  out  to  out  of  the  caisson,  none  of  which  are 
shown.  All  the  timber  used,  except  the  planking  and  outside  posts 
and  the  bracing  in  the  working  chamber,  was  12  X 12  inch.  Iron 
straps,  extending  6  feet  on  the  sides  and  ends,  were  placed  at  the 
corners  and  bolted  to  the  caisson  timbers.  These  straps  were  made 
of  bar-iron  3x1  inch  and  prevented  spreading  of  the  walls  of  the 
caisson  under  excessive  pressure  within.  Planks  were  spiked  to  the 
lower  part  of  the  posts  ;  and  also  a  narrow  plank,  called  a  shoe,  was 
spiked  under  the  bottom  of  the  posts  (see  Fig.  66). 

440.  "The  construction  was  simple  and  strong  ;  in  no  case  was 
there  any  bending  or  springing  of  the  w^alls.  The  arrangement  of 
the  cutting  edge  with  square  shoulders  was  a  departure  from  the 
ordinary  V-shape  (compare  Figs.  65  and  66,  pages  285  and  293), 
and  was  found  to  possess  many  advantages.  It  enabled  the  men  to 
better  regulate  the  sinking  of  the  caisson  by  giving  an  increased  bear- 
ing surface.  With  this  support,  the  material  could  be  cleaned  out 
from  under  one  side  or  end  ;  the  caisson  could  be  leveled  ;  and,  if 
the  material  was  softer  in  one  spot  than  another,  the  caisson  could 
be  prevented  from  tipping.  It  further  afforded  a  good  surface  for 
blocking  up  when  it  was  found  desirable  to  support  the  caisson 
during  the  removal  of  the  material  ;  and  it  gave  also  greater  security 
in  case  of  a  '  blow-out '  or  the  failure  of  air-pressure. "  * 

When  it  is  anticipated  that  gravel  or  bowlders  will  be  met  with 
in  sinking,  the  cutting  edge  is  usually  shod  with  iron.     The  iron 
cutting  edge  was  omitted  in  all  the  caissons  for  this  bridge,  and  it  is 
*  Col.  Wm.  M.  Patton,  engineer  in  charge  for  the  railroad  company. 


ART.   -i.]  PNEUMATIC    PROCESS.  289 

claimed  that  the  experience  here  shows  that  ''in  no  case  is  an  iron 
shoe  either  advantageous  or  necessary." 

441.  The  Crib.  The  construction  of  the  crib  is  shown  very  fully 
in  Plate  I.  The  timbers  were  all  12  X  12  inches  square,  bolted  to 
each  other  by  22-inch  drift-bolts — spaced  5  or  6  feet  apart, — and 
were  dovetailed  at  the  corners  aud  connections.  The  parts  of  all 
the  walls  of  the  crib  were  firmly  bolted  to  the  deck  of  the  caisson. 

Ordinarily  the  division  walls  of  the  crib  are  built  vertically  from 
top  to  bottom  ;  but  in  this  case,  they  were  off-set,  as  shown,  to 
secure  a  better  bond  in  the  mass  of  concrete.  If  the  walls  are  built 
solid  from  top  to  bottom,  the  concrete  filling  is  thereby  divided  into 
a  number  of  separate  monolithic  columns  ;  but  in  the  construction 
as  above,  the  concrete  forms  practically  a  single  solid  mass.  The 
walls  are  built  solid,  owing  to  the  difficulty  of  getting  the  concrete 
thoroughly  packed  in  around  so  many  timbers.  Large  stones,  such 
as  could  be  handled  by  one  man,  were  bedded  in  mortar  as  the  suc- 
cessive layers  of  concrete  were  formed,  and  over  and  around  these 
another  layer  of  concrete  was  rammed.  In  most  localities  there  is 
but  little  difference  in  cost  between  a  solid  timber  crib  and  one  with 
timber  pockets  filled  with  concrete. 

442.  The  Coffer-dam.  Uprights  were  first  placed  at  intervals  of 
about  D^  feet,  and  connected  by  mortise  and  tenon  to  caps  and 
sills.  This  frame-work  was  held  down  to  the  crib  by  rods  2  inches 
in  diameter,  having  hooks  at  the  lower  end  which  passed  into  eye- 
bolts  in  the  sides  of  the  crib.  On  the  sides  of  the  dam,  the  upper 
end  of  these  rods  passed  through  12  X  12  inch  timbers  resting  on 
the  sides  of  the  dam  and  projecting  about  2  feet  outside  ;  and  at  the 
ends  of  the  dam,  they  passed  through  short  pieces  bolted  to  one  of 
the  cross  timbers  and  projecting  beyond  the  end  of  the  dam. 

Owing  to  the  great  depth  required,  the  coffer-dam  was  built  in 
sections,  the  connecting  rods  being  made  in  sections  with  swivel 
connections.  The  second  section  was  not  added  until  the  depth 
sunk  required  it.  When  the  top  section  of  the  dam  was  put  on, 
the  projecting  ends  of  the  timbers  through  which  the  connecting 
rods  passed  were  sawed  off.  The  bottom  section  was  sheeted  with 
three  courses  of  3-inch  plank,  and  the  top  section  with  two  thick- 
nesses. The  joint  between  the  coffer-dam  and  the  crib,  and  also 
'he  sheeting,  were  well  calked. 

The  sides  of  the  coffer-dam  were  braced  against  the  pressure  of 


290 


FOUNDATIONS   UNDER  WATER. 


[chap.  XII. 


the  water,  by  13  X  12  inch  timbers  resting  on  the  top  of  each  sec- 
tion, and  by  a  system  of  bracing  in  the  middle  of  each  section. 
"When  the  masonry  was  completed,  the  coffer-dam  was  removed  by 
disconnecting  the  vertical  rods. 

443.  Machinery  Barge.  The  machinery  barge  was  an  ordinary 
flat-boat  fitted  up  for  the  purpose.  At  one  end  of  the  barge  there 
were  three  boilers  each  of  fifty  horse-power.  In  the  middle  were 
two  large  air-compressors,  designed  by  the  contracting  engineer. 
Gen.  Wm.  Sooy  Smith.  One  furnished  all  the  compressed  air  re- 
quired, the  other  being  ready  for  use  in  case  of  any  accident  or 
break-down.  At  the  other  end  of  the  boat  were  two  Worth  iugton 
steam  pumps  to  furnish  water  for  the  excavating  plant  used  in  the 
caisson.  There  were  also  a  small  engine  and  a  dynamo  which  fur- 
nished the  current  for  the  electric  lamps  used  in  the  caisson  and,  at 
night,  on  the  outside. 

444.  Material  Required.  Table  31  gives  the  dimensions  and 
quantities  of  materials  in  the  pneumatic  foundations  of  this  bridge, 
and  Table  32  (page  302)  gives  the  cost. 

TABLE  31. 

Dimensions  and  Quantities  of  Materials  in  Foundations  of 

Havre  de  Grace  Bridge.* 


Number  of  the  Pier. 

Description. 

n. 

m. 

IV. 

vm. 

IX. 

Dimensions: 

Caissons:— length  at  bottom,  in  feet 

widtii    "        "         "     " 

height  from  cutting  edge,  in  feet. . 

height  of  working  chamber,  in  feet 
Crib: — length,  in  feet 

63.3 
25.9 
17.2 
9.2 
61.5 
24.2 
40.0 

203.473 

179,939 

2,068 

330 

1,649 

000 

11,313 

34,181 

4,638 

2,472 

67.3 
25.9 
17.2 
9.2 
61.5 
24.2 
42.0 

215,565 

197.910 

31,517 

401 

1,893 

623 

15,651 

36,832 

700 

2,572 

79.4 
32.8 
17.2 
9.2 
77.3 
31.1 
22.2 

316,689 

143.993 

108,518 

631 

1,635 

126 

32,881 

40.909 

11.730 

3,392 

70.9 
32.6 
17.2 
9.2 
69.1 
30.8 
41.0 

281,540 

219,680 

85,759 

559 

2,581 

526 

31.026 

44,861 

10.0:M 

3,235 

78.2 
42.3 
19.3 
9.2 
76.4 
40.5 

32.8 

Quantities- 

Timber  in  the  caisson,    feet,  board  measure. 

"       "    "    crib, 

"       "    "    coffer-dam."        " 
Concrete  in  working  chamber,  cubic  yards. . . 

"         "  crib,  shafts,  etc.,           "        '"     ... 

"        below  cutting  edge,            "        "     ... 

465.125 

203,824 

126,532 

839 

3,172 

624 

33,435 

59,245 

11,237 

3,535 

•  The  data  by  courtesy  of  Sooysmith  &  Co.,  contractors  for  the  pneumatic  foundations. 

445.  Position  of  the  Air-lock.    Before  the  construction  of 
the  St.  Louis  bridge,  the  air-lock  had  always  been  placed  at  the  top 


ART.   4.]  PNEUMATIC    PROCESS.  291 

of  the  air-shaft,  and  was  of  such  construction  that  to  lengthen  the 
shaft,  as  the  caisson  sunk,  it  was  necessary  to  detacli  the  lock,  add 
a  section  to  the  shaft,  and  then  replace  the  lock  on  top.  This  was 
not  only  inconvenient  and  an  interruption  to  the  other  work,  but 
required  the  men  to  climb  the  entire  distance  under  compressed 
air,  which  is  exceedingly  fatiguing  (see  §  460).  To  overcome  these 
objections,  Eads  placed  the  air-lock  at  the  bottom  of  the  shaft. 
This  position  is  objectionable,  since  in  case  of  a  "  blow-out,"  i.  e., 
a  rapid  leakage  of  air, — not  an  unfrequent  occurrence, — the  men 
may  not  be  able  to  get  into  the  lock  in  time  to  escape  drowning.  If 
the  lock  is  at  the  top,  they  can  get  out  of  the  way  of  the  water  by 
climbing  up  in  the  shaft. 

At  the  Havre  de  Grace  bridge,  the  air-shaft  was  constructed  of 
wrought  iron,  in  sections  15  feet  long.  The  air-lock  was  made  by 
placing  diaphragms  on  the  inside  flanges  of  the  opposite  ends  of  the 
top  section.  A  new  section  and  a  third  diaphragm  could  be  added 
without  disturbing  the  air-lock ;  and  when  the  third  diajihragm 
was  in  place,  the  lower  one  was  removed  preparatory  to  using  it 
again.  Some  engineers  compromise  between  these  two  positions, 
and  leave  the  air-lock  permanently  at  some  intermediate  point  in 
the  pier  (see  Fig.  65,  page  285). 

446.  Excavators.  In  the  early  application  of  the  pneumatic 
method,  the  material  was  excavated  with  shovel  and  pick,  elevated 
in  buckets  or  bags  by  a  windlass,  and  stored  in  the  air-lock.  When 
the  air-lock  was  full,  the  lower  door  was  closed,  and  the  air  in  the 
lock  was  allowed  to  escape  until  the  upper  door  could  be  opened, 
and  then  the  material  was  thrown  out.  This  method  was  expensive 
and  slow. 

In  the  first  application  of  the  pneumatic  process  in  America 
(§  430),  Gen.  Wm.  Sooy  Smith  invented  the  auxiliar}'  air-lock,  ^ /, 
Fig.  63  (page  282),  through  which  to  let  out  the  excavated  mate- 
rial. The  doors,  /'  and  g,  are  so  connected  together  that  only  one 
of  them  can  be  opened  at  a  time.  The  excavated  material  being 
thrown  into  the  chute,  the  closing  of  the  door  /  opens  g,  and  the 
material  slides  out.  This  simple  device  is  said  to  have  increased 
threefold  the  amount  of  work  that  could  be  done. 

447.  Sand-lift.  This  is  a  device,  first  used  by  Gen.  Wm.  Sooy 
Smith,  for  forcing  the  sand  and  mud  out  of  the  caisson  by  means 
of  the  pressure  in  the  working  chamber.     It  consists  of  a  pipe. 


292  FOUNDATIONS    UNDER   WATER.  [CHAP.  XII. 

reaching  from  the  working  chamber  to  the  surface  (see  Fig.  63  and 
Plate  I),  controlled  by  a  valve  in  the  working  chamber.  The  sand 
is  heaped  up  around  the  lower  end  of  the  pipe,  the  valve  opened, 
and  the  pressure  forces  a  continuous  stream  of  air,  sand,  and  water 
up  and  out.     For  another  application  of  this  principle,  see  §  413. 

In  sand,  this  method  of  excavating  is  very  efficient,  being  eight 
to  ten  times  as  expeditious  as  the  auxiliary  air-lock.  Of  course, 
the  efficiency  varies  with  the  depth,  i.  e.,  with  the  pressure.  When 
the  soil  is  so  impervious  that  the  water  in  the  working  chamber  can 
not  be  forced  out  under  the  edge  of  the  caisson,  it  is  made  to  pass 
through  the  sand-lift  pipe. 

The  "goose-neck,^'  or  elbow  at  the  top  of  the  discharge  pipe,  is 
worn  away  very  rapidly  by  the  impact  of  the  ascending  sand  and 
pebbles.  At  the  Havre  de  Grace  bridge,  it  was  of  chilled  iron  4 
inches  thick  on  the  convex  side  of  the  curve,  and  even  then  lasted 
only  two  days.  At  the  Brooklyn  bridge,  the  discharge  pipe  ter- 
minated with  a  straight  top,  and  the  sand  was  discharged  against  a 
block  of  granite  placed  in  an  inclined  position  over  the  upper  end. 

Although  the  sand-lift  is  efficient,  there  are  some  objections  to 
it :  (1)  forcing  the  sand  out  by  the  pressure  in  the  cylinder  de- 
creases the  pressure,  which  causes,  particularly  in  pneumatic  piles  or 
small  caissons,  the  formation  of  vapors  so  thick  as  to  prevent  the 
workmen  from  seeing ;  (2)  the  diminished  pressure  allows  the 
water  to  flow  in  under  the  cutting  edge  ;  and  (3)  if  there  is  much 
leakage,  the  air-compressors  are  unable  to  supply  the  air  fast 
enough. 

448.  Mud-pump.  During  the  construction  of  the  St.  Louis 
bridge,  Capt.  Eads  invented  a  mud-pump,  which  is  free  from  the 
above  objections  to  the  sand-lift,  and  which  in  mud  or  silt  is  more 
efficient  than  it.  This  device  is  generally  called  a  sand-pump,  but 
is  more  jiroperly  a  mud-pump. 

The  principle  involved  in  the  Eads  pump  is  the  same  as  that 
employed  in  the  atomizer,  the  inspirator,  and  the  injector;  viz.,  the 
principle  of  the  induced  current.  This  principle  is  utilized  by  dis- 
charging a  stream  of  water  with  a  high  velocity  on  the  outside  of  a 
small  pipe,  which  produces  a  partial  vacuum  in  the  latter ;  when 
the  pressure  of  the  air  on  the  outside  forces  the  mud  through  the 
small  pipe  and  into  the  current  of  water  by  which  the  mud  is 
carried  away.     The  current  of  water  is  the  motive  power. 


ART.  4.] 


PNEUMATIC    PROCESS. 


293 


Fig.  6G  is  an  interior  view  of  the  caisson  of  the  Baltimore  and 
Ohio  R.  E.  bridge  at  Havre  de  Grace,  Md.,  and  shows  the  general 
aoTangement  of  the  pipes  and  mud-pump.     The  pump  itself  is  a 


294  FOUNDATIONS    UXDER   WATER.  [CHAP.  XII. 

hollow  pear-shaped  casting,  about  15  inches  in  diameter  and  15 
inches  long,  a  section  of  which  is  shown  in  the  corner  of  Fig.  66. 
The  water  is  forced  into  the  pump  at  a,  impinges  against  the  coni- 
cal casing,  d,  flows  around  this  lining  and  escapes  upwards  through 
a  narrow  annular  space,  /.  The  interior  casing  gives  the  water  an 
even  distribution  around  the  end  of  the  suction  pipe.  The  flow  of 
the  water  through  the  pump  can  be  regulated  by  screwing  the  suc- 
tion pipe  in  or  out,  thus  closing  or  opening  the  annular  space,  /. 
To  prevent  the  too  rapid  feeding  or  the  entrance  of  lumps,  which 
might  choke  the  pipe,  a  strainer — simply  a  short  piece  of  pipe, 
plugged  at  the  end,  having  a  series  of  ^-inch  to  |-inch  holes  bored 
in  it — was  put  on  the  bottom  of  the  suction  pipe.  The  discharge 
pipe  of  the  mud-pump  terminates  in  a  "  goose-neck  ^'  through 
which  the  material  is  discharged  horizontally. 

The  darkly  shaded  portions  of  the  section  of  the  pump  wear 
away  rapidly  ;  and  hence  they  are  made  of  the  hardest  steel  and 
conscructed  so  as  to  be  readily  removed.  Difllerent  engineers  have 
different  methods  of  providing  for  the  renewal  of  these  parts,  the 
outline  form  of  the  pump  varying  with  the  method  employed.  The 
pump  used  at  the  St.  Louis  bridge  was  cylindrical  in  outline,  but 
otherwise  essentially  the  same  as  the  above. 

449.  In  order  to  use  the  mud-pump,  the  material  to  be  exca- 
vated is  first  mixed  into  a  thin  paste  by  playing  upon  it  with  a  jet 
of  water.  This  pump  is  used  only  for  removing  mud,  silt,  and  soil 
containing  small  quantities  of  sand  ;  pure  sand  or  soil  containing 
large  quantities  of  sand  is  "  blown  out  •"  with  the  sand-lift. 

The  water  is  delivered  to  the  mud-pump  under  a  pressure,  ordi- 
narily, of  80  or  90  pounds  to  the  square  inch.  At  the  St.  Louis 
bridge,  it  was  found  that  a  mud-pump  of  3^-incli  bore  was  capable 
of  raising  20  cubic  yards  of  material  120  feet  per  hour,  the  water 
pressure  being  150  pounds  per  square  inch.* 

450.  Water-column.  A  combination  of  the  pneumatic  process 
and  that  of  dredging  in  the  open  air  through  tubes  has  been  em- 
ployed extensively  in  Europe.  It  seems  to  have  been  used  first  at 
the  bridge  across  the  Ehine  at  Kehl.  The  same  method  was  used 
at  the  Brooklyn  bridge.      The  principle  is  rudely  illustrated  in 

*  History  of  the  St.  Louis  Bridge,  p.  213. 


ART.  4.] 


PNEUMATIC    PROCESS. 


295 


J^tr 

Jiir 

Loci 

Lock 

k 

^ 

k 

-^ 

/     tTor^in^ 



Cfiamher     \ 

r- 

Fig.  67. 


Pig.  67.  The  central  shaft,  which  is  open  top  and  bottom,  projects 
a  little  below  the  cutting  edge, 
and  is  kept  full  of  water,  the 
greater  height  of  water  in  the 
column  balancing  the  pressure 
of  the  air  in  the  chamber.  The 
workmen  simply  push  the  mate- 
rial under  the  edge  of  a  water- 
shaft,  from  whence  it  is  exca- 
vated by  a  dredge  (§  412). 

451.  Blasting.  Bowlders  or 
points  of  rock  may  be  blasted  in 
compressed  air  without  any  ap- 
preciable danger  of  a  "blow- 
out "  or  of  injuring  the  ear- 
drums of  the  workmen.  This 
point  was  settled  in  sinking  the  foundations  of  the  Brooklyn  bridge  ; 
and  since  then  blasting  has  been  resorted  to  in  many  cases.  Bowl- 
ders are  sometimes  "carried  down,"  i.  e.,  allowed  to  remain  on  the 
surface  of  the  soil  in  the  working  chamber  as  the  excavation  pro- 
ceeds, and  subsequently  imbedded  in  the  concrete  with  which  the 
air-chamber  is  filled. 

452.  Rate  of  Sinking.  The  work  in  the  caisson  usually  con- 
tinues day  and  night,  winter  and  summer.  The  rate  of  progress 
varies,  of  course,  with  the  kind  of  soil,  and  particularly  with  the 
number  of  bowlders  encountered.  At  the  Havre  de  Grace  bridge, 
the  average  rate  of  progress  was  1.37  ft.  per  day;  at  Plattsmouth, 
2.22  ft.  ;  and  at  Blair,  1.75  ft.  per  day. 

453.  Guiding  the  Caisson.  Formerly  it  was  the  custom  to 
control  the  descent  of  the  caisson  by  suspension  screws  connected 
with  a  frame-work  resting  upon  piles  or  pontoons.  In  a  strong 
current  or  in  deep  water,  it  may  be  necessary  to  support  the  caisson 
partially  in  order  to  govern  its  descent ;  but  ordinarily  the  suspension 
is  needed  only  until  the  caisson  is  well  imbedded  in  the  soil.  The 
caisson  may  be  protected  from  the  current  by  constructing  a  break- 
water above  and  producing  dead  water  at  the  pier  site. 

After  the  soil  has  been  reached,  the  caisson  can  be  kept  in  its 
course  by  removing  the  soil  from  the  cutting  edge  on  one  side  or 
the  other  of  the  caisson.     In  case  the  caisson  does  not  settle  down 


296  FOUNDATIONS   UNDER   WATER.  [CHAP.  XII. 

after  the  soil  has  been  removed  from  under  the  cutting  edge,  a  re- 
duction of  a  few  pounds  in  the  air  pressure  in  the  working  chamber 
is  usually  sufficient  to  produce  the  desired  result.  At  the  Havre  de 
Grace  bridge,  it  was  found  that  by  allowing  the  discharged  mate- 
rial to  pile  up  against  the  outside  of  the  caisson,  the  latter  could 
bemoved  laterally  almost  at  will.  The  top  of  the  caisson  was  made 
3  feet  larger,  all  round,  than  the  lower  course  of  masonry,  to  allow 
for  deviation  in  sinking.  The  deviation  of  the  caisson,  which  was 
founded  90  feet  below  the  water,  was  less  than  18  inches,  even 
thougli  neither  suspension  screws  nor  guide  piles  were  employed. 

In  sinking  the  foundations  for  the  bridge  over  the  Missouri 
Kiver  near  Sibley,  Mo.,  it  was  necessary  to  move  the  caisson  con- 
siderably horizontally  without  sinking  it  much  farther.  This  was 
accomplished  by  placing  a  number  of  posts — 12  inches  square — 
in  an  inclined  position  between  the  roof  of  the  working  chamber 
and  a  temporary  timber  platform  resting  on  the  ground  below. 
When  these  posts  had  been  wedged  up  to  a  firm  bearing,  the 
air  pressure  was  released.  The  water  flowing  into  the  caisson 
loosened  the  soil  on  the  outside,  and  the  weight  of  the  caisson  com- 
ing on  the  inclined  posts  caused  them  to  rotate  about  their  lower 
ends,  which  forced  the  caisson  in  the  desired  direction.  In  this 
way,  a  lateral  movement  of  3  or  4  feet  was  secured  while  sinking 
about  the  same  distance. 

A  caisson  is  also  sometimes  moved  laterally,  while  sinking, 
by  attaching  a  cable  which  is  anchored  off  to  one  side  and  kept 
taut. 

454.  A  new  method  of  controlling  the  descent  of  the  caisson  has 
been  recently  introduced,  which  is  specially  valuable  in  swift  cur- 
rents or  in  rivers  subject  to  sudden  rises.  It  was  used  first  in  the 
construction  of  the  piers  for  a  bridge  across  the  Yazoo  River  near 
Vicksburg,  Miss.  A  group  of  72  piles,  each  40  feet  long,  was  driven 
into  the  river  bed,  and  sawed  off  under  the  water  ;  the  caisson  was 
then  floated  into  place,  and  lowered  until  the  heads  of  the  piles 
rested  against  the  roof  of  the  working  chamber.  As  the  work 
proceeded,  the  piles  were  sawed  off  to  allow  the  caisson  to  sink. 
One  of  the  reasons  for  employing  piles  in  this  case,  was  that,  if  the 
caisson  did  not  finally  rest  upon  bed-rock,  they  would  assist  in  sup- 
porting the  pier. 

That  such  ponderous  masses  can  be  so  certainly  guided  in  their 


ART.   4.  J  PNEUMATIC    PROCESS.  297 

descent  to  bed-rock,  is  not  the  least  valuable  nor  least  interesting 
fact  connected  with  this  method  of  sinking  foundations. 

455.  Fkictional  Resistance.  At  the  Havre  de  Grace  bridge, 
the  normal  frictional  resistance  on  the  timber  sides  of  the  pneumatic 
caisson  was  280  to  350  lbs.  per  sq.  ft.  for  depths  of  40  to  80  feet, 
the  soil  being  silt,  sand,  and  mud  ;  when  bowlders  were  encoun- 
tered, the  resistance  was  greater,  and  when  the  air  escaped  in  large 
quantities  the  resistance  was  less.  At  the  bridge  over  the  Missouri 
Eiver  near  Blair,  Neb.,  the  frictional  resistance  usually  ranged  be- 
tween 350  and  450  lbs.  per  sq.  ft.,  the  soil  being  mostly  fine  sand 
with  some  coarse  sand  and  gravel  and  a  little  clay.  At  the  Brook- 
lyn bridge  the  frictional  resistance  at  times  was  600  lbs.  per  sq.  ft. 
At  Cairo,  in  sand  and  gravel,  the  normal  friction  was  about  600  lbs. 
per  sq.  ft. 

For  data  on  the  friction  of  iron  cylinders  and  masonry  shafts, 
see  §§  418-19,  pages  275-77;  and  for  data  on  the  friction  of  ordi- 
nary piles,  see  §§  370-72,  pages  247-48. 

456.  Filling  the  Air-chamber.  When  the  caisson  has 
reached  the  required  depth,  the  bottom  is  leveled  off — by  blasting, 
if  necessary, — and  the  working  chamber  and  shafts  are  filled  with 
concrete.  Sometimes  only  enough  concrete  is  placed  in  the  bottom 
to  seal  the  chamber  water-tight,  and  the  remaining  space  is  filled 
with  sand.  This  was  done  at  the  east  abutment  of  the  St.  Louis 
bridge,  the  sand  being  pumped  in  from  the  river  with  the  sand- 
pump  previously  used  for  excavating  the  material  from  under  the 
caisson. 

457.  Noted  Examples.  The  St.  Louis  Bridge.  The  founda- 
tions of  the  steel-arch  bridge  over  the  Mississippi  at  St.  Louis  are 
the  deepest  ever  sunk  by  the  pneumatic  process,  and  at  the  time  of 
construction  (1870)  they  were  also  very  much  the  largest.  The 
caisson  of  the  east  abutment  was  an  irregular  hexagon  in  plan, 
83  X  70  feet  at  the  base,  and  64  X  48  feet  at  the  top— 14  feet  above 
the  cutting  edge.  The  working  chamber  was  9  feet  high.  The 
cutting  edge  finally  rested  on  the  solid  rock  94  feet  below  low 
water.  The  maximum  emersion  was  109  feet  8-|^  inches,  the  greatest 
depth  at  which  pneumatic  work  has  yet  been  done.  The  other 
caissons  were  almost  as  large  as  the  one  mentioned  above,  but  were 
not  sunk  as  deep. 

The  caissons  were  constructed  mainly  of  wood ;   but  the  side 


298  FOUNDATIONS    UNDEK   "WATER.  [CHAP.   XII. 

walls  and  the  roof  were  covered  with  plate  iron  to  prevent  leakage, 
and  strengthened  by  iron  girders  on  the  inside.  This  was  the  first 
pneumatic  caisson  constructed  in  America  ;  and  the  use  of  large 
quantities  of  timber  was  an  important  innovation,  and  has  become 
one  of  the  distinguishing  characteristics  of  American  practice.  In 
all  subsequent  experience  in  this  country  (except  as  mentioned  in 
§  458),  the  iron  lining  for  the  working  chamber  has  been  dispensed 
with.  The  masonry  rested  directly  upon  the  roof  of  the  caisson, 
i.  e.,  no  crib-work  was  employed.  In  sinking  the  first  pneumatic 
foundation  an  iron  coffer-dam  was  built  upon  the  top  of  the  caisson  ; 
but  the  last — the  largest  and  deepest — was  sunk  without  a  coffer- 
dam,— a  departure  from  ordinary  European  practice,  which  is  occa- 
sionally followed  in  this  country  (see  §  436). 

458.  The  Brooklyn  Bridge.  The  foundations  of  the  towers  of 
the  suspension  bridge  over  the  East  Eiver,  between  New  York  City 
and  Brooklyn,  are  the  largest  ever  sunk  by  the  pneumatic  process. 
The  foundation  of  the  New  York  tower,  which  was  a  little  larger 
and  deeper  than  the  other,  was  rectangular,  172  X  102  feet  at  the 
bottom  of  the  foundation,  and  157  X  77  feet  at  the  bottom  of  the 
masonry.  The  caisson  proper  was  31|^  feet  high,  the  roof  being  a 
solid  mass  of  timber  22  feet  thick.  The  working  chamber  was  9| 
feet  high.  The  bottom  of  the  foundation  is  78  feet  below  mean 
high  tide,  and  the  bottom  of  the  masonry  is  46J  feet  below  the 
same.  From  the  bottom  of  the  foundation  to  the  top  of  the 
balustrade  on  the  tower  is  354  feet,  the  top  of  the  tower  being  276 
feet  above  mean  high  tide. 

To  make  the  working  chamber  air-tight,  the  timbers  were  laid 
in  pitch  and  all  seams  calked  ;  and  in  addition,  the  sides  and  the 
roof  were  covered  with  plate  iron.  As  a  still  further  precaution, 
the  inside  of  the  air-chamber  was  coated  with  varnish  made  of  rosin, 
menhaden  oil,  and  Spanish  brown. 

For  additional  details  see  the  several  annual  reports  of  the  en- 
gineers in  charge,  and  also  numerous  articles  in  the  engineering 
newspapers  and  magazines  from  1869  to  1872. 

459.  Forth  Bridge.  For  an  illustrated  account  of  the  pneumatic 
foundation  work  of  the  bridge  across  the  Frith  of  Forth,  Eng- 
land, see  Engineering  Neivs,  vol.  xiii.  pages  242-43.  The  caissons 
employed  there  differed  fro-m  those  described  above  (1)  in  being 
made  almost  wholly  of  iron,  (2)  in  an  elaborate  system  of  cages  for 


ART.  4.]  PNEUMATIC    PROCESS.  299 

hoisting  the  material  from  the  inside,  and  (3)  in  the  use  of  inter- 
locked hydraulic  apparatus  to  open  and  close  the  air-locks.  Each 
of  the  two  deep-water  piers  consists  of  four  cylindrical  caissons 
70  feet  in  diameter  the  deepest  of  which  rests  96  feet  below  high 
tide. 

460.  Physiological  Effect  of  Compressed  Air.  In  the  ap- 
plication of  the  compressed-air  process,  the  question  of  the  ability 
of  the  human  system  to  bear  the  increased  pressure  of  the  air  be- 
comes very  important. 

After  entering  the  air-lock,  as  the  pressure  increases,  the  first 
sensation  experienced  is  one  of  great  heat.  As  the  pressure  is  still 
further  increased  a  pain  is  felt  in  the  ear,  arising  from  the  abnormal 
pressure  upon  the  ear-drum.  The  tubes  extending  from  the  back 
of  the  mouth  to  the  bony  cavities  over  which  this  membrane  is 
stretched,  are  so  very  minute  that  compressed  air  can  not  pass 
through  them  with  a  rapidity  sufficient  to  keep  up  the  equilibrium 
of  pressure  on  both  sides  of  the  drum  (for  which  purpose  the  tubes 
were  designed  by  nature),  and  the  excess  of  pressure  on  the  outside 
causes  the  pain.  These  tubes  can  be  distended,  thus  relieving  the 
pain,  by  the  act  of  swallowing,  or  by  closing  the  nostrils  with  the 
thumb  and  finger,  shutting  the  lips  tightly,  and  inflating  the 
cheeks.  Either  action  facilitates  the  passage  of  the  air  through 
these  tubes,  and  establishes  the  equilibrium  desired.  The  relief  is 
only  momentary,  and  the  act  must  be  repeated  from  time  to  time, 
as  the  pressure  in  the  air-lock  increases.  This  pain  is  felt  only 
while  the  air  in  the  lock  is  being  "equalized,"  i.  e.,  while  the  air  is 
being  admitted,  and  is  most  severe  the  first  time  compressed  air  is 
encountered,  a  little  experience  generally  removing  all  unpleasant 
sensations.  The  passage  through  the  lock,  both  going  in  and  com- 
ing out,  should  be  slow  ;  that  is  to  say,  the  compressed  air  should 
be  let  in  and  out  gradually,  to  give  the  pressure  time  to  equalize 
itself  throughout  the  various  parts  of  the  body. 

When  the  lungs  and  whole  system  are  filled  thoroughly  with 
the  denser  air,  the  general  effect  is  rather  bracing  and  exhilarating. 
The  increased  amount  of  oxygen  breathed  in  compressed  air  very 
much  accelerates  the  organic  functions  of  the  body,  and  hence  labor 
in  the  caisson  is  more  exhaustive  than  in  the  open  air ;  and  on  get- 
ting outside  again,  a  reaction  with  a  general  feeling  of  prostration 
sets  in.     At  moderate  depths,  however,  the  laborers  in  the  caisson. 


300  FOUNDATIONS   UNDER   WATER.  [CHAP.  XII. 

after  a  little  experience,  feel  no  bad  effects  from  the  compressed  air, 
either  while  at  work  or  afterwards. 

Eemaining  too  long  in  the  working  chamber  causes  a  form  of 
paralysis,  recently  named  caisson  disease,  which  is  sometimes  fatal. 
The  injarious  effect  of  compressed  air  is  much  greater  on  men  ad- 
dicted to  the  use  of  intoxicating  liquors  than  on  others.  Only 
sound,  able-bodied  men  should  be  permitted  to  work  in  the  caisson. 

In  passing  through  the  air-lock  on  leaving  the  air-chamber,  the 
workman  experiences  a  great  loss  of  heat  owing  (1)  to  the  expan- 
sion of  the  atmosphere  in  the  lock,  (3)  to  the  expansion  of  the  free 
gases  in  the  cavities  of  the  body,  and  (3)  to  the  liberation  of  the 
gases  held  in  solution  by  the  liquids  of  the  body.  Hence,  on  com- 
ing out  the  men  should  be  protected  from  currents  of  air,  should 
drink  a  cup  of  strong  hot  coffee,  dress  warmly,  and  lie  down  for  a 
short  time. 

461.  No  physiological  difficulty  is  encountered  at  small  depths  ; 
but  this  method  is  limited  to  depths  not  much  exceeding  100  feet, 
owing  to  the  deleterious  effect  of  the  compressed  air  upon  the  work- 
men. At  the  east  abutment  of  the  St.  Louis  bridge  (§  457),  the 
caisson  was  sunk  110  feet  below  the  surface  of  the  water.  Except 
in  this  instance,  the  compressed-air  process  has  never  been  apjjlied 
at  a  greater  depth  than  about  90  feet.  Theoretically,  the  depth,  in 
feet,  of  the  lower  edge  of  the  caisson  below  the  surface  divided  by 
33  is  equal  to  the  number  of  atmospheres  of  pressure.  The  press- 
ure is  never  more  than  this,  and  sometimes,  owing  to  the  fric- 
tional  resistance  to  the  flow  of  the  water  through  the  soil,  it  is  a 
little  less.  Therefore  the  depth  does  not  exactly  indicate  the 
pressure  ;  but  the  rule  is  sufficiently  exact  for  this  purpose.  At  St. 
Louis,  at  a  depth  of  110  feet,  the  men  were  able  to  work  in  the 
compressed  air  only  four  hours  per  day  in  shifts  of  two  hours  each, 
and  even  then  worked  only  part  of  the  time  they  were  in  the  air- 
chamber. 

With  reasonable  care,  the  pneumatic  process  can  be  applied  at 
depths  less  than  80  or  90  feet  without  serious  consequences.  At 
great  depths  the  danger  can  be  greatly  decreased  by  observing  the 
following  precautions,  in  addition  to  those  referred  to  above  :  (1)  In 
hot  weather  cool  the  air  before  it  enters  the  caisson  ;  *  (2)  in  cold 

*  This  was  done  in  1888  at  the  bridge  over  the  Ohio  River  at  Cairo,  111. — prob- 
ably the  first  example.     The  temperature  of  the  air  was  reduced  20°  F. 


AKT.  4.]  PNEUMATIC    PKOCESS.  301 

weather  warm  the  air  in  the  lock  when  the  men  come  out ;  and 
(3)  raise  and  lower  them  by  machinery. 

For  an  exhaustive  account  of  the  various  aspects  of  this  subject, 
see  Dr.  Smithes  article  on  the  "  Physiological  Effect  of  Compressed 
Air,"  in  the  Report  of  the  Engineer  of  the  Brooklyn  Bridge.* 

462.  Cost.  The  contract  for  pneumatic  foundation  is  usually 
let  at  specified  prices  per  unit  for  the  materials  left  permanently  in 
the  structure  and  for  the  material  excavated,  including  the  neces- 
sary labor  and  tools.  The  prices  for  material  in  place  are  about  as 
follows  :  Timber  in  caisson  proper,  from  $40  to  $50  per  thousand 
feet,  board  measure,  according  to  the  locality  in  which  the  work  is 
done  ;  and  the  timber  in  the  crib- work  and  coffer-dam  about  $5  to 
$7  per  thousand  less.  The  concrete,  which  is  usually  composed  of 
broken  stone  and  suflBcient  1  to  2  or  1  to  3  Portland  cement  mortar 
to  completely  fill  the  voids,  costs,  exclusive  of  the  cement,  from  $5  to 
$7  per  cubic  yard  for  that  in  the  crib,  and  about  twice  this  sum  for 
that  in  the  air-chamber  and  under  the  cutting  edge.  The  wrought- 
iron  spikes,  drift-bolts,  screw-bolts,  and  cast-iron  washers  cost  from 
3^  to  6  cents  per  pound,  f  The  caisson  and  filling  costs  from  $14  to 
$20  per  cubic  yard  ;  and  the  crib  and  filling  from  $8  to  $10. 

The  price  for  sinking,  including  labor,  tools,  machinery,  etc., 
ranges,  according  to  the  kind  of  soil,  from  18  to  40,  or  even  50, 
cents  per  cubic  foot  of  the  volume  found  by  multiplying  the  area 
of  the  caisson  at  the  cutting  edge  by  the  final  depth  of  the  latter 
below  low  water.  In  sand  or  silt  the  cost  is  18  to  20  cents,  and  in 
stiff  clay  and  bowlders  40  to  50  cents. 

463.  Examples.  The  table  on  page  302  gives  the  details  of  the 
cost  of  the  pneumatic  foundation  of  the  Havre  de  Grace  bridge,  as 
fully  described  in  §§  438-44. 

The  table  on  page  303  gives  the  details  of  the  cost  of  the  pneu- 
matic caissons  of  the  bridge  across  the  Missouri  River  near  Blair, 
Neb.  The  caissons  (Fig.  65,  page  285)  were  54  feet  long,  24  feet  wide, 
and  17  feet  high.  In  the  two  shore  piers,  Nos.  I  and  IV  of  the 
table,  the  caissons  were  surmounted  by  cribs  20  feet  high  ;  but  in 
the  channel  piers,  the  masonry  rested  directly  upon  the  roof  of  the 

*  Priae  Essay  of  the  Alumni  Association  of  the  College  of  Physicians  and  Sur- 
geons of  New  York  City,  1S73. 

t  There  are  usually  from  140  to  150  pounds  of  iron  per  thousand  feet  (board  meas- 
ure) of  timber. 


302 


FOUNDATIONS  UNDER  WATER. 


[chap.  XII. 


TABLE  32. 
Cost,  to  the  R.  R.  Co.,  op  Foundations  op  Havre  de  Grace  Bridge. 


N0MBKR  OP  THE  Pier. 

Items. 

II. 

III. 

IV. 

VIII. 

IX. 

Depth  of  cutting  edge  below  low  water, 
feet  

Depth  of  cutting  edge  below  mud  line, 
feet 

Displacement   below  low  water,  cu.  ft. 
"        mud  line,  cu.  ft . . 

Caisson :  timber,  @  $46.80  per  M 

68.3 

55.5 
112,124 
94,504 

$9,522.54 

1,456.12 

5,775.00 

16.82 

8,421.14 

1.291.14 

14,016..50 

10.76 

96.78 

14.45 

22,424.80 
000 

70.7 

58.7 
123,402 
106,269 

$10,088.44 

1,587.15 

7,017.50 

18.37 

9,262.19 

1,4.54.85 

16,090.50 

11.10 

1.375.00 

236.15 

24.680.40 
10,902.50 

59.9 

.32.3 
159.588 
84.014 

$14,820.94 

2.596.23 

13,247.50 

19.19 

6,738.87 

1,179.36 

13,897.50 

10.91 

5,078.64 

892.29 

31.917.60 
2,205.00 

76.0 

55  2 

189,578 
127,586 

$13,176.07 

2.242.40 

18,987.50 

24.34 

8,936.58 

1,749.35 

21,943.50 

9.91 

4,013.52 

684.20 

37,915.60 
9,205.00 

65.0 

32.6 
231.691 
107,836 

$21,767.85 

iron,  ®  534  cts.  per  lb 

concrete.  @  $17.50  per  cu.  yd. 

total  eo.st,  per  net  cu.  yd 

Crib  •  timber,  @,  $46.80  per  M 

3.295.38 

25,602.50 

22.10 

9  538.96 

1,445.93 

concrete,  ®  $8.50  per  net  cu.  yd . . 
total  cost,  per  cu.  yd 

26,962.00 
10.09 

CoCEer-dam:  timber,  ©  $40  80  per  M 

iron,  @  514  cts.  per  lb 

Cost  of  sinlting.  @  20  cts.  per  cu.  ft.  of 

displacement  below  low  water 

Concrete  below  cutting  edge,  @  $17.50  . . 

5.921.70 
899.22 

46,.3.38.20 
10,920.00 

63,018.47 
19.93 

71,792.18 
21.58 

90,368.93 
25.20 

109,648.72 
23.30 

141,772.44 

Total  cost  per  cu.  yd.  of  foundation  be- 
low masonry,  including  coffer-dams. 

23.44 

Average  total  cost  of  the  foundation,  to  R.  B.  Co.,  per  net  cubic  yard $22.69. 

caisson.     The  work  was  done,  in  1882-83,  by  the  bridge  company^s 
men  under  the  direction  of  the  engineer. 

464.  In  1869-72,  thirteen  cylinders  were  sunk  by  the  plenum- 
pneumatic  process  for  the  piers  of  a  bridge  over  the  Schuylkill 
Eiver  at  South  Sti-eet,  Philadelphia.  There  were  three  piers,  one 
of  which  was  a  pivot  pier.  There  were  two  cylinders,  8  feet  in 
diameter  and  82  feet  long,  sunk  through  22  feet  of  water  and  30 
feet  of  "  sand  and  tough  compact  mud  intermingled  with  bowlders  ;" 
two  cylinders,  8  feet  in  diameter  and  57  feet  long,  sunk  through  23 
feet  of  water  and  5  feet  of  soil  as  above  ;  one  cylinder,  G  feet  in 
diameter  and  64  feet  long,  sunk  through  22  feet  of  water  and  18 
feet  of  soil  as  above  ;  and  8  columns,  4  feet  in  diameter  and  aggre- 
gating 507  feet,  sunk  through  22  feet  of  water  and  18  feet  of  soil 
as  above.  A  10-foot  section  of  the  8-foot  cylinder  weighed  14,600 
pounds,  of  the  6-foot,  10,800  pounds,  and  of  the  4-foot,  6,800 
pounds.     The  cylinders  rested  upon  bed-rock,  and  were  bolted  to 

*  Data  by  courtesy  of  Sooysmith  &  Co.,  contracting  engineers  for  the  pneumatic 
foundations. 


AKT.   4.] 


PNEUMATIC    PKOCESS. 


303 


it.     The  actual  cost  to  the  contractor,  exclusive  of  tools  and  ma- 
chinery, was  as  in  Table  34  (page  304). 

TABLE  33. 
Cost  of  Pneumatic  Foundations  of  Blair  Bridge* 


Items. 


Number  of  the  Pier. 


III. 


IV. 


Total  distance  caisson  was  lowered  after  comple 

tion 

Final  depth  of  cutting  edge  below  surface  of  water 
"       "        '■  '■         ■'      mud  line 


55.6  ft. 
51.9    '• 

47.7  " 


54.5  ft. 
52.3  " 
51.0    " 


Caisson  and  filling,  cost  of 

"        '•  "      "  per  cubic  yard  

Crib  and  filling,  cost  of 

"      "        •'         '•     ■■  per  cubic  yard  

Air-lock,  shafts,  etc.,  cost  of 

Sinking  caisson,  cost  of,  including  erection  and  re- 
moval of  machiiierj-  

Sinking  caisson,  cost  of,  per  cubic  foot  of  displace- 
ment below  position  of  cutting  edge  when 
caisson  was  completed 

Sinking  caisson,  cost  of.  i)er  cubic  foot  of  displace- 
ment below  surface  of  water 

Sinking  caisson,  cost  of,  per  cubic  foot  of  displace- 
ment below  mud  line 

Total  cost  t  of  foundation 

"        "        "  "  per  cubic  yard 


1,753.51 '$12,386.56 


14.31 

7,368.16 

8.85 

1,481.60 

5,772.52 


8.0  cts. 
8.6    " 


15.12 


1,567.42 
5,629.37 

8.0  cts. 
8.3    " 


9.3    "  8.5 

(26,375  79  $19,583.35 

15.98,  23.87 


56.2  ft. 
53.4  " 
49.4    •' 

$13,819.34 
16  74 


1,536.80 
6,888.16 

9.5  cts. 

9.9    " 

10.8    "     ! 

$22.244..30, 

27.081 


68.5  ft. 
57.0  •' 
54.7    " 

$11,252.45 

13.77 

6.303.46 

7.59 

1,. 02 1. 08 

7.084.26 


7.1  cts. 

9.6    '• 

10.0    " 
$26,161.25 
15.85 


Average  costt  of  the  foundations,  per  cubic  yard $20.70. 

465.  ''Excavation  in  the  Brooklyn  caisson  J  cost  for  labor 
only,  including  the  men  on  top,  about  §5.25  per  cubic  yard 
[19  cents  per  cubic  foot].  Kunniug  the  six  air-compressors 
added  to  this  $3.60  j^er  hour,  or  about  47  cents  per  yard;  lights 
added  $0.56  more:  and  these  with  other  contingencies  nearly 
equaled  the  cost  of  labor.  The  great  cost  was  due  to  the  excessive 
hardness  of  the  material  over  much  of  the  surface,  the  caisson  finally 
resting,  for  nearly  its  whole  extent,  on  a  mass  of  bowlders  or  hard- 
pan.     The  concrete  in  the  caisson  cost,  for  every  expense,  about 

*  Compiled  from  the  report  of  Geo.  S.  Morison,  chief  engineer  of  the  bridge. 

t  Exclusive  of  engineering  expenses  and  cost  of  tools,  machinery,  and  buildings. 
In  a  note  to  the  author,  Mr.  Morison,  the  engineer  of  the  bridge  says  :  "  It  is  impos- 
sible to  divide  the  buildings,  tools,  and  engineering  expenses  between  the  substruct- 
ure and  other  portions  of  the  work.  The  bulk  of  the  items  of  tools  and  machinery 
[$12,369.88],  however,  relates  to  the  foundations."  The  engineering  expenses  and 
buildings  were  nearly  3  per  cent,  of  the  total  cost  of  the  entire  bridge.  The  cost  of 
tools  and  machinery  was  equal  to  a  little  over  13  per  cent,  of  the  cost  of  the  founda- 
tions as  above.  Including  these  items  would  add  nearly  one  sixth  to  the  amounts  in 
the  last  three  lines. 

X  For  a  brief  description,  see  §  458. 


S04 


FOUNDATIONS   UNDER   WATER. 


[chap.  XII. 


TABLE  34. 
Cost   of  Pneumatic  Piles  at  Philadelphia  in  1869-73.* 


Diameter  of  Cylinders. 

Items  of  Expense. 

4-ft. 

6-ft. 

8- ft. 

Cost  of  cast  iron,  @  $59. .50  per  ton   

'•     •'  bolts,  @  9%  cents  per  lb 

"     "  grouted  rubble  masonry  (exclusive  of  labor),  @  $5.40 

$11.239  36    S2,0.53.75 
-189.84            93.31 

1,266  79!         3.58.40 
6,693.50}         911.88 

$19,689.49    $3,417.34 

$23.10         $33.  .54 
2.50             5.60 
13.20            14.25 

$13,577.90 
670.02 

2,779.97 

9.036.51 

$26,064.40 

$51.35 

■'     '•  materials  for  mason/y  per  lineal  foot  of  cylinder  — 
"     "  sinking  and  laying  masonry  per  lineal  foot  of  cylinder 

10.00 
32.51 

Total  cost,t  per  lineal  foot,  of  cylinder  in  place  

$38.80         $53.39 

$93.76 

$15.50  per  cubic  yard.  The  caisson  and  filling  together  aggregated 
16,898  cubic  yards  ;  and  the  approximate  cost  per  yard  for  every 
expense  was  $20.71."  J  The  foundation  therefore  cost  about  $30 
per  cubic  yard. 

The  pneumatic  foundations  for  the  channel  piers  of  the  bridge 
over  the  Missouri  at  Plattsmouth,  Neb.,  cost  as  follows:  One 
foundation,  consisting  of  a  caisson  50  ft.  long,  20  ft.  wide,  and  15.5 
ft.  high,  surmounted  by  a  crib  14.15  ft.  high,  sunk  through  13  ft. 
of  water  and  20  ft.  of  soil,  cost  $19.29  per  cubic  yard  of  net  volume. 
Another,  consisting  of  a  caisson  50  ft.  long,  20  ft.  wide,  and  15.5 
ft.  high,  surmounted  by  a  crib  3G.25  ft.  high,  sunk  through  10 
ft.  of  water  and  44  ft.  of  soil,  cost  $14.45  per  cubic  yard  of  net 
volume.  § 

466.  European  Examples.  The  following  •([  is  interesting  as 
showing  the  cost  of  pneumatic  work  in  Europe  : 

"  At  Moulins,  cast-iron  cylinders,  8  feet  2^  inches  in  diameter, 
with  a  filling  of  concrete  and  sunk  83  feet  below  water  into  marl, 
cost  $62.94  per  lineal  foot,  or  $29.71  for  the  iron  work,  and  $33.23 


*  Compiled  from  an  article  by  D.  McN.  Stauffer,  engineer  in  charge,  in  Trans.  Am. 
Soc.  of  C.  E.,  vol.  Y\\.  pp.  287-309. 

t  Exclusive  of  tools  and  machinery. 

X  F.  Collingwood,  assistant  engineer  Broolvlyn  bridge,  in  Trans.  Am.  Soc.  of  C.  E. 

§  Compiled  from  the  report  of  Geo.  S.  Morison,  chief  engineer  of  the  bridge. 

T[  By  Jules  Gaudard,  as  translated  from  the  French  by  L.  F.  Vernon-Haj'court  for 
the  Proceedings  of  the  Institute  of  Civil  Engineers  (London). 


ART.  4.]  PXEUMATIC    PEOCESS.  305 

for  sinking  and  concrete.  At  Argenteuil,  with  cylinders  11  feet  10 
inches  in  diametei',  the  sinking  alone  cost  $42.12  per  lineal  foot 
[nearly  $10  per  cubic  yard],  where  a  cylinder  was  sunk  53|^  feet  in 
three  hundred  and  ninety  hours.  [The  total  cost  of  this  founda- 
tion was  $3-4.09  per  cubic  yard,  see  table  on  page  310.]  At  Orival, 
where  a  cylinder  was  sunk  49  feet  in  twenty  days,  the  cost  of  sinking 
was  $36.83  per  lineal  foot.  At  Bordeaux,  with  the  same-sized 
cylinders,  a  gang  of  eight  men  conducted  the  sinking  of  one  cylin- 
der, and  usually  34  cubic  yards  were  excavated  every  twenty-four 
houi's.  The  greatest  depth  reached  was  55f  feet  below  the  ground, 
and  7 1  feet  below  high  Avater.  In  the  regular  course  of  working,  a 
cylinder  was  sunk  in  from  nine  to  fifteen  days,  and  the  whole  opera- 
tion, including  preparations  and  filling  with  concrete,  occupied  on 
the  average  25  days.  One  cylinder,  or  a  half  pier,  cost  on  the  aver- 
age $11,298.40,  of  which  $1,461  was  for  sinking.  M.  Morandiere 
estimates  the  total  cost  of  a  cylinder  sunk  like  those  at  Argenteuil, 
to  a  depth  of  50  feet,  at  $7,012.80. 

467.  ''  Considering  next  the  cost  of  piers  of  masonry  on  wrought- 
iron  caissons  of  excavation,  the  foundations  of  the  Lorient  viaduct 
over  the  ScorfE  cost  the  large  sum  of  $24.11  per  cubic  yard,  owing  to 
difficulties  caused  by  the  tides,  the  labor  of  removing  the  bowlders 
from  underneath  the  caisson,  and  the  large  cost  of  plant  for  only 
tAvo  piers.  The  foundation  of  the  Kehl  bridge  cost  still  more,  about 
$28.23  *  per  cubic  yard  ;  but  this  can  not  be  regarded  as  a  fair  in- 
stance, being  the  first  attempt  [see  §  429]  of  the  kind. 

"The  foundations  of  the  Xantes  bridges,  sunk  56  feet  below 
low-water  level,  cost  about  $14.84  per  cubic  yard.  The  average 
cost  per  pier  was  as  follows  : 

Caisson  (41  feet  4  inches  by  14  feet  5  inclies),  50  tons  of  wrought 

iron  @  $116.88 $5,844 

Coffer-dam,  3  tons  of  wrought  iron  @  $58.44 175 

Excavation,  916  cubic  yards®  $4.47 4,091 

Concrete 4, 188 

Masonry,  plant,  etc 1,870 

Average  cost  per  pier $16,168 

"One  pier  of  the  bridge  over  the  Meuse  at  Rotterdam,  with  a 

*  Notice  the  slight  inconsistency  between  this  qiiantity  and  the  one  in  the  third 
line  from  the  last  of  the  table  on  page  310,  both  being  from  the  same  article. 


306  FOUNDATIONS   UNDER   WATER.  [CHAP.  XII. 

caisson  of  222  tons  and  a  coffer-dam  casing  of  94  tons,  and  sunk  75 
feet  below  high  water,  cost  $70,858,  or  $13.97  per  cubic  yard. 

"  The  Vichy  bridge  has  five  piers  built  on  caissons  34  feet  by  13 
feet,  and  two  abutments  on  caissons  26  feet  by  24  feet.  The  foun- 
dations were  sunk  23  feet  in  the  ground,  the  upper  portion  con- 
sisting of  shingle  and  conglomerated  gravel,  and  the  last  10  feet  of 
marl.     The  cost  of  the  bridge  was  as  follows  : 

Interest  for  eight  months,  and  depreciation  of  plant  worth  $19,480. .  $3,896 

Cost  of  preparations,  approach  bridge,  and  staging 4, 904 

Caissons  (seven),  150i  tons  @  $113.38 17,108 

Sinking 9,823 

Concrete  and  masonry 5, 303 

Contractor's  bonus  and  general  expenses 6,107 

Total  cost  of  five  foundations $47,141 

The  cost  per  cubic  yard  of  the  foundation  below  low  water  was 
$16.69,  of  which  the  sinking  alone  cost  $3.50  in  gravel,  and  $4.37 
in  marl. 

"At  St.  Maurice,  the  cost  per  cubic  yard  of  foundation  was 
$15.94,  exclusive  of  staging." 

468.  Conclusion.  Except  in  very  shallow  or  very  deep  water, 
the  compressed-air  process  has  almost  entirely  superseded  all  others. 
The  following  are  some  of  the  advantages  of  this  method.  1.  It  is 
reliable,  since  there  is  no  danger  of  the  caisson's  being  stopped, 
before  reaching  the  desired  depth,  by  sunken  logs,  bowlders,  etc., 
or  by  excessive  friction,  as  in  dredging  through  tubes  or  shafts  in 
cribs.  2.  It  can  be  used  regardless  of  the  kind  of  soil  overlying  the 
rock  or  ultimate  foundation.  3.  It  is  comparatively  rapid,  since 
the  sinking  of  the  caisson  and  the  building  up  of  the  pier  go  on  at 
the  same  time.  4.  It  is  comparatively  economical,  since  the  weight 
added  in  sinking  is  a  part  of  the  foundation  and  is  permanent,  and 
the  removal  of  the  material  by  blowing  out  or  by  pumping  is  as 
uniform  and  rapid  at  one  depth  as  at  another, — the  cost  only  being 
increased  somewhat  by  the  greater  depth.  5.  This  method  allows 
ample  opportunity  to  examine  the  ultimate  foundation,  to  level  the 
bottom,  and  to  remove  any  disintegrated  rock.  6.  Since  the  rock 
can  be  laid  bare  and  be  thoroughly  washed,  the  concrete  can  be  com- 
menced upon  a  perfectly  clean  surface  ;  and  hence  there  need  be  no 
question  as  to  the  stability  of  the  foundation. 


ART.  5.]  THE    FREEZING    PROCESS.  307 


Art.  5.  The  Freezing  Process. 

469.  Principle.  The  presence  of  water  has  always  been  the 
great  obstacle  in  foundation  work  and  in  shaft  sinking,  and  it 
is  only  very  recently  that  any  one  thought  of  transforming  the 
liquid  soil  into  a  solid  wall  of  ice  about  the  space  to  be  excavated. 
The  method  of  doing  this  consists  in  inclosing  the  site  to  be  ex- 
cavated, by  driving  into  the  ground  a  number  of  tubes  through 
which  a  freezing  mixture  is  made  to  circulate.  These  consist  of  a 
large  tube,  closed  at  the  lower  end,  inclosing  a  smaller  one,  open  at 
the  lower  end.  The  freezing  mixture  is  forced  down  the  inner 
tube,  and  rises  through  the  outer  one.  At  the  top,  these  tubes 
connect  with  a  reservoir,  a  refrigerating  machine,  and  a  pump. 
The  freezing  liquid  is  cooled  by  an  ice-making  machine,  and  then 
forced  through  the  tubes  until  a  wall  of  earth  is  frozen  around 
them  of  suflBcient  thickness  to  stand  the  external  pressure,  when 
the  excavation  can  proceed  as  in  dry  ground. 

470.  History.  This  method  was  invented  by  F.  H.  Poetsch, 
M.D.,  of  Aschersleben,  Prussia,  in  1883.  It  has  been  applied  in 
but  three  cases.  The  first  was  at  the  Archibald  colliery,  near 
Schweidlingen,  Prussia,  where  a  vein  of  quicksand,  20  feet  thick,  was 
encountered  at  a  depth  of  about  150  feet  below  the  surface.  Here 
twenty-three  pipes  were  used,  and  35  days  consumed  in  the  freezing 
process,  under  local  difficulties.  The  second  was  at  the  Centrum 
mine,  near  Berlin,  where  about  107  feet  of  quicksand,  etc.,  was 
penetrated.  Engineers  had  been  baffled  for  years  in  their  attempts 
to  sink  a  shaft  here  ;  but  in  33  days  Mr.  Poetsch  had,  with  only  16 
freezing  tubes,  secured  a  6-foot  wall  of  ice  around  the  shaft  area,  and 
the  shaft  was  excavated  and  curbed  without  difficulty.  The  third 
piece  of  work  was  at  the  Eimilia  mine,  Fensterwalde,  Austria,  in 
1885,  where  an  8^-foot  shaft  was  sunk  through  115  feet  of  quick- 
sand.* 

471.  Details  of  the  Process.  In  the  last  case  mentioned 
above,  "  12  circulatory  tubes  were  used,  sunk  in  a  circle  about  14 
feet  in  diameter,  from  12  to  15  days  being  required  to  sink  them  t 
depth  of  about  100  feet.     The  outside  tubes  were  8^  inches  in 

*  As  this  volume  is  going  through  the  press,  this  method  is  being  applied  in  two 
places  in  tliis  country^Iron  Mountain,  Mich.,  and  Wyoming,  Penn. — in  sinking 
mine-shafts. 


308  FOUNDATION'S   UNDER   WATER.  [CHAP.   XII. 

diameter,  and  made  of  plate  iron  0.15  inch  thick.  The  tubes  were 
snnk  by  aid  of  the  water-jet.  They  were  given  a  very  slight  incli- 
nation outward  at  the  bottom  to  avoid  any  deviation  in  sinking 
that  might  interfere  with  the  line  of  the  shaft.  The  fi-eezing 
liquid  employed  was  a  solution  of  chloride  of  calcium,  which  con- 
geals at  a  temperature  of  —35°  C.  (  —31°  F.).  The  circulation  of 
the  liquid  through  the  tubes  was  secured  by  a  small  pump  with 
a  piston  6  inches  in  diameter  and  a  12-inch  stroke.  At  the  begin- 
ning of  the  operation,  this  pump  made  30  double  strokes  per  min- 
ute, which  was  equivalent  to  the  passage  of  0.6  gallon  of  the  liquid 
through  each  tube  per  minute  ;  at  the  end  of  the  oi3eration,  when 
it  was  only  necessary  to  maintain  the  low  temperature,  the  pump 
strokes  were  reduced  to  15  per  minute.  The  refrigerating  machine 
employed  was  one  of  a  model  guaranteed  by  the  maker  to  produce 
1,100  pounds  of  ice  per  hour.  The  motive-power  was  supplied  by 
a  small  engine  of  about  5  horse-power.  The  ammoniac  pump  had 
a  piston  2.8  inches  in  diameter  and  a  9.2-inch  stroke,  and  made  30 
strokes  per  minute.  The  pressure  maintained  was  about  10  atmos- 
pheres. The  quantity  of  ammoniacal  liquid  necessary  to  charge 
the  apparatus  was  281  gallons ;  and  under  normal  conditions  the 
daily  consumption  of  this  liquid  was  0.78  gallon, 

"The  actual  shaft  excavation  was  commenced  63  days  after  the 
freezing  apparatus  had  been  set  in  motion.  The  freezing  machine 
was  in  operation  240  days.  The  work  was  done  without  difficulty, 
and  a  progress  of  1.64  feet  per  day  was  made.  The  timbering  was 
very  light,  but  no  internal  pressure  of  any  kind  was  observed.  The 
brick  masonry  used  for  finally  lining  the  shaft  was  about  11  inches 
thick.  When  the  shaft  was  finished,  the  tubes  were  withdrawn 
without  difficulty,  by  circulating  through  them  a  hot,  instead  of  a 
cold,  solution  of  the  chloride  of  calcium,  thus  thawing  them  loose 
from  the  surrounding  ice.  The  tubes  were  entirely  uninjured,  and 
Gould  be  used  again  in  another  similar  operation. 

472.  "  The  material  in  the  above  plant  is  estimated  to  have  cost 
$15,000,  and  $4,800  more  for  mounting  and  installation.  The  daily 
expense  of  conducting  the  freezing  process  is  estimated  at  $11.  The 
total  expense  for  putting  down  the  shaft  is  estimated  at  $128.66  per 
linear  foot."*     The  last  is  equivalent  to  about  $2.25  per  cubic  foot. 

*  Engineering  News,  vol.  xiv.  pp.  24,  25,  translated  from  Le  O&nie  Civil  of  June  13, 

1885. 


AKT.   6.]  COMPAKISON"   OF   METHODS.  30& 

473.  Modification  for  Foundations  under  Water.  For  sinking 
foundations  under  water,  two  methods  of  applying  this  process  have 
been  proposed.  One  of  these  consists  in  combining  the  pneumatic 
and  freezing  processes.  A  pneumatic  caisson  is  to  be  sunk  a  short 
distance  into  the  river-bed,  and  then  the  congealing  tubes  are 
applied,  and  the  entire  mass  between  the  caisson  and  the  rock  is 
frozen  solid.  When  the  freezing  is  completed,  the  caisson  will  be 
practically  sealed  against  the  entrance  of  water,  and  the  air-lock  can 
be  removed  and  the  masonry  built  up  as  in  the  open  air. 

The  other  method  consists  in  sinking  an  open  caisson  to  the 
river-bed,  and  putting  the  freezing  tubes  down  through  the  water. 
When  the  congelation  is  completed,  the  water  can  be  pumped  out 
and  the  work  conducted  in  the  open  air. 

474.  Advantages  Claimed.  It  is  claimed  for  this  process  that 
it  is  expeditious  and  economical,  and  also  that  it  is  particularly 
valuable  in  that  it  makes  possible  an  accurate  estimate  of  the  total 
cost  before  the  work  is  commenced, — a  condition  of  affairs  unat- 
tainable by  any  other  known  method  in  equally  difficult  ground.  It 
has  an  advantage  over  the  pneumatic  process  in  that  it  is  not  limited 
by  depth.  It  can  be  applied  horizontally  as  well  as  vertically,  and 
hence  is  specially  useful  in  sub-aqueous  tunneling,  particularly  in 
soils  which,  with  compressed  air,  are  treacherous. 

475.  Difficulties  Anticipated.  So  far  it  has  been  used  only 
in  sinking  shafts  for  mines.  Two  difficulties  are  anticipated  in  ap- 
plying it  to  sink  foundations  for  bridge  piers  in  river  beds  ;  viz., 
(1)  the  difficulty  in  sinking  the  pipes,  owing  to  striking  sunken  logs, 
bowlders,  etc.;  and  (3)  the  possibility  of  encountering  running 
water,  which  will  thaw  the  ice-wall.  These  difficulties  are  not  in- 
surmountable, but  experience  only  can  demonstrate  how  serious 
they  are. 

476.  Cost.     See  §  472,  and  compare  with  table  on  page  310. 

Art.  6.     Comparison  of  Methods. 

477.  The  following  comparison  of  the  different  methods  is  fron: 
an  article  by  Jules  Gaudard  on  Foundations,  as  translated  by  L.  F. 
Vernon-Harcourt  for  the  proceedings  of  the  Institute  of  Civil  En- 
gineers (London).  Except  as  showing  approximate  relative  costs  in 
Europe,  it  is  not  of  much  value,  owing  to  improvement  made  since 
the  article  was  written,  to  the  differences  between  European  and 


310 


FOUNDATIONS    UNDER   WATER. 


[chap.    XII. 


American  practice,  and  to  differences  in  cost  of  materials  in  the  two 
countries. 

478.  "  M.  Croizette  Desnoyers  has  framed  a  classification  of  the 
methods  of  foundations  most  suitable  for  different  depths,  and  also 
an  estimate  of  the  cost  of  each.  These  estimates,  however,  must  be 
considered  merely  approximate,  as  unforeseen  circumstances  pro- 
duce considerable  variations  in  works  of  this  nature. 

TABLE   35. 
Cost  of  Vabious  Kinds  op  Foundations  in  Europe. 


Kind  of  Foundation. 


Depth 

IN  Feet. 


Min.    Max 


Cost  per 
Cubic  Yard. 


Min.      Max 


On  piles  after  compression  of  the  ground,  siiallow  depth. 
"      "        "  "  greater  depth  . 

By  sinking  wells 

By  pumping 


.33 


under  favorable  circumstances 

"  "      unfavorable  circumstances 

On  concrete  under  water,  small  amount  of  silt 

*'        "  "  "      large       '•         "    "  

By  means  of  compressed  air*  under  favorable  circumstances. . . 
^       "       "  "  "  "       unfavorable  circumstances: 

Lorient  viaduct 

Kehl  bridge  t 

Argenteuil  bridge 

Bordeaux  bridge 


^2.92 
4.39 


2.92 
4.39 
14.85 

4.37 
9.00 

13.39 


[50  ft.] 
[70  "  ] 
[50  "  ] 


$24, 
29 
34, 
40 


$4.39 
7.30 

9.00 

4.39 
13.39 

17.77 

9.00 
11.93 

16.17 

11 

71 
09 
17 


*  See  also  §§  466-67.  t  See  foot-note  on  page  305. 

"When  the  foundations  consist  of  disconnected  pillars  or  piles, 
the  above  prices  must  be  applied  to  the  whole  cubic  content,  includ- 
ing the  intervals  between  the  parts  ;  but  of  course  at  an  equal  cost 
Bolid  piers  are  the  best. 

479.  "  For  pile-work  foundations  the  square  yard  of  base  is  prob- 
ably a  better  unit  than  the  cubic  yard.  Thus  the  foundations  of 
the  Vernon  bridge,  with  piles  from  24  to  31  feet  long,  and  with 
cross-timbering,  concrete,  and  caisson,  cost  $70  per  square  yard  of 
base.  According  to  estimates  made  by  M.  Picquenot,  if  the  foun- 
dations had  been  put  in  by  means  of  compressed  air,  the  cost  would 
have  been  $159.64  ;  with  a  caisson,  not  water-tight,  sunk  down, 
$66.27  ;  with  concrete  poured  into  a  space  inclosed  with  sheeting, 
$62.23  ;  and  by  pumping,  $83.56  per  square  yard  of  base." 


PART    IV. 

MASONRY  STRUCTURES. 


CHAPTER  XIII. 
MASONRY    DAMS. 

480.  It  is  not  the  intention  here  to  discuss  every  feature  of 
masonry  dams ;  that  has  been  done  in  the  special  reports  and  arti- 
cles referred  to  in  §  520,  page  334.  The  fundamental  principles 
will  be  considered,  particularly  with  reference  to  their  applica- 
tion in  the  subsequent  study  of  retaining  walls,  bridge  abutments, 
bridge  piers,  and  arches.  The  discussions  of  this  chapter  are 
applicable  to  masonry  dams,  reservoir  walls,  or  to  any  wall  which 
counteracts  the  pressure  of  water  mainly  by  its  weight. 

There  are  two  ways  in  which  a  masonry  dam  may  resist  the 
thrust  of  the  water  ;  viz.,  (1)  by  the  inertia  of  its  masonry,  and 
(2)  as  an  arch.  1.  The  horizontal  thrust  of  the  water  may  be  held 
in  equilibrium  by  the  resistance  of  the  masonry  to  sliding  forward 
or  to  overturning.  A  dam  which  acts  in  this  way  is  called  a  gravity 
dam.  2.  The  thrust  of  the  water  may  be  resisted  by  being  trans- 
mitted laterally  to  the  side-hills  (abutments)  by  the  arch-like  action 
of  the  masonry.  A  dam  which  acts  in  this  way  is  called  an  arched 
dam. 

Only  two  dams  of  the  pure  arch  type  have  ever  been  built.  The 
almost  exclusive  use  of  the  gravity  type  is  due  to  the  uncertainty 
of  our  knowledge  concerning  the  laws  governing  the  stability  of 
masonry  arches.  This  chapter  will  be  devoted  mainly  to  gravity 
dams,  those  of  the  arch  type  being  considered  only  incidentally. 
Arches  will  be  discussed  fully  in  Chapter  XVIII. 

811 


313  MASOXRY    DAMS.  [CHAP.   XIII, 


Art.  1.  Stability  of  Gravity  Dams. 

481.  Principles.  By  the  principles  of  liydrostatics  we  know 
(1)  that  the  pressure  of  a  liquid  upon  any  surface  is  equal  to  the 
weight  of  a  volume  of  the  liquid  whose  base  is  the  area  of  the  im- 
mersed surface  and  whose  height  is  the  vertical  distance  of  the  center 
of  gravity  of  that  surface  below  the  upper  surface  of  the  water  ;  (2) 
that  this  pressure  is  always  perpendicular  to  the  pressed  surface ; 
and  (3)  that,  for  rectangular  surfaces,  this  pressure  may  be  con- 
sidered as  a  single  force  applied  at  a  distance  below  the  upper 
surface  of  the  liquid  equal  to  f  of  the  depth. 

482.  A  gravity  dam  may  fail  (1)  by  sliding  along  a  horizontal 
joint,  or  (2)  by  overturning  about  the  front  of  a  horizontal  joint, 
or  (3)  by  crushing  the  masonr}^,  particularly  at  the  front  of 
any  horizontal  joint.  However,  it  is  admitted  that  by  far  the 
greater  number  of  failures  of  dams  is  due  to  defects  in  the  founda- 
tion. The  method  of  securing  a  firm  foundation  has  already  been 
discussed  in  Part  III ;  and,  hence,  this  subject  will  be  considered 
here  only  incidentally.  There  is  not  much  probability  that  a  dam 
will  fail  by  sliding  forward,  but  it  may  fail  by  overturning  or  by  the 
crushing  of  the  masonry.  These  three  methods  of  failure  will  be 
considered  separately  and  in  the  above  order. 

483.  In  the  discussions  of  this  article  it  will  be  necessary  to 
consider  only  a  section  of  the  wall  included  between  two  vertical 
planes — a  unit  distance  apart — perpendicular  to  the  face  of  the 
wall,  and  then  so  arrange  this  section  that  it  will  resist  the  loads  and 
pressure  put  upon  it ;  that  is,  it  is  sufficient,  and  more  convenient, 
to  consider  the  dam  as  only  a  unit,  say  1  foot,  long. 

484.  NOMENCLATIJEE.  The  following  nomenclature  will  be  used 
throughout  this  chapter  : 

H  =  the  horizontal  pressure,  in  pounds,  of  the  water  against  a 
section  of  the  back  of  the  wall  1  foot  long  and  of  a  height 
equal  to  the  height  of  the  wall. 
Fr=  the  weight,  in  pounds,  of  a  section  of  the  wall  1  foot  long. 
w  =  the  weight,  in  pounds,  of  a  cubic  foot  of  the  masonry. 
h  =  the  height,  in  feet,  of  the  wall ;  i.  e. ,  h  =  E F,  Fig.  68. 
I  =  the  length  of  the  base  of  the  cross  section ;  i.  e.,  I  =  A  B, 

Fig.  68. 
t  =  the  width  of  the  wall  on  tojj  ;  i.  e.,  t  =  D  E,  Fig.  68. 


AKT.   1.] 


STABILITY    OF    GEATITT   DAMS. 


313 


h  =  the  batter  of  the  wall,  i.  e.,  the  inclination  of  the  surface 
per  foot  of  rise — h'  being  used  for  the  batter  of  the  up- 
stream face  and  h^  for  that  of  the  down-stream  face. 
x-=  A  C  =  the  distance  from  the  down-stream  face  of  any  joint  to 
the  point  in  which  a  vertical  through  the  center  of  gravity 
of  the  wall  pierces  the  plane  of  the  base. 
d  =  the  distance  the  center  of  pressure  deviates  from  the  center 
of  the  base. 
62.5  =  the  weight,  in  pounds,  of  a  cubic  foot  of  water, 

485.  Stability  against  Sliding.  The  horizontal  pressure  of 
the  water  tends  to  slide  the  dam  forward,  and  is  resisted  by  the 
friction  due  to  the  weight  of  the  wall. 

486.  Sliding  Force.  The  horizontal  pressure  of  the  water 
against  an  elementary  section  of  the  wall,  by  principle  (1)  of  §  481, 
is  equal  to  the  area  of  the  section  multiplied  by  half  the  height  of 
the  wall,  and  that  product  by  the  weight  of  a  cubic  unit  of  water;  or 


H=h  XlXih  X  62.5  =  31.25  h\ 


(1) 


Notice  that  II  is  the  same  whether  the  pressed  area  is  inclined  or 
vertical ;  that  is  to  say,  H  is  the  horizontal  component  of  the  total 
pressure  on  the  surface. 

487.  Resisting  Forces.     The  weight  of  an  elementary  section  of 
the  wall  is  equal  to  the  area  of  the  vertical  n         E 

cross  section  multiplied  by  the  weight  of  a 
cubic  unit  of  the  masonry.  The  area  of 
the  cross  section,  ABED,  Fig.  68,  equals 

EFxDE+^EFxFB+^D  GxAG 

=  ht+^h'h'+^h'h^    ...     (2) 

Then  the  weight  of  the  elementary  sec- 
tion of  the  wall  is 

W=w{ht  +  \h'h' +  yi'b;)    .     (3) 

The  vertical  pressure  of  the  water  on 
the  inclined  face  increases  the  pressure  on  the  foundation,  and, 
consequently,  adds  to  the  resistance  against  sliding.  The  vertical 
pressure  on  E  B  is  equal  to  the  horizontal  projection  of  that  area 
multiplied  by  the  distance  of  the  center  of  gravity  of  the  surface 
below  the  top  of  the  water  and  by  the  weight  of  a  cubic  unit  of 


ac  F 

Fig.  68. 


314  MASONRY   DAMS.  [CHAP.  XIII. 

water,  or,  the  vertical  pressure  =  FB  X  1  X  ^  h  X  Q2.5  =  h  b'  X 
^h  X  62.5  =  31.25  h'b'. 

488.  If  the  earth  rests  against  the  heel  of  the  dam  (the  bot- 
tom of  the  inside  face),  it  will  increase  the  pressure  on  the  foun- 
dation, since  earth  weighs  more  than  water  ;  on  the  other  hand,  the 
horizontal  pressure  of  the  earth  will  be  a  little  greater  than  that  of 
an  equal  height  of  water.  However,  since  the  net  resistance  with 
the  earth  upon  the  heel  of  the  wall  is  greater  than  with  an  equal 
depth  of  water,  it  will  be  assumed  that  the  water  extends  to  the 
bottom  of  the  wall. 

If  the  water  finds  its  way  under  and  around  the  foundation  of 
the  wall,  even  in  very  thin  sheets,  it  will  decrease  the  pressure  of 
the  wall  on  the  foundation,  and,  consequently,  decrease  the 
stability  of  the  wall.  The  effective  weight  of  the  submerged  por- 
tion of  the  wall  will  be  decreased  62 1-  lbs.  per  cu.  ft.  However,  the 
assumption  that  water  in  hydrostatic  condition  finds  its  way  under 
or  into  a  dam  is  hardly  admissible ;  hence  the  effect  of  buoyancy 
will  not  be  considered.  * 

489.  Co-efficient  of  Friction.  The  values  of  the  co-efficient  of 
friction  most  frequently  required  in  masonry  computations  are  given 
in  the  table  on  page  315.  There  wiU  be  frequent  reference  to  this 
table  in  subsequent  chapters  ;  and  therefore  it  is  made  more  full 
than  is  required  in  this  connection.  The  values  have  been  collected 
from  the  best  authorities,  and  are  believed  to  be  fair  averages.  See 
also  the  table  on  page  276. 

490.  Condition  for  Equilibrium.  In  order  that  the  wall  may 
not  slide,  it  is  necessary  that  the  product  found  by  multiplying  the 
co-efficient  of  friction  by  the  sum  of  the  weight  of  the  wall  and  the 
vertical  pressure  of  the  water  shall  be  greater  than  the  horizontal 
pressure  of  the  water.  That  is  to  say,  in  order  that  the  dam  may 
not  slide  it  is  necessary  that  /x  {W -\- 31.25  h^  h')  shall  be  greater 
than  H;  or,  in  mathematical  language, 

H  31.25  h' 


W  H-  31.25  h'  b'^  w{ht  +  ih'y  -\-^  ?i'  b,)  H-  31.25  A'  b' ' 

*  Since  the  above  was  written,  Jas.  B.  Francis  presented  a  paper  (May  16,  1888) 
before  the  American  Society  of  Civil  Engineers,  which  seems  to  show  that  water 
pressure  is  communicated,  almost  undiminished,  through  a  layer  of  Portland  cement 
mortar  (1  part  cement  and  2  parts  sand)  1  foot  thick. 


A«T.  L] 


STABILITY    OF    GRAVITY    DAMS. 


315 


TABLE   36. 
Co-efficients  of  Friction  fob  Dbt  Masonry. 


Description  of  the  Masonry. 


Soft  limestone  on  soft  limestone,  both  well  dressed 

Brick-work  ou  brick-work,  with  slightly  damp  mortar 

Hard  brick-work  on  hard  brick-work,  with  slightly  damp  mortar 

Point-dressed  granite  on  like  granite 

"  "  "         "  well-dressed  granite 

Common  brick  on  common  brick 

"  "       "  hard  limestone ^ 

Hard  limestone  on  hard  limestone,  with  moist  mortar 

Beton  blocks  (pressed)  on  like  beton  blocks 

Fine-cut  granite  on  pressed  "  " 

Well-dressed  granite  ou  well  dressed  granite 

Polished  limestone  on  polished  limestone 

Well-dressed  granite  on  like  grMnite.  with  fresh  mortar 

Common  brick  on  common  brick,  with  wet  mortar 

Polished  marble  ou  common  brick 

Point-dressed  granite  on  gravel 

"  "        "  dry  clay 

"  "  "        "sand , 

'•  "  "         "  moist  clay 

Wrought  iron  on  well-dressed  limestone 

"  "      "  hard,  well  dressed  limestone,  wet 

Oak,  flatwise,  on  limestone 

"      endwise,  on  limestone 


CO-KFFICIENT. 


0.75 
0.75 
0.70 
0.70 
0.65 
0.65 
0.65 
0.65 
0.65 
0.60 
0.60 
0.60 
0.50 
0.50 
0.45 

0.60 
0.50 
0.40 
0.33 
0.50 
0.25 
0.65 
0.40 


which  reduced  becomes 


62.5  A 

^  ^  w  (2  ^  +  h  {b'  +  bj)  +  62.5  h  b'' 


.     .     (4) 


The  weight  of  a  cubic  foot  of  masonry,  w,  varies  between  125  lbs. 
for  concrete  or  poor  brick-work,  and  160  lbs.  for  granite  ashlar. 
Dams  are  usually  built  of  rubble,  which  weighs  about  150  lbs.  per 
cu.  ft.  To  simplify  the  formula,  we  will  assume  that  the  masonry 
weighs  125  lbs.  per  cu.  ft.;  i.  e.,  that  the  weight  of  a  cubic  foot  of 
masonry  is  twice  that  of  water.  This  assumption  is  on  the  safe  side, 
whatever  the  kind  of  masonry.*     Making  this  substitution  in  (4) 


*  Increased  safety  generally  requires  increased  cost  of  construction,  and  hence 
it  is  not  permissible  to  use  approximate  data  simply  because  the  error  is  on  the  side 
toward  safety.  It  will  be  shown  that  there  is  no  probability  of  any  dam's  failing  by 
sliding,  and  that  the  size,  and  consequently  the  volume  and  cost,  are  determined  by 


316  MASOKRY   DAMS.  [CHAP.  XIII. 

gives 

other  things  being  the  same,  the  thinner  the  wall  at  the  top, 
the  easier  it  will  slide.  If  the  section  of  the  wall  is  a  triangle,  i.  e., 
if  /  =  0,  then  by  equation  (5)  we  see  that  the  dam  is  safe  against 
sliding  when 

''^"•^wk) (^^ 

An  examination  of  the  table  on  page  315  shows  that  there  is  no 
probability  that  the  co-eflBcient  of  friction  will  be  less  than  0.5;  and 
inserting  this  vdue  of  jj.  in  (6)  shows  that  sliding  can  not  take 
place  if  (f  b'  +  b^  >  or  =  1.  To  prevent  overturning,  {b'  +  ^i) 
is  usually  =  or  >  1  (see  Fig.  72,  page  328);  and,  besides,  a  con- 
siderable thickness  at  the  top  (see  §  509)  is  needed  to  resist  the 
shock  of  waves,  etc.  Hence  there  is  no  probability  of  the  dam's 
failing  by  sliding  forward.  Further,  the  co-efficient  of  friction  in 
the  table  on  page  315  takes  no  account  of  the  cohesion  of  the  mor- 
tar, which  ma}^  have  a  possible  maximum  value,  for  best  Portland 
mortar,  of  36  tons  per  sq.  ft.  (500  lbs.  per  sq.  in.);  and  this  gives 
still  greater  security.  Again,  the  earth  on,  and  also  in  front  of,  the 
toe  of  the  wall  adds  greatly  to  the  resistance  against  sliding.  Fi- 
nally, it  is  customary  to  build  masonry  dams  of  uncoursed  rubble 
(§§  213-17),  to  prevent  the  bed- joints  from  becoming  channels  for 
the  leakage  of  water;  and  hence  the  stones  are  thoroughly  inter- 
locked,— which  adds  still  further  resistance.  Therefore  it  is  certain 
that  there  is  no  danger  of  any  masonry  dam's  failing  by  sliding  for- 
ward under  the  pressure  of  still  water. 

491.  It  has  occasionally  happened  that  dams  and  retaining  walls 
have  been  moved  bodily  forward,  sliding  on  their  base;  but  such  an 
occurrence  is  certainly  unusual,  and  is  probably  the  result  of  the 
wall's  having  been  founded  on  an  unstable  material,  perhaps  on  an 
inclined  bed  of  moist  and  uncertain  soil.  In  most  that  was  said  in 
Part  III  concerning  foundations,  it  was  assumed  that  the  founda* 

the  dimensions  required  to  prevent  crushing  and  overturning;  hence  this  approxima- 
tion involves  no  increase  in  the  cost. 


AET.  1.]  STABILITY   OF    GKAYITY    DAMS.  31? 

tion  was  required  to  support  only  a  vertical  load.  AVlien  the  struct- 
ure is  subjected  also  to  a  lateral  pressure,  as  in  dams,  additional 
means  of  security  are  demanded  to  prevent  lateral  yielding. 

When  the  foundation  rests  upon  piles  a  simple  expedient  is  to 
drive  piles  in  front  of  and  against  the  edge  of  the  bed  of  the  founda- 
tion; but  obviously  this  is  not  of  much  value  except  when  the  piles 
reach  a  firmer  soil  than  that  on  which  the  foundation  directly  rests. 
If  the  piles  reach  a  firm  subsoil,  it  will  help  matters  a  little  if  the 
upper  and  more  yielding  soil  is  removed  from  around  the  top  of  the 
pile,  and  the  place  filled  with  broken  stone,  etc.  Or  a  wall  of  piles 
may  be  driven  around  the  foundation  at  some  distance  from  it,  and 
timber  braces  or  horizontal  buttresses  of  masonry  may  be  jjlaced  at 
intervals  from  the  foundation '  to  the  piles.  A  low  masonry  wall  is 
sometimes  used,  instead  of  the  wall  of  piles,  and  connected  with  the 
foot  of  the  main  wall  by  horizontal  buttresses,  whose  feet,  on  the 
counter-wall,  are  connected  by  arches  in  a  horizontal  plane  in  order 
to  distribute  the  pressure  more  evenly. 

In  founding  a  dam  upon  bed-rock,  the  resistance  to  sliding  on 
the  foundation  may  be  greatly  increased  by  leaving  the  bed  rough  ; 
and,  in  case  the  rock  quarries  out  with  smooth  surfaces,  one  or  more 
longitudinal  trenches  may  be  excavated  in  the  bed  of  the  foundation, 
and  afterwards  be  filled  with  the  masonry. 

In  the  proposed  Quaker  Bridge  dam  the  maximum  horizontal 
thrust  of  the  water  is  equal  to  0.597  of  the  weight  of  the  masonry. 

492.  Stability  against  Overturning.  The  horizontal  pres- 
sure of  the  water  tends  to  tijD  the  wall  forward  about  the  front  of 
any  joint,  and  is  resisted  by  the  moment  of  the  weight  of  the  wall. 
Eor  the  present,  it  will  be  assumed  that  the  wall  rests  upon  a  rigid 
base,  and  therefore  can  fail  only  by  overturniug  as  a  whole. 

The  conditions  necessary  for  stability  against  overturning  can  be 
completely  determined  either  by  considering  the  moments  of  the 
several  forces,  or  by  the  principle  of  resolution  of  forces.  In  the 
following  discussion,  the  conditions  will  be  first  determined  by  mo- 
ments, and  afterward  by  resolution  of  forces. 

493.  A.  By  Moments.  The  Overturning  Moment.  The  pressure 
of  the  water  is  perpendicular  to  the  pressed  surface.  If  the  water 
presses  against  an  inclined  face,  then  the  pressure  makes  the  same 
angle  with  the  horizontal  that  the  surface  does  with  the  vertical. 
Since  there  is  a  little  difficulty  in  finding  the  arm  of  this  force,  it  is 


318 


MASONEY   DAMS. 


[chap.  XIIL 


more  convenient  to  deal  with  the  horizontal  and  vertical  components 
of  the  pressure. 

The  horizontal  pressure  of  the  water  can  be  found  by  equation 
(1),  page  313.  The  arm  of  this  force  is  equal  to  -^  1i  (principle  3, 
§  481).     Hence  the  moment  tending  to  overturn  the  wall  is  equal  to 


\Hh  =  ^  31.25  ¥  =  10.42  h\ 


(7) 


which,  for  convenience,  represent  by  J/j . 

494.  The  Resisting  Moments.  The  forces  resisting  the  over- 
turning are  (1)  the  weight  of  the  wall  and  (2)  the  vertical  pressure 
of  the  water  on  the  inclined  face. 

The  weight  of  the  wall  can  be  computed  by  equation  (3),  page 
313.  It  acts  vertically  through  the  center  of  gravity  of  the  cross 
section. 

The  center  of  gravity  can  be  found  algebraically  or  graphically. 
There  are  several  ways  in  each  case,  but 
the  following  graphical  solution  is  the  sim- 
plest. In  Fig.  69,  draw  the  diagonals  D  B 
and  A  E,  and  lay  off  J[  /  =  ^  / ;  then 
draw  D  J,  and  mark  the  middle  of  it  Q. 
The  center  of  gravity,  0,  of  the  area 
ABED  is  at  a  distance  from  Q  towards 
B  equal  to  ^  ^  ^.  This  method  is  appli- 
cable to  any  four-sided  figure. 

The  position  of  the  center  of  gravity  can 
also  be  found  algebraically  by  the  principle 
that  the  moment  of  the  entire  mass  about 
any  point,  as  ^,  is  equal  to  the  moment  of  the  part  A  D  G,  plus 
che  moment  of  the  portion  D  E  F  G,  plus  the  moment  of  the  part 
E  B  F, — all  about  the  same  point,  A.  Stating  this  principle  alge- 
braically gives 

=  i^h'y-\-ht-^ih'b,)x,  .      ...   (8) 

m  which  x  =  the  distance  A  C.     Solving  (8)  gives 


z  =  — 


^h{b'  +  bj  +  i 


(9) 


ART.   I. J  STABILITY    OF    GRAVITY   DAMS.  310 

The  arm  of  the  weight  is  ^  C  (:=  x),  and  therefore  the  mo- 
ment is 

Wx  AC=w[ht-\-^h'{y +  b^)]x,  .     .     .     (10) 

which,  for  convenience,  represent  by  M, . 

495.  The  vertical  pressure  of  the  water  on  the  inclined  face, 
E  B,  has  been  computed  in  §  487,  which  see.  This  force  acts  ver- 
tically between  i^^and  B,  at  a  distance  from  B  equal  to  \  F  B;  the 
arm  of  this  force  \s  A  B  -  \  F  B  =  I  -  I  hh'  =  h  h^  +  t  ^  ^Jih'. 
Therefore,  the  moment  of  the  vertical  pressure  on  the  inclined 
face  is 

31.25  h'  h'  (li  b^  +  t  +  ^k  V),     ....     (11) 

which,  for  convenience,  represent  by  J/3 .     Of  course,  if  the  pressed 
face  is  vertical,  J/3  will  be  equal  to  zero. 

496.  The  moment  to  resist  overturning  is  equal  to  the  sum  of 
(10)  and  (11)  above,  or  J/,  +  J/3 . 

The  moment  represented  by  the  sum  of  J/^  and  J/3  can  be  deter- 
mined directly  by  considering  the  pressure  of  the  water  as  acting 
perpendicular  to  E  B  at  ^  F  B  from  B ;  the  arm  of  this  force  is  a 
line  from  A  perpendicular  to  the  line  of  action  of  the  pressure.  If 
the  cross  section  were  known,  it  would  be  an  easy  matter  to  measure 
this  arm  on  a  diagram;  but,  in  designing  a  dam,  it  is  necessary  to 
know  the  conditions  requisite  for  stability  before  the  cross  section 
can  be  determined,  and  hence  the  above  method  of  solution  is  the 
better. 

497.  Condition  for  Equilibrium.  In  order  that  the  wall  may 
not  turn  about  the  front  edge  of  a  joint,  it  is  necessary  that  the 
overturning  moment,  J/, ,  as  found  by  equation  (7),  shall  be  less 
than  the  sum  of  the  resisting  moments,  J/^  and  J/3 ,  as  found  by 
equations  (10)  and  (11);  or,  in  other  words,  the  factor  against  over-. 

turning  =  -^-^^ — '- (12) 

498.  Factor  of  Safety  against  Overturning.  In  computing  the 
stability  against  overturning,  the  vertical  pressure  of  the  water 
against  the  inside  face  is  frequently  neglected;  i.e.,  it  is  assumed 
that  J/3 ,  as  above,  is  zero.  This  assumption  is  always  on  the  safe 
side.  Computed  in  this  way,  the  factor  of  safety  against  overturn- 
ing for  the  proposed  Quaker  Bridge  dam,  which  when  completed 


320  MASOXRY    DAMS  [CHAP.  XIII. 

will  be  considerably  the  largest  dam  in  the  world,  varies  between 
2.07  and  3.68.  Krantz,*  who  included  the  vertical  component  in 
his  computations,  considers  a  factor  of  2.5  to  5.55  as  safe,  the  larger 
value  being  for  the  largest  dam,  owing  to  the  more  serious  conse- 
quences of  failure.  The  greater  the  factor  of  safety  provided  for, 
the  greater  is  the  first  cost;  and  the  less  the  factor  of  safety,  the 
greater  the  expense  of  maintenance,  including  a  possible  reconstruc- 
tion of  the  structure. 

499.  B.  By  Resolution  of  Forces.  In  Fig.  70,  K  is  the  center 
of  pressure  of  the  water  on  the  back  of 
the  wall.  K  B  =\  E  B.  o  is  the  center 
of  gravity  of  the  wall, — found  as  already 
described.  Through  K  draw  a  line,  K  a, 
perpendicular  to  E  B;  through  o  draw  a 
vertical  line  o  a.  To  any  convenient 
scale  lay  off  ab  equal  to  the  total  pressure 
of  the  water  against  E  B,  and  to  the 
same  scale  make  af  equal  to  the  weight 
of  an  elementary  section  of  the  wall. 
Complete  the  parallelogram  a  h  ef.  The 
diagonal    ae    intersects   the  base  of  the 

wall  at  N. 

500.  On  the  assumption    that    the    masonry   and    foundation 

are  absolutely  incompressible  (the  compressibility  will  be  considered 

presently),  it  is  clear  that  the  wall  will  not  overturn  as  long  as  the 

resultant  ae  intersects  the  base  AB  between  A  and  B.      The  factor 

A  C 
against  overturning  is  -^^--„  which  is  the  equivalent  of  equation  (12). 

The  wall  can  not  slide  horizontally  on  the  base,  when  the  angle 
JVaCis  less  than  the  angle  of  repose,  i.  e.,  when  tan  NaC  is,  less 
than  the  co-eflacient  of  friction.  The  factor  against  sliding  is  equal 
to  the  co-efficient  of  friction  divided  by  fan  NaC,  which  is  only 
another  way  of  stating  the  conclusion  drawn  from  equation  (4), 
page  315. 

501.  Stability  against  Crushing.  The  preceding  discussion 
of  the  stability  against  overturning  is  on  the  assumption  that  the 
masonry  does  not  crush.     This  method  of  failure  will  now  be  con- 

*  "  Study  of  Reservoir  Walls,"  Mahan's  translation,  p.  53. 


ART.  1.] 


STABILITY    OF   GRAVITY    DAMS. 


321 


sidered.  When  the  reservoir  is  empty,  the  pressure  tending  to 
produce  crushing  is  tlie  weight  of  the  dam  alone,  which  pressure  is 
distributed  uniformly  over  the  horizontal  area  of  the  wall.  When 
the  reservoir  is  full,  the  thrust  of  the  water  modifies  the  distribution 
of  the  pressure,  increasing  the  pressure  at  the  front  of  the  wall  and 
decreasing  it  at  the  back.  We  will  now  determine  the  law  of  the 
variation  of  the  pressure. 

Let  A  B,  Fig.  71,  represent  the  base  of  a  vertical  section  of  the 
dam ;    or   A  B   may   represent   the   rect- 
angular base  (whose  width  is   a  unit)  of 
any  two  bodies  which  are  pressed  against 
each  other  by  any  forces  whatever. 


FiQ.  71. 


Jlf=the  resulting  moment  (about  ^)  of 

all   the   external   forces.     In   the 

case  of  a  dam,  M  =  M^  —  M^, — see 

equations  (7)  and  (11). 
W  =  the  total  normal  pressure  on  A  B. 

In  the  case  of  a  dam,  W  =  the  weight  of  the  masonry. 
P  =  the  maximum  pressure,  per  unit  of  area,  at  A. 
p  =  the  change  in  unit  pressure,  per  unit  of  distance,  from  A 

towards  B. 
X  =  any  distance  from  A  towards  B. 
P  —  J}  X  =  the  pressure  per  unit  at  a  distance  x  from  A. 
Y  =  a,  general  expression  for  a  vertical  force. 

The  remainder  of  the  nomenclature  is  as  in  §  484,  page  313. 
Taking  moments  about  A  gives 

M-  Wx+    r  (P -px)dx.x  =  0;  .     .     .     (13) 

M-Wx^^Pf-\pV  =  ^ (14) 

For  equilibrium,  the  sum  of  the  forces  normal  to  -4  ^  must  also 
be  equal  to  zero  ;  or 

:^^F=-r+    r  {P  -px)dx  =  0,    ,     .     .     (15) 

from  which 

pT  =  'iPl-2W.     .     ,     .     .     ,     ,     (16) 


323  MASONRY    DAMS.  [CHAP.   XIIL 


502.  Maximnm  Pressure.     Combining  (16)  with  (14)  and  re 
ducing,  _ 

4r_6J^       6^ 

I        r    '^  r ^  ^ 

tf  the  stability  against  overturning  be  determined  algebraically,' ■?'.  e., 
by  equation  (12),  then  JIfand  x  are  known,  and  P  can  be  computed 
by  equation  (17). 

If  the  wall  is  symmetrical  x  =  ^l,  and  (17)  becomes 

W       6  M 

Equation  (18)  is  a  more  general  form  of  equation  (1),  page  205, 
since  in  the  latter  there  is  but  one  external  force  acting,  and  that 
is  horizontal. 

W 

In  equation  (18),  notice  that  —  is  the  uniform  pressure  ouAB 

due  to  the  weight  of  the  wall ;  also  that  -^  is  the  increase  of  pres- 

sure  at  A  due  to  the  tendency  to  overturn,  and  that  consequently 
the  uniform  pressure  at  B  is  decreased  a  like  amount. 

503.  The  maximum  pressure  may  be  found  also  in  another  way. 
Assume  that  N,  Fig.  71,  is  the  center  of  pressure.  Let  j9j  {—  B L) 
represent  the  pressure  at  B,  and^^  (=  ^-S')that  at  A  ;  and  any 
intermediate  ordinate  of  the  trapezoid  A  B  L  K  will  represent  the 
pressure  at  the  corresponding  point.  Then,  since  the  forces  acting 
on  A  B  must  be  in  equilibrium  for  translation,  the  area  of  the 
trapezoid  will  represent  the  entire  pressure  on  the  base  A  B.  Stated 
algebraically,  this  is 

Pi^^  l=W. (19) 

ifJi&o,  since  the  forces  acting  on  A  B  must  be  in  equilibrium  for 
rotation,  the  moment  of  the  pressure  to  the  right  of  N  must  be 
equal  to  that  to  the  left ;  that  is  to  say,  the  center  of  gravity  of  the 
trapezoid  A  B  L  ^must  lie  in  the  line  N  J.  By  the  principles  of 
analytical  mechanics,  the  ordinate  ^  ^  to  the  center  of  gravity 
AB  LEis, 

-^IP_P^\     ......     (20) 


ART.   1.]  STABILITY    OF   GRAVITY   DAMS.  323 

Solvmg  (19)  and  (20)  gives 

If  the  wall  is  a  right-angled  triangle  with  the  right  angle  at  A, 

X  =  -^I,  which,  substituted  in  the  above  expression,  shows  that  the 

2  W 
pressure  at  A  is  — - — ,  and  also  that  the  pressure  at  B  is  zero, — all 

of  which  is  as  it  should  be.  Equation  (21)  is  a  jjef'fectly  general 
expression  for  the  pressure  between  any  tioo  plane  surfaces  pressed 
t ofj ether  hy  normal  forces.  Notice  that  equation  (21)  is  identical 
with  the  first  two  terms  of  the  right-hand  side  of  equation  (17). 

The  form  of  (21)  can  be  changed  by  substituting  for  x  its  value 
\l  —  d\  then 

^'^  -  /  +    r ^  ^ 

Equation  (22)  gives  the  pressure  at  A  due  to  the  weight  of  the 
wall ;  but  it  will  also  give  the  maximum  pressure  on  the  base  due 
to  both  the  vertical  and  the  horizontal  forces,  provided  d  be  taken 
as  the  distance  from  the  middle  of  the  base  to  the  point  in  which 
the  resultant  of  all  the  forces  cuts  the  base.  Therefore  we  may 
write 

P-^^'^. (23) 

504.  Equation  (23)  is  the  equivalent  of  equation  (17),  page  322. 
It  is  well  to  notice  that  equation  (23)  is  limited  to  rectangular  hori- 
zontal cross-sections,  since  it  was  assumed  that  the  pressure  on  the 
section  varies  as  the  distance  back  from  the  toe.  If  the  stability 
against  overturning  is  determined  algebraically,  as  by  equation  (12), 
then  equation  (17)  is  the  more  convenient ;  but  if  the  stability  is 
determined  graphically,  as  in  Fig.   70,  then  equation    (23)  is  the 

2  W 
simpler.     Notice  that  \i  d  =  \l,  P  —  —r-,  which  is  in  accordance 

with  what  is  known  in  the  theory  of  arches  as  the  principle  of  the 
middle  third  ;  that  is,  as  long  as  the  center  of  pressure  lies  within 
the  middle  third  of  the  joint,  the  maximum  pressure  is  not  more 
than  twice  the  mean,  and  there  is  no  tension  in  any  part  of  the 
joint. 


324:  MASONET   DAMS.  [CHAP.  XIII. 

IF 

Notice,  in  equation  (23),  that  -y-  is  the  uniform  load  on  the  base; 

6  Wd 
and  also  that  — y. —  is  the  increase  of  pressure  due  to  the  eccentric- 
ity of  the  load.     It  is  immaterial  whether  the  deviation  d  is  caused 
by  the  form  of  the  wall  or  by  forces  tending  to  produce  overturn- 
ing. 

505.  Tension  on  the  Masonry.  By  an  analysis  similar  to  that 
above,  it  can  be  shown  that  the  decrease  in  pressure  at  B,  due  to 
the  overturning  moment,  is  equal  to  the  increase  at  ^4.  If  d  —  ^I, 
then  by  equation  (23)  the  increase  at  A  and  decrease  at  i?  is  W, 
that  is  to  say,  the  pressure  at  ^  is  2  W  and  that  at  B  is  zero. 
Therefore,  if  the  center  of  pressure  departs  more  than  ^  I  from  the 
center  of  the  base,  there  will  be  a  minus  pressure,  i.  e.  tension,  at 
B.  Under  this  condition,  the  triangle  A  V K' ,  in  Fig.  71,  page 
321,  represents  the  total  pressure,  and  the  triangle  J5  FX'the  total 
tension  on  the  masonry, — A  K'  being  the  maximum  pressure  at  A, 
and  B  L'  the  maximum  tension  at  B. 

If  a  good  quality  of  cement  mortar  is  used,  it  is  not  unreason- 
able to  count  upon  a  little  ■  resistance  from  tension.  As  a  general 
rule,  it  is  more  economical  to  increase  the  quantity  of  stone  than  the 
quality  of  the  mortar  ;  but  in  dams  it  is  necessary  to  use  a  good 
mortar  to  prevent  (1)  leakage,  (2)  disintegration  on  the  water  side, 
and  (3)  crushing.  If  the  resistance  due  to  tension  is  not  included 
in  the  computation,  it  is  an  increment  to  the  computed  margin  of 
safety. 

506.  If  the  masonry  be  considered  as  incapable  of  resisting  by 
tension,  then  when  d  in  equation  (23)  exceeds^?  the  total  pres- 
sure will  be  borne  on  A  V,  Fig.  71.  In  this  case  A  N'  (the  distance 
from  A  to  the  point  where  the  resultant  pierces  the  base)  will  be 
less  than  ^  L  It  A  K"  represents  the  maximum  pressure  P,  then 
the  area  of  the  triangle  A  V K"  will  represent  the  total  weight  W. 
The  area  of  A  V  K"  ^  \  A  K"  Y.  AV  =  \P  Y  Z  A  N'\  Hence 
\Py'^AN'^  W,ox 

2  W              2  W 
^^TAW'^^'^iilr-d) ^^^^ 


To  illustrate  the  difference    between  equations  (23)  and  (24)^ 


ART.   1.]  STABILITY    OF   GRAVITY   DAMS.  525 

assume  that  the  distance  from  the  resultant  to  the  center  of  the  base 
is  one  quarter  of  tlie  length  of  the  base,  /.  e.,  assume  that  d  =■  \l. 
Then,  by  equation  (23),  the  maximum  pressure  at  A  is 

^  -  I  ^  r-4.  ~^^  r ^'^^^ 

and  by  equation  (2-4)  it  is 

2  W                 W 
^-3(^/_x/)--3    ; ^-^) 

That  is  to  say,  if  the  masonry  is  capable  of  resisting  tension,  equa- 
tion (25)  shows  that  the  maximum  pressure  is  2^  times  the  pressure 
due  to  the  weight  alone  ;  and  if  the  masonry  is  incapable  of  resist- 
ing tension,  equation  (20)  shows  that  the  maximum  pressure  is  2f 
times  the  pressure  due  to  the  weight  alone. 

Notice  that  equation  (24)  is  not  applicable  when  d  is  less  than 
^I ;  in  that  case,  equation  (23)  must  be  nsed. 

507.  Limiting  Pressure.  As  a  preliminary  to  the  actual  design- 
ing of  the  section,  it  is  necessary  to  fix  upon  the  maximum  pressure 
per  square  foot  to  which  it  is  proposed  to  subject  the  masonry.  Of 
course,  the  alloAvalile  pressure  depends  upon  the  quality  of  the 
masonry,  and  also  upon  the  conditions  assumed  in  making  the  com- 
putations. It  appears  to  be  the  custom,  in  practical  computations, 
to  neglect  the  vertical  pressure  on  the  inside  face  of  the  dam,  i.  e., 
to  assume  that  J/j ,  equation  (11),  page  319,  is  zero  ;  this  assumption 
is  always  on  the  safe  side,  and  makes  the  maximum  pressure  on  the 
outside  toe  appear  greater  than  it  really  is.  Computed  in  this  way, 
the  maximum  pressure  on  rubble  masonry  in  cement  mortar  in 
some  of  the  great  dams  of  the  world  is  from  11  to  14  tons  per  sq. 
ft.  The  proposed  Quaker  Bridge  dam  is  designed  for  a  maximum 
pressure  of  16.6  tons  per  sq.  ft.  on  massive  rubble  in  Portland 
cement  mortar. 

For  data  on  the  strength  of  stone  and  brick  masonry,  see  §§ 
221-23  and  §§  246-48,  respectively. 

508.  The  actual  pressure  at  the  toe  will  probably  be  less  than 
that  computed  as  above.  It  was  assumed  that  the  weight  of  the 
wall  was  uniformly  distributed  over  the  base  ;  but  if  the  batter  is 
considerable,  it  is  probable  that  the  pressure  due  to  the  weight  of 
the  wall  will  not  vary  uniformly  from  one  side  of  the  base  to  the 


326  MASOXRY    DAMS.  [CHAP.  XIII. 

other,  but  will  be  greater  on  the  central  portions.  The  actual 
maximum  will,  therefore,  probably  occur  at  some  distance  back 
from  the  toe.  Neither  the  actual  maximum  nor  the  point  at  which 
it  occurs  can  be  determined. 

Professor  Kankine  claims  that  the  limiting  pressure  for  the  out- 
side toe  should  be  less  than  for  the  inside  toe.  Xotice  that  the 
preceding  method  determines  the  maximum  vertical  pressure. 
When  the  maximum  pressure  on  the  inside  toe  occurs,  the  only 
force  acting  is  the  vertical  pressure  ;  but  when  the  maximum  on 
the  outside  occurs,  the  thrust  of  the  water  also  is  acting,  and  there- 
fore the  actual  pressure  is  the  resultant  of  the  two.  With  the  pres- 
ent state  of  our  knowledge,  we  can  not  determine  the  effect  of  a 
horizontal  component  upon  the  vertical  resistance  of  a  block  of  stone, 
but  it  must  weaken  it  somewhat. 

AkT.    2.    OUTLIXES   OF   THE    DeSIGX. 

509.  Width  on  Top.  As  far  as  the  forces  already  considered 
are  concerned,  the  width  of  the  wall  at  the  top  might  be  nothing, 
since  at  this  point  there  is  neither  a  pressure  of  water  nor  any 
weight  of  masonry.  But  in  practice  we  must  consider  the  shock  of 
waves  and  ice,  Avhich  in  certain  cases  may  acquire  great  force  and 
prove  very  destructive  to  the  upper  portion  of  the  dam.  This  force 
can  not  be  computed,  and  hence  the  width  on  top  must  be  assumed. 
This  width  depends  to  a  certain  extent  upon  the  height  and  length 
of  the  dam.  The  top  of  large  dams  may  be  used  as  a  roadway. 
Krantz  *  says  that  it  is  "  scarcely  possible  to  reduce  the  top  vridth 
below  2  metres  (6.5  ft.)  for  small  ponds,  nor  necessary  to  make  it 
more  than  5  metres  (16.4  ft.)  for  the  largest." 

Fig.  72,  page  .328,  gives  the  width  on  top  of  Krantz's  profile  type, 
and  also  of  the  profile  recommended  by  the  engineers  of  the 
Aqueduct  Commission  for  the  proposed  Quaker  Bridge  dam. 

510.  The  Profile.  In  designing  the  vertical  cross  section  of  a 
gravity  dam  to  resist  still  water,  it  is  necessary  to  fulfill  three  con- 
ditions :  (] )  To  prevent  sliding  forward,  equation  (4),  page  315, 
must  be  satisfied;  (2)  to  resist  overturning,  equation  (12),  page  319, 
must  be  satisfied  ;  and  (3)  to  resist  crushing,  equation  (23),  page 
323,  or  (24),  page  324,  must  be  satisfied.     As  these  equations  really 

*  "  Study  of  Reservoir  Walls,"  Mahan's  translation,  p.  35. 


AET.  2.]  OUTLINES   OF   THE    DESIGN".  327 

involve  only  three  variables,  viz. :  h,  bx,  and  h', — the  height  of  the 
dam  and  the  batter  of  the  two  faces, — they  can  be  satisfied  exactly. 
It  has  been  shown  that  there  is  no  danger  of  the  dam's  sliding  for- 
ward even  if  the  width  on  top  is  zero  ;  and  hence  there  are  practi- 
cally but  two  conditions  to  be  fulfilled  and  two  variables  to  be 
determined.  To  prevent  overturning  when  the  reservoir  is  full, 
equation  (12)  must  be  satisfied  ;  and  to  prevent  crushing,  equation 
(23) — or  (2i) — must  be  satisfied  for  the  points  (Figs.  09,  70,  etc.) 
when  the  reservoir  is  full,  and  for  B  when  the  reservoir  is  empty. 

Although  it  is  possible  to  satisfy  these  conditions  exactly,  the 
theoretical  profile  can  be  obtained  only  by  successive  approxima- 
tions. This  is  done  by  dividing  the  profile  into  elementary  hori- 
zontal layers,  beginning  at  the  top,  and  determining  the  dimension 
of  the  base  of  each  layer  separately.  The  theoretical  width  at  the 
top  being  zero  and  the  actual  width  being  considerable,  a  portion  of 
the  section  at  the  top  of  the  dam  will  be  rectangular.  A  layer  being 
given,  and  the  profile  of  the  portion  above  it  being  known,  certaiii 
dimensions  are  assumed  for  the  lower  base  of  the  layer ;  and  the 
stability  against  overturning  is  then  determined  by  appl^'iug  equa- 
tion (12),  or  by  the  method  of  Fig.  70  (page  320).  The  maximum 
pressure  at  A  is  then  found  by  applying  equation  (17)  or  (^3),  after 
which  the  maximum  pressure  at  B  when  the  reservoir  is  empty 
must  be  determined  by  applying  equation  (23).  If  the  first  dimen- 
sions do  not  give  results  in  accordance  with  the  limiting  conditions, 
others  must  be  assumed  and  the  computations  repeated.  A  third 
approximation  will  probably  rarely  be  needed. 

It  is  not  necessary  to  attempt  to  satisfy  these  equations  precisely, 
since  there  are  a  number  of  unknown  and  unknowable  factors,  as  the 
weight  of  the  stone,  the  quality  of  the  mortar,  the  character 
of  the  foundation,  the  quality  of  the  masonry,  the  hydrostatic 
pressure  under  the  mass,  the  amount  of  elastic  yielding,  the 
force  of  the  waves  and  of  the  ice,  etc.,  which  have  more  to  do 
with  the  ultimate  stability  of  a  dam  than  the  mathematically  exact 
profile.  It  is  therefore  sufficient  to  assume  a  trial  profile,  being 
guided  in  this  by  the  matters  referred  to  in  §  511  and  §  512,  and 
test  it  at  a  few  points  by  applying  the  preceding  equations  ;  a  few 
modifications  to  more  nearly  satisfy  the  mathematical  conditions  cr 
to  simplify  the  profile  is  as  far  as  it  is  wise  to  carry  the  theoretical 
determination  of  the  profile. 


328 


MASOXRY    DAMS. 


[chap.   XIII. 


511.  Krautz's  Study  of  Reservoir  Walls,  translated  from  the 
French  by  Capt.  F.  A,  Mahan,  U.  S.  A.,  gives  the  theoretical  pro- 
files for  dams  from  16.40  ft.  (5  metres)  to  164  ft.  (50  metres)  high. 
The  faces  are  arcs  of  circles.  The  mathematical  Avork  of  determin- 
ing the  profiles  is  not  given  ;  luit  it  is  evident  that  the  polygonal 
profile  was  deduced  as  above  described,  and  that  an  arc  of  a  circle 
was  then  drawn  to  average  the  irregularities.  The  largest  of  these 
profiles  is  shown  in  Fig.  72  by  the  broken  line.  The  others  are 
simply  the  upper  portion  of  the  largest,  with  the  thickness  and  the 
height  of  the  portion  above  the  water  decreased  somewhat  and  the 
radius  of  the  faces  modified  correspondingly. 


..53.'i2..... 


Ji?±t3.iC 


^ibi^^a.17 

-^---r 


The  larger  profile  of  Fig.  72  is  that  recommended  by  the  engi- 
neers of  the  Aqueduct  Commission  for  the  proposed  Quaker  Bridge 
dam.     The  profiles  of  most  of  the  high  masonry  dams  of  the  world 


AET.   2.]  OUTLINES   OF   THE   DESIGN.  329 

are  exceedingly  extravagant,  and  hence  it  is  not  worth  while  to  give 
examples. 

512.  Prof.  Wm.  Cain  has  shown  *  that  the  equations  of  condi- 
tion are  nearly  satisfied  by  a  cross  section  composed  of  two  tra- 
pezoids, the  lower  and  larger  of  which  is  the  lower  part  of  a  triangle 
having  its  base  on  the  foundation  of  the  dam  and  its  apex  at  the 
surface  of  the  water,  and  the  upper  trapezoid  having  for  its  top  the 
predetermined  width  of  the  dam  on  top  (§  509),  and  for  its  sides 
nearly  vertical  lines  which  intei'sect  the  sides  of  the  lower  trapezoid. 
The  width  of  the  dam  at  the  bottom  is  obtained  by  applying  the 
equations  of  condition  as  above.  The  relative  batter  of  the  up- 
stream and  down-stream  faces  depends  upon  the  relative  factors 
of  safety  for  crushing  and  overturning.  This  section  gives  a 
factor  of  safety  which  increases  from  bottom  to  top, — an  important 
feature. 

513.  The  Plan.  If  the  wall  is  to  be  one  side  of  a  rectangular 
reservoir,  all  the  vertical  sections  will  be  alike ;  and  therefore  the 
heel,  the  toe,  and  the  crest  will  all  be  straight.  If  the  wall  is  to  be 
a  dam  across  a  narrow  valley,  the  height  of  the  masonry,  and  conse- 
quently its  thickness  at  the  bottom,  will  be  greater  at  the  center 
than  at  the  sides.  In  this  case  the  several  vertical  cross  sections 
may  be  placed  so  that  (1)  the  crest  Avill  be  straight,  or  (2)  so  that 
the  heel  will  be  straight  in  plan,  or  (3)  so  that  the  toe  will  be 
straight  in  plan.  Since  the  up-stream  face  of  the  theoretical  pro- 
file is  nearly  vertical  (see  Fig.  72),  there  will  be  very  little  difference 
in  the  form  of  the  dam  whether  the  several  cross  sections  are 
placed  in  the  first  or  the  secoud  position  as  above.  If  the  crest  is 
straight,  the  heel,  in  plan,  will  be  nearly  so  ;  if  the  crest  is  straight, 
the  toe,  in  plan,  will  be  the  arc  of  a  circle  such  that  the  middle 
ordinate  to  a  chord  equal  to  the  span  (length  of  the  crest)  will  be 
equal  to  the  maximum  thickness  of  the  dam ;  and  if  the  toe  is 
made  straight,  the  crest  will  become  a  circle  of  the  same  radius. 
This  shows  that  strictly  speaking  it  is  impossible  to  have  a  straight 
gravity  dam  across  a  valley,  since  either  the  crest  or  toe  must  be 
curved.  The  question  then  arises  as  to  the  relative  merits  of  these 
two  forms. 

514.  Straight  Crest  vs.  Straight  Toe.     The  amount  of  masonry 

*  Engineering  News,  voL  xix.  pp.  51^13. 


330  MASONEY   DAMS.  [CHAP.  XIII. 

in  the  two  forms  is  the  same,  since  the  vertical  sections  at  all  points 
are  alike  in  both.* 

The  stability  of  the  two  forms,  considered  only  as  gravity  dams, 
is  the  same,  since  the  cross  sections  at  like  distances  from  the  center 
are  the  same. 

The  form  with  a  curved  crest  and  straight  toe  will  have  a  slight 
advantage  due  to  its  possible  action  as  an  arch.  However,  it  is  not 
necessary  to  discuss  further  the  relative  advantages  of  these  two 
types,  since  it  will  presently  be  shown  that  both  the  toe  and  the  crest 
of  a  gravity  dam  should  be  curved. 

515.  Gravity  vs.  Arch  Dams.  A  dam  of  the  pure  gravity  type 
is  one  in  which  the  sole  reliance  for  stability  is  the  weight  of  the 
masonry.  A  dam  of  the  pure  arch  type  is  one  relying  solely  upon 
the  arched  form  for  stability.  With  the  arched  dam,  the  pressure 
of  the  water  is  transmitted  laterally  through  the  horizontal  sections 
to  the  abutments  (side  hills).  The  thickness  of  the  masonry  is  so 
small  that  the  resultant  of  the  horizontal  pressure  of  the  Avater  and 
the  weight  of  the  masonry  passes  outside  of  the  toe  ;  and  hence, 
considered  only  as  a  gravity  dam,  is  in  a  state  of  unstable  equilib- 
rium. If  such  a  dam  fails,  it  will  probably  be  by  the  crushing 
of  the  masonry  at  the  ends  of  the  horizontal  arches.  In  the 
present  state  of  our  knowledge  concerning  the  elastic  yielding  of 
masonry,  we  can  not  determine,  with  any  considerable  degree  of 
accuracy,  the  distribution  of  the  pressure  over  the  cross  section  of 
the  arch  (see  Art.  1,  Chap.  XVIII). 

If  it  were  not  for  the  incompleteness  of  our  knowledge  of  the 
laws  governing  the  stability  of  masonry  arches,  the  arch  dam  would 
doubtless  be  the  best  type  form,  since  it  requires  less  masonry  for 
any  particular  case  than  the  pure  gravity  form.  The  best  infor- 
mation we  have  in  regard  to  the  stability  of  masonry  arches  is  de- 
rived from  expe}-ience.  The  largest  vertical  masonry  arch  in  the 
world  has  a  span  of  only  220  feet.  There  are  but  two  dams  of  the 
pure  arch  type  in  the  world,  viz. :  the  Zola  f  in  France  and  the 

*  If  the  valley  across  which  the  dam  is  built  has  any  considerable  longitudinal  slope, 
as  it  usually  will  have,  there  will  be  a  slight  difference  according  to  the  relative  posi- 
tion of  the  two  forms.  If  two  ends  remain  at  the  same  place,  the  straight  toe  throws 
the  dam  farther  up  the  valley,  makes  the  base  higher,  and  consequently  slightly  de- 
creases the  amount  of  raasonrj^. 

+  For  description,  see  Report  on  Quaker  Bridge  Dam,  Engineering  Neios,  vol.  xix. 
p.  6  et  seq. 


ART.  2.]  OUTLINES   OF   THE    DESIGN.  331 

Bear  Valley*  in  Southern  California.  The  length  of  the  former  is 
205  feet  on  top,  height  122  feet,  and  radius  158  feet;  the  length 
of  the  latter  is  230  feet  on  top,  height  64  feet,  radius  of  top 
335  feet  and  of  the  bottom  226  feet.  The  experience  with  large 
arches  is  so  limited  (see  Table  63,  page  502),  as  to  render  it  un- 
wise to  make  the  stability  of  a  dam  depend  wholly  upon  its  action 
as  an  arch,  except  under  the  most  favorable  conditions  as  to  rigid 
side-hills  and  also  under  the  most  unfavorable  conditions  as  to  cost 
of  masonry.  Notice  that  with  a  dam  of  the  pure  arch  type,  the 
failure  of  one  joart  is  liable  to  cause  the  failure  of  the  whole ;  while 
with  a  gravity  section,  there  is  much  less  danger  of  this.  Further, 
since  the  average  pressure  on  the  end  arch  stones  increases  with  the 
span,  the  arch  form  is  most  suitable  for  short  dams. 

516.  Curved  Gravity  Dams.  Although  it  is  not  generally  wise 
to  make  the  stability  of  a  dam  depend  entirely  upon  its  action  as 
an  arch,  a  gravity  dam  should  be  built  in  the  form  of  an  arch,  /.  e., 
with  both  crest  and  toe  curved,  and  thus  secure  some  of  the  advan- 
tages of  the  arch  type.  The  vertical  cross  section  should  be  so  jjro- 
portioned  as  to  resist  the  water  pressure  by  the  weight  of  the 
masonry  alone,  and  then  any  arch-like  action  will  give  an  addi- 
tional margin  for  safety.  If  the  section  is  proportioned  to  resist 
by  its  weight  alone,  arch  action  can  take  place  only  by  the  elastic 
5nelding  of  the  masonry  under  the  water  pressure  ;  but  it  is  known 
that  masonry  will  yield  somewhat,  and  that  therefore  there  will  be 
some  arch  action  in  a  curved  gravity  dam.  Since  but  little  is  known 
about  the  elasticity  of  stone,  brick,  and  mortar  (see  §  16),  and  noth- 
ing at  all  about  the  elasticity  of  actual  masonry,  it  is  impossible 
to  determine  the  amount  of  arch  action,  i.  e.,  the  amount  of  pres- 
sure that  is  transmitted  laterally  to  the  abutments  (side-hills). 

That  it  is  possible  for  a  dam  to  act  as  an  arch  and  a  gravity  dam 
at  the  same  time  is  shown  as  follows  :  "  Conceive  a  dam  of  the 
pure  arch  type,  of  thin  rectangular  cross  section  so  as  to  have  no 
appreciable  gravity  stability.  Conceive  the  dam  to  be  made  up  of 
successive  horizontal  arches  with  key-stones  vertically  over  each 
other.  The  thrust  in  each  arch  will  increase  with  the  depth,  but 
the  spans  will,  under  the  ordinary  practical  conditions,  decrease 
with  the  depth,  so  that  the  tendency  to  'settle  at  the  crown '  (move 
horizontally)  will  be  approximately  equal  in  each.     If  now  we  adopt 

*  For  description,  see  Engineering  News,  vol.  six.  pp.  513-15. 


332  MASOXRY    DAMS.  [CHAP.    XIII. 

a  triangular  in  place  of  a  rectangular  cross  section,  we  increase  the 
areas  and  decrease  the  unit  pressures  from  arch-thrust  as  we  go 
down,  and  hence  decrease  compression  and  consequent  horizontal 
'  settlement '  of  the  arches  ;  in  other  words,  we  introduce  a  tendency 
in  the  water  face  of  the  dam  to  rotate  about  its  lower  edge.  But 
this  is  precisely  the  tendency  which  results  from  the  elastic  action 
of  the  mass  in  respect  to  gravity  stability,  which  latter  we  have  at 
the  same  time  introduced  by  adojDting  the  gravity  section.  Hence 
the  two  act  in  perfect  harmony,  and  there  will  be  a  certain  size  of 
triangular  section  (theoretically, — practically  it  could  not  be  exact) 
at  which  precisely  half  the  stability  will  be  due  to  arch  action  and 
half  to  gravity  action,  each  acting  without  any  appreciable  conflict 
or  interference  with  the  other.'"  * 

517.  In  addition  to  the  increased  stability  of  a  curved  gravity 
dam  due  to  arch  action,  the  curved  form  has  another  advantage. 
The  pressure  of  the  water  on  the  back  of  the  arch  is  everywhere 
perpendicular  to  the  up-stream  face,  and  can  be  decomposed  into 
two  components — one  perpendicular  to  the  chord  (the  span)  of  the 
arch,  and  the  other  parallel  to  the  chord  of  the  arc.  The  first 
component  is  resisted  by  the  gravity  and  arch  stability  of  the  dam, 
and  the  second  throws  the  entire  up-stream  face  into  compression. 
The  aggregate  of  this  lateral  pressure  is  equal  to  the  water  pressure 
on  the  projection  of  the  up-stream  face  on  a  vertical  plane  perpen- 
dicular to  the  span  of  the  dam.  This  pressure  has  a  tendency  to 
close  all  vertical  cracks  and  to  consolidate  the  masonry  transversely, 
— which  effect  is  very  desirable,  as  the  vertical  joints  are  always  less 
perfectly  filled  than  the  horizontal  ones.  This  pressure  also  pre- 
pares  the  dam  to  act  as  an  arch  eai'lior  than  it  would  otlierwise  do, 
and  hence  makes  available  a  larger  amount  of  stability  due  to  arch 
action. 

The  compression  due  to  these  lateral  components  is  entirely  in- 
dependent of  the  arch  action  of  the  dam,  since  the  arch  action 
would  take  place  if  the  pressure  on  the  dam  were  everywhere  per- 
pendicular to  the  chord  of  the  arch.  Further,  it  in  no  way  weakens 
the  dam,  since  considered  as  a  gravity  dam  the  effect  of  the  thrust 
of  the  water  is  to  relieve  the  pressure  on  the  back  face,  aiid  con- 
sidered as  an  arch  the  maximum  pressure  occurs  at  the  sides  of  the 
down-stream  face. 


*  Editorial  in  Engineering  News,  vol.  six.  p.  272. 


4.KT.   2.]  OUTLINES   OF   THE    DESIGN.  333 

The  curved  dam  is  a  little  longer  than  a  straight  one,  and  hence 
would  cost  a  little  more.  The  difference  in  length  between  a  chord 
and  its  arc  is  given,  to  a  close  degree  of  approximation,  by  the  formula 


in  which  a  =  the  length  of  the  arc,  c  =  the  length  of  the  chord, 
and  r  =  the  radius.  This  shows  that  the  increase  in  length  due  to 
the  arched  form  is  comparatively  slight.  For  example,  if  the  chord 
is  equal  to  the  radius,  the  arch  is  only  ^\,  or  4  per  cent.,  longer  than 
the  chord.  Furthermore,  the  additional  cost  is  less,  proportionally, 
than  the  additional  quantity  of  masonry  ;  for  example,  10  per  cent, 
additional  masonry  will  add  less  than  10  per  cent,  to  the  cost. 

518.  Of  the  twent^^-five  most  important  masonry  dams  in  the 
world,  two  are  of  the  pure  arch  type,  fifteen  are  of  the  curved 
gravity  type,  and  eight  are  of  the  straight  gravity  type.  The  eight 
highest  dams  are  of  the  curved  gravity  type.* 

519.  dUALITY  OF  THE  Masoney.  It  is  a  well  settled  principle 
that  any  masonry  structure  which  sustains  a  vertical  load  should 
have  no  continuous  vertical  joints.  Dams  support  both  a  horizontal 
and  a  vertical  pressure,  and  hence  neither  the  vertical  nor  the  hori- 
zontal joints  should  be  continuous.  This  requires  that  the  masonry 
shall  be  broken  ashlar  (Fig.  39,  jiage  136)  or  random  squared-stone 
masonry  (Fig.  44,  page  137),  or  uncoursed  rubble  (Fig.  45,  page  13 7). 
The  last  is  generally  employed,  particularly  for  large  dams.  The 
joints  on  the  faces  should  be  as  thin  as  possible,  to  diminish  the 
effect  of  the  weather  on  the  mortar  and  also  the  cost  of  repointing. 
In  ordinary  walls  much  more  care  is  given  to  filling  comjsletely 
the  horizontal  than  the  vertical  ones  ;  but  in  dams  and  reservoir 
walls  it  is  important  that  the  vertical  joints  also  shall  be  completely 
filled. 

To  prevent  leakage,  it  is  very  important  that  all  spaces  between 
the  stones  should  be  filled  completely  with  good  mortar,  or  better, 
witii  mortar  impervious  to  water  (see  §  141).  If  the  stone  itself  is 
not  impervious,  the  wall  may  be  made  water  tight  by  the  ap- 
plication of  Sylvester's  washes  (§  263)  to  the  inside  face  of  the 
dam. 

*  For  source  of  information  concerning  these  dam?,  see  §  520 — Bibliography  of 
Masonrj-  UdLis. 


334  MASOJSTEY   DAMS.  [CHAP.  XIII. 

520.  BiBLlOGKAPHY  OF  MASONRY  Dams.  Design  and  Construc- 
tion of  Masonry  Dams,  Rankiiie,  (Miscellaneous  Scientific  Papers, 
pp.  550-61.)  Study  of  Reservoir  Walls,  Krantz,  (translated  from 
the  French  by  Capt.  F.  A.  Mahan,  U.  S.  A.)  Profiles  of  High 
Masonry  Dams,  McMaster,  (published  in  Van  Nostraud's  Engineer- 
ing Magazine  and  also  as  No.  6  of  Van  Nostrand's  Science  Series. ) 
Strains  in  High  Masonry  Dams,  E.  Sherman  Gould,  (Van 
Nostrand's  Engineering  Magazine,  vol.  30,  p.  265  et  seq.).  Histori- 
cal and  Descriptive  Revieio  of  Earth  and  Masonry  Dams,  with 
Plans,  David  Gravel,  (Scientific  American  Supplement,  No.  595 
(May  28,  1887),  pp.  9496-9500.)  Wegmann's  Design  and  Con- 
struction  of  Masonry  Da?)is  gives  an  account  of  methods  em- 
ployed in  determining  the  profile  of  the  proposed  Quaker  Bridge 
dam,  and  also  contains  illustrations  of  the  high  masonry  dams 
of  the  world.  For  a  general  discussion  of  high  masonry  dams, 
including  a  consideration  of  the  best  form  for  the  horizontal 
cross  section,  a  full  description  of  the  proposed  Quaker  Bridge 
dam  and  a  comparison  of  it  with  other  great  dams,  and  many 
valuable  points  concerning  practical  details,  see  numerous  re- 
ports, correspondence,  and  editorials  in  Engineering  Neivs,  Jan- 
uary to  June,  1888  (vol.  19).  The  above  articles  contain  many 
references  to  the  literature,  mostly  French,  of  high  masonry  dams. 

Aet.  3.  Rock  Fill  Dams. 

521.  There  are  three  well-known  types  of  dams,  which  have 
been  in  use  from  time  immemorial  :  earth  bank,  timber  crib-work, 
and  masonry.  Eecent  engineering  practice  on  the  Pacific  coast  has 
introduced  another  type,  viz.:  the  Rock  Fill  Dam,  which  is  of  too 
much  importance  to  pass  by  without  a  mention  here,  although 
strictly  it  can  not  be  classed  as  masonry  construction. 

A  rock  fill  dam  consists  of  an  embankment  of  irregular  stones 
thrown  in  loosely,  except  that  sometimes  the  faces  are  laid  by  hand. 
If  the  overflow  is  to  discharge  over  the  crest,  the  largest  stones 
should  be  placed  on  the  down-stream  slope.  The  dam  may  be  made 
practically  water  tight  (1)  by  filling  the  voids  with  smaller  stones, 
gravel,  sand,  and  earth,  or  (2)  by  placing  any  desired  thickness  of 
earth  and  puddle  on  the  up-stream  face,  or  (3)  by  covering  the 
water  slope  with  one  or  more  thicknesses  of  planking,  which  is  calked 
and  sometimes  also  pointed.     Either  the  first   or   second  method 


ART.   2.]  OUTLINES   OF   THE   DESIGX.  335 

would  make  a  dam  practically  water  tight  from  the  beginning,  and 
it  would  grow  tighter  with  age  ;  the  third  method,  if  carefully  exe- 
cuted, would  make  the  dam  absolutely  water  tight  at  the  beginning, 
but  would  decay,  since  the  upper  part  of  the  sheeting  would  ordi- 
narily be  alternately  wet  and  dry. 

A  great  number  of  rock  fill  dams  have  been  built  on  the  Pacific 
slope  in  the  past  few  years,  for  mining  and  irrigating  purposes.  A 
dam  of  this  character  has  recently  been  completed  on  the  Hassa- 
\-ampa  Eiver  in  Arizona,  of  the  following  dimensions  :  "  Height, 
110  ft.;  base,  135  ft.;  top  width,  10  ft.;  length  on  top,  400  ft.;, 
water  slope,  20  ft.  horizontal  to  47  ft.  vertical  (^  to  1);  back  slopes, 
70  ft.  horizontal  to  180  ft.  vertical  (f  to  1);  contents,  4G,000  cu.  yds. ;. 
cost,  by  contract,  S2.40  per  cu.  yd."  *  It  is  proposed  to  build  a  dam 
of  this  character  in  California  250  feet  high,  which  is  about  80  feet 
higher  than  any  existing  masonry  dam,  and  practically  is  nearly  the 
same  amount  higher  than  the  proposed  Quaker  Bridge  dam 
(Fig.  72,  page  328). 

522.  ''  Earth  dams  are  good  and  useful  when  only  still  water  not 
running  over  the  crest  is  to  be  dealt  with.  Counting  reservoir  walls 
as  dams,  which  they  are,  earth  dams  are  vastly  more  used  than  any 
other.  They  must  be  made  with  the  greatest  care,  and,  if  of  any 
considerable  height,  an  inner  wall  of  puddle  is  necessary  to  their 
integrity.  They  must  be  carried  down  to  firm  and  impervious  sub- 
soil of  some  kind,  or  they  are  worthless.  Any  considerable  leak  is 
at  once  fatal  to  them  ;  and  they  are  also  subject  to  serious  injury 
from  muskrats,  crabs,  etc.  Nevertheless,  many  earth  dams  of 
great  age  and  great  height  exist,  and  bid  fair  to  exist  for  ages, 
showing  that  it  is  entirely  possible  to  make  them  secure." 

Stone-filled  timber  cribs  have  been  very  much  used  for  dams  ; 
but  such  structures  are  sure  to  rot  in  time,  since  the  timber  can  not 
always  be  kept  wet.  It  seems  probable  that  in  most  instances  where 
cribs  have  been  used  a  rock-fill  dam  would  have  been  better, 
cheaper,  and  more  durable. 

Masonry  dams  of  all  sizes,  proportions,  and  ages  exist  in  great 
abundance,  and  the  entire  suitability  of  masonry  for  the  construction 
of  dams  is  well  established.  This  class  of  dams  is  to  be  preferred 
where  large  quantities  of  stone  are  not  near  at  hand,  or  where  leak- 
age is  undesirable  because  of  loss  of  water  or  of  injury  to  land  be- 

*  Engineering  News,  vol.  xx.  p.  232. 


336  MASOXRY   DAMS,  [CHAP.  XIII. 

low,  or  where  space  is  valuable,  or  where  the  surroundings  require 
a  dam  of  good  appearance. 

523.  "  These  three  types  afford  an  adequate  choice  for  nearly  all 
i'equirenieuts,  but  it  is  obvious  that  they  are  open  to  certain  com- 
mon objections  from  which  the  fourth  type — a  rock-fill  dam — is 
ffee.  They  are  all  comparatively  costly  ;  they  require  a  good  deal 
of  labor,  and  much  of  it  skilled  and  faithful  labor,  for  their  con- 
struction ;  they  can  only  with  great  inconvenience  be  constructed 
with  water  around  them,  wliich  for  the  most  part  must  be  kept  away 
by  costly  coifer-dams  or  diversions  of  channels  ;  above  all,  a  leak  is 
always  a  source  of  danger,  and  is  apt  to  be  destructive.  They  are 
all  of  them,  as  it  were,  during  all  their  existence,  in  unstable 
equilibrium — all  right  so  long  as  the  balance  of  forces  remains  un- 
disturbed, and  seriously  endangered  by  a  variety  of  causes  which 
may  disturb  it.  On  the  other  hand  a  rock-fill  dam  is  by  the  very 
process  of  its  construction,  if  conducted  with  reasonable  judgment, 
a  structure  which  tends  to  improve  with  time,  and  which  can  not 
be  injured  but  may  be  benefited  by  causes  which  threaten  the 
other  and  more  artificial  types  ;  in  other  words,  it  is  a  structure 
which  may  not  be  very  tight,  but  which  is  in  stable  equilibrium  as 
respects  all  disturbing  causes,  being  improved  and  never  injured  by 
them. 

"A  rock-fill  dam  is  appropriate  where  the  bed  on  which  it  rests 
is  either  rock,  hard-pan,  stiff  clay,  or  some  other  imjDervious  and 
almost  unwashable  material.  The  bed  may  be  more  or  less  over- 
laid with  gravel  or  loose  material  without  harm,  if  it  be  possible 
to  remove  the  loose  material  in  advance,  and  if  there  be  current 
enough  to  remove  it  from  under  the  foot  of  the  dam,  as  the  work 
of  construction  progresses,  it  will  not  even  involve  extra  expense  or 
delay,  and  the  dam  may  be  begun  on  top  of  the  stratum  without 
apparent  regard  to  it  ;  but  whenever  there  is  any  considerable 
stratum  of  loose  material,  a  rock-fill  dam  can  only  be  built  by  back- 
ing it  with  earth  or  puddle  as  a  timber  dam  would  be,  and  the 
necessity  of  providing  a  proper  apron  to  receive  the  overflow  may 
make  a  timber  or  crib  dam  the  more  economical.  It  is  obvious 
that  the  place  of  all  places  for  the  proper  use  of  such  a  rock-fill 
dam  is  where  leakage  is  of  no  importance,  either  from  the  loss  of 
"water  or  from  injury  to  land  below  ;  where  skilled  labor  is  scarce 
and  costly,  and  simplicity  of  work  rather  than  aggregate  quantities 


ART.   2.]  OUTLINES   OF   THE    DESIGN.  337 

the  important  consideration  ;  where  good  materials  for  masonry  are 
scarce  or  absent ;  and  where  the  surroundings  do  not  demand  at- 
tention to  the  question  of  appearance."  * 

The  greatest  economy  in  this  form  of  dam  occurs  when  the  fill 
is  made  in  water  ;  and  it  is  particularly  advantageous  in  the  canali- 
zation of  rivers,  i.  e.,  in  forming  pools  in  rivers  for  the  benefit  of 
navigation.  It  has  been  proposed  to  use  rock-fill  dams  exclusively 
in  the  construction  of  the  Nicaragua  canal. 

524.  In  California  the  cost  of  this  class  of  dams  varies  from  $2 
to  $3  per  cubic  yard,  including  all  accessories,  which  is  said  to  be 
about  50  per  cent,  cheaper  than  for  earth  dams  of  equal  area  of 
transverse  cross  section. 

*  Editorial  in  Engimerirm  News,  vol.  xx.  p.  70. 


CHAPTER  XIV. 
RETAINING  WALLS. 

625.  Definitions.  Retaining  loall  is  a  wall  of  masonry  for 
sustaining  the  pressure  of  earth  deposited  behind  it  after  it  is  built. 
A  retaining  wall  is  sometimes  called  a  sustaining  wall. 

Face  toall,  or  dujje  wall,  is  a  s]3ecies  of  retaining  wall  built 
against  the  face  of  earth  in  its  undisturbed  and  natural  position. 
Obviously  it  is  much  less  important  and  involves  less  difficulties 
than  a  true  retaining  wall. 

Buttresses  are  projections  in  the  front  of  the  wall  to  strengthen 
it.  They  are  not  often  used,  on  account  of  their  unsightliness,  ex- 
cept as  a  remedy  when  a  wall  is  seen  to  be  failing. 

Counterforts  are  projections  at  the  rear  of  the  wall  to  increase 
its  strength.  They  are  of  doubtful  economy,  and  were  much  more 
frequently  used  formerly  than  now. 

Land-ties  are  long  iron  rods  which  connect  the  face  of  the  wall 
with  a  mass  of  masonr}^,  a  large  iron  plate,  or  a  large  wooden  post 
bedded  in  the  earth  behind  the  wall,  to  give  additional  resistance  to 
overturning. 

Surcharge.  If  the  material  to  be  supported  slopes  up  and  back 
from  the  top  of  the  wall,  the  earth  above  the  top  is  called  the  sur- 
charge. 

Eetaining  walls  are  frequently  employed  in  railroad  work,  on 
canals,  about  harbors,  etc.;  and  the  principles  involved  in  their 
construction  have  more  or  less  direct  application  in  arches,  in  tun- 
neling and  mining,  in  timbering  of  shafts,  and  in  the  excavation  of 
deep  trenches  for  sewers,  etc.,  and  in  military  engineering. 

526.  Method  of  Failtjee.  A  retaining  wall  may  fail  (1)  by 
revolving  about  the  front  of  any  horizontal  joint,  or  (2)  by  sliding 
on  the  plane  of  any  horizontal  joint,  or  (3)  by  the  bulging  of  the 
body  of  the  masonry.     The  first  is  much  the  most  frequent  mode  of 

338 


DIFFICULTIES.  339 


failure,  and  the  second  is  the  least  frequent.  The  wall  can  not  fail 
by  the  center's  bulging  out,  unless  some  force  acts  to  keep  the  top 
from  moving  forward, — as  in  a  cellar  wall,  the  abutments  of  arches, 
etc. 

527.  Difficulties.  In  the  discussion  of  the  stability  of  dams, 
it  was  shown  that  in  order  to  completely  determine  the  effect  of  the 
thrust  of  the  water  against  the  wall,  it  is  necessary  to  know  (1) 
the  amount  of  the  pressure,  (2)  its  point  of  application,  and  (3) 
the  direction  of  its  line  of  action.  Similarly,  to  determine  the 
effect  of  the  thrust  of  a  bank  of  earth  against  a  wall,  it  is  necessary 
to  know  (1)  the  amount  of  the  pressure,  (2)  its  point  of  application, 
and  (3)  its  line  of  action.  The  determination  of  these  three  quan- 
tities requires  three  equations.  The  resistance  of  the  wall  both  to 
sliding  and  to  overturning  can  be  found  with  sufficient  accuracy,  as 
has  already  been  explained  in  Chapter  XIII — Dams;— ^but  the 
other  elements  of  the  problem  are,  in  the  present  state  of  our 
knowledge,  indeterminate. 

The  origin  of  the  difficulties  may  be  explained  briefly  as  follows. 
A  B  represents  a  retaining  wall  ;  A  D  \s,  the  sur- 
face of  the  ground.  The  earth  has  a  tendency  to 
break  away  and  come  down  some  line  as  CD.  The 
force  tending  to  bring  the  earth  down  is  its  weight ; 
the  forces  tending  to  keep  it  from  coming  down  are 
the  friction  and  cohesion  along  the  line  CD.  The 
pressure  against  the  wall  depends  upon  the  form  of  B  C 
the  line  CD.     If  the  constants  of  weight,  friction,  Fig.  73. 

and  cohesion  of  any  particular  ground  were  known,  the  form  of  CD 
and  also  the  amount  of  the  thrust  on  the  wall  could  be  determined. 
Notwithstanding  the  fact  that  since  the  earliest  ages  constructors 
have  known  by  practical  experience  that  a  mass  of  earthwork 
will  exert  a  severe  lateral  pressure  upon  a  wall  or  other  retaining 
structure,  there  is  probably  no  other  subject  connected  with  the 
constructor's  art  in  which  there  exists  the  same  lack  of  exact  ex- 
perimental data.  This  lack  is  doubtless  due,  in  part  at  least,  to  a 
reliance  upon  theoretical  investigations.  Of  course,  mathematical 
investigations  unsupported  by  experiments  or  experience  are  a  very 
uncertain  guide. 

This  subject  will  be  discussed  further  under  the  heads  (1) 
Theoretical  Formulas,  and  (2)  Empirical  Rules. 


yf 


340  RETAINING    WALLS.  [CHAP.    XIV. 


Art.  1.    Theoretical  Formulas. 

528.  A  great  variety  of  theories  have  been  presented,  but  all  rest 
upon  an  uncertain  foundation  of  assumption,  and  all  are  more  or 
less  defective  and  self-contradictory.  All  theories  of  the  stability 
of  retaining  walls  involve  the  three  following  assumptions  : 

529.  First  Assumption.  All  theories  assume  that  the  surface 
of  rupture,  C  D,  Fig.  73,  is  a  plane.  This  is  equivalent  to  assum- 
inof  that  the  soil  is  devoid  of  cohesion,  and  is  inelastic  and  homo- 
geneous,  and  also  that  if  a  mass  of  such  material  be  sustained  by  a 
wall,  there  is  a  certain  plane,  called  the  plane  of  rupture,  along 
which  the  particles  are  m  equilibrium,  i.  e.,  are  just  on  the  point  of 
moving.  This  assumption  would  be  nearly  correct  in  the  case  of 
clean,  sharp  sand,  but  would  be  considerably  in  error  with  a  tough, 
tenacious  soil. 

This  assumption  gives  the  data  by  which  the  amount  of  the 
thrust  of  the  earth  can  be  computed;  that  is  to  say,  this  assumption 
furnishes  the  conditions  from  which  one  of  the  equations  may  be 
established. 

530.  Second  Assumption.  A  second  assumption  which  is  always 
made  is  that  the  point  of  application  of  the  lateral  pressure  of  the 
earth  is  one  third  of  the  height  of  the  wall  from  the  bottom.  The 
total  pressure  on  the  wall  varies  as  some  function  of  the  height ; 
and  it  is  assumed  to  vary  as  the  square  of  the  height,  and  that 
therefore  the  center  of  pressure  is  at  a  point  two  thirds  of  the 
depth  below  the  top.  This  is  equivalent  to  assuming  that  the  varia- 
tion of  the  pressure  in  a  mass  of  earth  is  the  same  as  in  a  liquid, 
I.  e.,  that  the  material  is  devoid  of  internal  friction. 

This  assumption  furnishes  the  second  of  the  equations  required 
to  determine  the  effect  of  the  thrust  of  earth  against  a  retaining 
wall. 

531.  Third  Assumption.  The  third  equation  is  obtained  by 
assuming  the  direction  of  the  pressure.  There  are  different  theories 
based  on  different  assumptions  as  to  this  direction. 

The  theories  of  the  stability  of  retaining  walls  in  most  frequent 
use  will  now  be  stated,  and  the  underlying  assumptions  and  the 
defects  of  each  will  be  pointed  out. 


ART.   1.]  THEORETICAL   FORMULAS.  341 

532.  Coulomb's  Theory.  The  theory  advanced  by  Coulomb  in 
1784  was  the  first  to  even  approximate  the  actual  conditions,  and 
his  method  is  the  basis  of  nearly  all  formulas  used  by  engineers  at 
the  present  time.  It  has  been  taken  up  and  followed  out  to  its 
consequences  by  Prony  (1802),  Mayniel  (1808),  Fran^aise  (1820), 
Navier  (1826),  Audoy  and  Poncelet  (1840),  Hagen  (1853),  Scheffler 
(1857),  and  Moseley,  as  well  as  a  host  of  others,  in  recent  times. 

Coulomb  assumed  (1)  that  the  line  D  C,  Fig.  73  (page  339),  is 
a  straight  line,  down  which  the  prism  A  C  D  tends  to  slide;  (3)  that 
the  resultant  pressure  is  applied  at  a  point  two  thirds  of  the  depth 
below  the  top;  and  (3)  that  the  pressure  exerted  by  this  mass  on  the 
wall  is  normal  to  its  back  face,  which  is  equivalent  to  neglecting  the 
friction  of  the  earth  against  the  back  of  the  wall.  He  decomposed  the 
weight,  W,  of  the  prism  A  C  D,  Fig.  74,  and  the 
reaction,  R,  of  the  wall  into  two  components 
respectively,  parallel  and  perpendicular  to  the 
surface  of  rupture,  D  C.  The  difference  of 
these  parallel  components,  P^—  P„,  he  placed 
equal  to  the  prism's  resistance  to  sliding;  and  ^  c 
assumed  the  latter  to  be  equal  to  /j.  iVj,  in  which  Fig.  74. 

/x  is  the  co-efficient  of  friction.  There  is  some  prism,  A  CD,  the 
pressure  of  which  against  the  wall  is  just  sufficient  to  cause  sliding. 
The  amount  of  this  pressure  will  depend  ujDon  the  weight,  w,  of  a 
unit  of  volume  of  the  backing;  upon  the  height,  h,  of  the  wall; 
upon  the  co-efficient  of  friction,  fx,  of  earth  on  earth;  and  upon  the 
distance  A  D,  which  call  x. 

Under  the  conditions  assumed,  it  is  possible  to  state  a  value  o\ 
R  in  terms  of  h,  iv,  //,  and  x.  Coulomb  assumed  R  to  vary  as  x, 
and  differentiated  the  value  of  R  to  find  the  position  of  the  surface 
of  rupture,  D  C,  for  a  maximum  pressure  on  the  wall.  This  leads 
to  the  simple  conclusion  that  the  lateral  pressure  exerted  by  a  bank 
of  earth  with  a  horizontal  top  is  simply  that  due  to  the  wedge-shaped 
mass  included  between  the  vertical  back  of  the  wall  and  a  line  bi- 
secting the  angle  between  the  vertical  and  the  slope  of  repose  of  the 
material;*  that  is,  the  pressure  of  the  earth  against  the  wall  A  B, 


*  For  an  algebraic  demonstration,  see  Moseley's  Mechanics  of  Engineering  (2(J 
Amer.  Ed.),  pp.  413-16;  for  a  graphical  demonstration,  see  Van  Nostrand's  Engineer- 
ing Magazine,  vol.  ix.  p.  202,  and  vol.  xxii.  p.  267. 


342  EETAINING   WALLS.  [CHAP.  XIV. 

Fig.  74,  is  equal  to  the  pressure  of  the  prism  ACE  sliding  along  a 

perfectly  smooth  plane  C E,  which  bisects  the  angle  of  repose,  A  CD. 
No  satisfactory  proof  lias  been  given  of  the  correctness  of  this 

procedure  by  either  Coulomb  or  any  one  else;  and  no  defense  has 
ever  been  made  against  a  number  of  serious  objections  to 
it  which  have  been  raised.  Experiments  show  that  the 
lateral  pressure  of  the  prism  A  B  C,  Fig.  75,  between  two 
boards  A  B  and  A  C,  against  A  B,  "  is  quite  as  much  when 
the  board  A  C  is  at  the  slope  of  repose,  It}  to  1,  as  when  it 
is  at  half  the  angle;  and  there  was  hardly  any  difference 

(vhether  the  board  was  horizontal,  or  at  a  slope  of  -|  to   1,  or  at 

any  intermediate  slope,"* 

533.   By  this  theory  the  pressure   of  the  wedge  A  C  D  (Fig. 

74)  is 

P  =  iiv  h'  tan=  iA CD, (1) 

in  which  w  is  the  weight  of  a  unit  of  the  material  to  be  supported, 
and  U  is  the  height  of  the  wall.  This  thrust  is  assumed  to  act  two 
thirds  of  A  C,  Fig.  74,  below  A.  Or,  in  other  words,  the  thrust  of 
the  prism  is  equivalent  to  the  pressure  of  a  liquid  Avhose  weight  per 
unit  of  volume  is  w  tan''  \  A  CD. 

Equating  the  moment  of  the  overturning  force  and  the  moments 
of  resistance  in  terms  of  the  unknown  thickness,  and  solving  the 
equation,  gives  the  thickness  which  the  wall  must  have  to  be  on  the 
point  of  overturning.  For  example,  assume  that  it  is  desired  to 
determine  the  thickness,  t,  of  a  vertical  rectangular  wall.  Eepre- 
sent  the  weight  of  a  cubic  foot  of  the  masonry  by  W.  Then  placing 
the  moment  of  the  wall  equal  to  the  amount  of  the  thrust  of  the 
earth,  gives 

Wilt  .\t  =  P.\li. (2) 

Solving  equations  (1)  and  (2)  gives 

t  =  h  tan  \A  CD  \/  -^ (3) 

*  Benj.  Baker,  an  eminent  English  engineer,  in  a  very  interesting  and  instructive 
article  on  "  The  Actual  Lateral  Pressure  of  Earthwork,"  reprinted  in  Van  Nostrand's 
Engineering  Magazine,  vol.  xxv.  pp.  333-42,  353-71,  and  492-505,  from  Proc.  of  the 
Inst,  of  C.  E.,  vol.  Ixv.  pp.  140-241. 


ART.   1.]  THEORETICAL   FORMULAS.  343 

Numerous  tables  have  been  computed  which  give,  to  a  great 
number  of  decimal  places,  tlie  thickness  of  a  rectangular  wall  in 
terms  of  its  height,  the  arguments  being  the  ratio  of  the  weights  of 
a  unit  of  volume  of  the  wall  and  backing,  and  the  angle  of  repose. 
Such  tables  are  of  but  little  practical  value,  as  will  appear  presently. 

534.  Surcharged  Walls.  The  rule  that  the  plane  of  rupture 
bisects  the  angle  between  the  natural  slope  of  the  earth  and  the  back 
of  the  wall,  holds  good  only  when  the  top  surface  of  the  bank  is 
horizontal  and  the  back  of  the  wall  vertical.  The  formula  for  a 
surcharged  wall,  or  for  the  case  in  which  the  back  is  not  vertical, 
or  for  both  combined,  may  be  deduced  *  in  the  same  general  way  as 
above;  but  the  results  for  each  case  are  too  complicated  for  ordinary 
use,  and  each  is  subject  to  the  same  errors  as  the  formula  for  a  ver- 
tical wall  and  level  top  surface.  There  are  a  number  of  exceedingly 
ingenious  gi'aphical  solutions  of  the  resulting  equations,  f 

535.  Reliability  of  Coulomb's  Theory.  It  is  generally  conceded 
that  the  results  obtained  by  this  method  have  but  little  practical 
value.  "■  Experiments  and  practical  experience  show  that  walls, 
which  according  to  this  theory  are  on  the  point  of  overturning, 
possess  on  the  average  a  factor  of  safety  of  about  tu'o."  %  One  of 
the  author's  students  experimented  with  fine  shot,  which  appear  to 
fulfill  the  fundamental  assumptions  of  this  theory,  and  found  that 
the  observed  resistance  was  1.97  times  that  computed  by  Coulomb's 
formula.§  The  uncertainties  of  the  fundamental  assumptions  and 
the  questionableness  of  some  of  the  mathematical  processes  are 
sufficient  explanation  of  the  difference  between  the  theory  and 
practice. 

536.  Weyrauch's  Theory.  This  is  the  latest  one,  having  been 
proposed  in  18T8.  It  was  first  brought  to  the  attention  of  American 
engineers  by  Professor  J.  A.  Du  Bois's  translations  of  Winkler's 
"  Neue  Theorie  des  Erddruckes,"  and  Weyrauch's  paper  on  retain- 
ing walls  published  in  "Zeitschrift  fiir  Baukunde,"  1878,  Band  i. 
Heft  2,  which  translation  was  published  in  the  Journal  of  the  Frank- 

*  See  Moseley's  Mechanics  of  Engineering,  pp.  424-26. 

+  See  Van  Nostrand's  Engineering  Magazine,  vol.  ix.  p.  304  ;  and  do.,  voL  xxv. 
p.  35.5.  For  references  to  elaborate  graphical  treatises  on  retaining  walls,  see  Du 
Bois's  Graphical  Statics,  pp.  Iv-lvi  of  Introduction. 

X  Benj.  Baker  in  "  The  Actual  Lateral  Pressure  of  Earthwork."  See  foot-note  on 
page  342. 

§  See  M.  Fargusson"s  Bachelor's  Thesis,  University  of  Illinois. 


344 


RETAIKIXG    AVALLS. 


[chap.   XIV. 


lin  Institute,  vol.  cviii.  pp.  361-87.     The  following  presentation  of 
this  theory  is  drawn  mainly  from  that  article. 

This  theory  assumes  (1)  that  the  surface  of  rupture  is  a  plane, 
(2)  that  the  point  of  application  of  the  resultant  of  the  lateral 
pressure  of  the  earth  is  at  a  point  one  third  of  the  height  of  the 
wall  from  the  bottom,  and  (3)  that  there  is  no  friction  between  the 
earth  and  the  back  of  the  wall.  It  is  claimed  that  these  three  are 
the  only  assumptions  involved  in  this  theory,  and  that  the  direction 
of  the  resultant  pressure  is  deduced  from  the  fundamental  rela- 
tions necessary  for  equilibrium  under  the  conditions  assumed. 

The  analysis  to  establish  the  equations  for  the  amount  and  direc- 
tion of  the  thrust  of  the  earth  is  too  long  and  too  complicated  to  be 
attempted  here ;  consequently,  only  the  final  equations  will  be 
given. 

Ta  Let  E  =  the  thrust  of  earth  against 
the  wall. 
10  =  the  weight  of  a  unit  of  the 

earth. 
h  =  the  height  of  the  wall. 
a  =  the  angle  the  back  of  wall 

makes  with  the  vertical. 
d  =  the   angle   which  E  makes 
with   the   normal  to  the 
back  of  the  wall. 
6  =  the  angle  of  the  upper  surface  with  the  horizontal. 
/3  =  the  angle  of  the  plane  of  rupture  with  the  vertical. 
0  =  the  angle  of  repose  with  the  horizontal. 
537.   General  Formulas.     For  a  plane  earth-surface,  horizontal 
or  sloping  up  at  any  angle,  and  the  back  of  the  wall  vertical  or 
leaning  forward  at  any  angle,  the  general  relations  are  * 


E  = 


¥  w 


cos  (0  —  a) 
_{n  +  1)  cos  a_\  2  cos  {a  +  6)' 


.      .      (*) 


in  which 


_  i /sin  (0  +  ^)  sin  (0  —  e) 
~       cos  {a  -\-  6)  cos  {a  —  e)' 


(5) 


*  See  Howe's  Retaining  Walls  for  Earth,  pp.  46,  47;  and  also  Van  Nostrand's 
Engineering  Magazine,  vol.  xxii.  pp.  26.5-77. 


T.   l.J  THEORETICAL   FORMULAS.  345 

The  value  of  d  required  in  (5)  can  be  deduced  from 

tan  6  -       siQ(3^-6)-A^sin2(n^-e) 

^  _  cos  (2  or  -  e)  +  A'  cos  2  {a  -  e)'    '     '     ^^^ 


in  which 


„       cos  e  —  V  cos"  e  —  cos*  d>  ,  , 

K= , -, ^ (7) 

cos     0  •        V'/ 


538.  Horizontal  Earth-surface.  If  the  upper  surface  of  the 
earth  is  horizontal,  then  6=0,  and 

_        tan  a  ¥  w 

~  sin  {a  +T)  •  ~Y'' (^) 

and  6  can  be  found  from 

sin  0  sin  2  a 
tan  d  = r^ ^r- (9) 

1  —  sm  0  cos  2  a  ^  ' 

If  the  back  of  the  wall  is  vertical,  a  =  0 ;  and  equation  (9) 
gives  6  =  0.     Therefore 

^=tan=(45°-|)^4^.* (10) 

539.  Surcharge  at  the  Natural  Slope.  If  the  upper  surface  of 
earth  has  the  natural  slope,  e  =  cp  ;  and  therefore 

L       cos  a      J  2  cos  [a  -\-  o)  ^     ' 

and  6  is  determined  from 

tan<?  =  /'°^7.<'^-^">, ^U) 

1  —  sm  0  sm  (0  —  2  «')  ^     ' 

If  the  back  of  the  wall  is  vertical,  a  =  0,  and  S  =  <p,  which 
shows  that  B  acts  parallel  to  the  top  surface  of  the  earth.  In  this 
case 

U=icos(p  hUv (13) 

*  Compare  with  equation  (1),  page  343. 


346  EETAINING  WALLS.  [CHAP.  XIV. 

540.  The  general  equations  for  Weyrauch's  theory,  viz.,  equa- 
tions (4),  (5),  (6),  and  (7),  have  not  been  solved  for  any  special 
case,  except  for  e  =  0,  and  e  =  <p.  The  reduction  is  very  long  and 
tedious. 

541.  The  formulas  for  each  of  the  above  cases  may  be  solved 
graphically,*  but  the  explanations  are  too  long  to  be  given  here. 

542.  Reliability  of  Weyrauch's  Theory,  On  behalf  of  this 
theory  it  is  claimed  f  that  the  only  errors  in  it  are  those  due  to  the 
neglect  of  the  cohesion  of  the  backing,  and  to  assuming  that  the 
surface  of  rupture  is  a  plane  ;  and  also  that  ''  it  is  free  from  all  the 
objections  which  may  be  urged  against  all  others,  and  can  be  used 
with  confidence,"     These  claims  are  not  supported  by  the  facts. 

Weyrauch's  theory  is  unquestionably  subject  to  any  errors  which 
may  be  involved  in  the  assumptions  that  the  surface  of  rupture  is  a 
plane  (see  §  529),  and  that  the  point  of  application  of  the  resultant 
pressure  of  the  earth  is  at  two  thirds  of  the  height  of  the  wall  from 
the  top  (see  §  530),  Second,  the  analysis  purports  to  be  perfectly 
general ;  I  but  it  is  evidently  inapplicable  to  a  wall  inclined  toward 
the  earth  to  be  supported,  since  the  formulas  make  the  thrust  of 
the  earth  increase  with  the  backward  inclination  of  the  wall,  Ic 
fact  the  theory  makes  no  difference  between  a  wall  leaning  forward 
and  one  leaning  backward.  For  a  wall  inclining  at  the  angle  of 
repose,  it  gives  a  very  great  lateral  pressure — see  eqs.  (8)  and  (9). 
Third,  the  mathematical  process  of  determining  the  position  of  the 
Burface  of  rupture  is  at  least  questionable.  Fourth,  the  theory  errs 
on  the  safe  side,  because  it  neglects  a  vertical  component  of  the 
earth  pressure  which  is  independent  of  friction.  § 

Weyrauch's  theory  differs  from  Coulomb's  only  in  the  form  of 
the  results  and  in  the  manner  of  deducing  them  ;  ||  and  hence  is  of 
no  practical  value. 

543.  Weyrauch's  method  of  deducing  the  direction  of  the  earth 

*  See  Jour.  Frank.  Inst.,  vol.  cviii.  pp.  380-85;  Van  Nostrand's  Engineering 
Magazine,  vol.  xxii.  pp.  266-73  ;  Howe's  Retaining  Walls  for  Earth,  pp.  7-13. 

+  By  its  authior.  Prof.  Wej-rauch,  and  also  by  the  translator,  Prof.  Du  Bois,— see 
Jour.  Frank.  Inst.,  vol.  cviii.  pp.  -486-87. 

X  See  Jour.  Frank.  Inst.,  vol.  cviii.  p.  377  ;  and  also  Howe's  Retaining  Walls  for 
Earth,  p.  2. 

§  In  proof  that  such  a  component  exists,  see  experiments  by  Siegler  in  Annales  de* 
fbnts  et  C/iausses,  reprinted  in  Scientific  American  Supplement,  vol.  xxiv.  pp.  9724-25, 

[  Van  Nostrand's  Engineering  Magazine,  vol.  xxii.  pp.  265-77. 


^RT.    1.]  THEORETICAL    FORMULAS.  34? 


pressure  assumes  that  there  is  no  friction  between  the  eai-th  and  the 
back  of  the  wall,  or,  in  other  words,  that  the  angle,  d,  which  the 
thrust  of  the  earth  makes  with  the  back  of  the  wall,  does  not  de- 
pend upon  the  structure  of  the  wall  for  its  value.  The  formula  in 
this  form  fails  to  agree  with  ordinary  experience  ;  and  hence  it 
has  been  proposed  *  to  modify  the  general  formula  by  considering 
that  the  angle  between  the  resultant  pressure  of  the  earth  and  the 
back  of  the  wall  is  never  less  than  the  angle  of  friction  between  the 
earth  and  the  wall.     The  method  of  doing  this  is  as  follows : 

If  0'  represents  the  co-efficient  of  friction  between  the  earth 
and  the  wall,  then  the  direction  of  E  must  make  an  angle  with  the 
normal  to  the  back  face  of  the  wall  equal  at  least  to  0'.  To  intro- 
duce 0'  into  Professor  Weyrauch's  theory,  it  is  necessary  to  find  the 
value  of  6  as  given  by  his  formula,  and  see  if  it  is  greater  or  less  than 
0'.  If  it  is  less,  use  the  value  of  0'  to  determine  the  direction  of 
E;  if  greater,  use  the  value  of  6  and  omit  0'  altogether.  The 
value  of  0'  can  not  be  determined  accurately  ;  but  unless  tlie  back 
of  the  wall  is  exceedingly  smooth,  0'  will  be  greater  than  0.  If 
the  back  of  the  wall  is  rough  rubble  (§  213)  or  is  stepped,  0'  will  be 
considerably  larger  than  0.  If  the  friction  between  the  earth  and 
the  wall  be  neglected,  i.  e.,  if  it  is  assumed  that  0'  =  0,  then  when 
rough  rubble  retaining  walls  are  proportioned  according  to  Wey- 
rauch's theory,  they  will  have  a  factor  of  safety  considerably  larger 
than  appears  from  the  computations. 

This  modification  is  inconsistent  with  the  general  theory,  since 
the  fundamental  equations  were  established  for  that  value  of  d  which 
would  produce  equilibrium,  and  the  corresponding  value  of  the 
thrust  was  deduced  accordingly.  It  is  certainly  incorrect  to  use  one 
direction  in  determining  the  value  of  the  thrust  and  another  in 
applying  it.  Further,  it  is  not  reasonable  to  believe  that  the  thrust 
ever  makes  an  angle  with  the  normal  to  the  back  of  the  wall 
greater  than  the  angle  of  friction,  since  one  of  the  fundamental 
conditions  of  statics  is  that  if  the  resultant  pressure  makes  an  angle 
with  the  normal  greater  than  the  angle  of  repose,  motion  takes 
place.  This  modification  of  Weyrauch's  theory  purports  to  give  the 
relations  for  a  state  of  equilibrium,  and  yet  violates  the  fundamental 
condition  necessary  for  equilibrium.  Xeither  the  original  theory 
nor  the  above  modification  of  it  are  of  any  practical  value. 

*  By  Prof.  Cain  in  Van  Nostrand's  Engineering  Magazine,  voL  xxv.  p.  93. 


348  DETAINING    WALLS.  [CHAP.   XIV. 

544.  Rankine'S  Theory.  There  is  another  class  of  theories, 
which,  in  addition  to  the  assumptions  of  §  530  and  §  531,  assume 
that  the  thrust  of  the  earth  makes  an  angle  with  the  back  of  the 
wall  equal  to  the  angle  of  repose  of  the  earth.  Different  writers 
arrive  at  their  results  in  different  ways,  but  most  of  them  proceed 
trom  a  consideration  of  the  conditions  of  equilibrium  of  the  earth 
particles,  and  arrive  at  their  results  by  integration.  Of  the  formulas 
deduced  in  the  latter  way,  Rankine's  *  are  the  best  known.  All  the 
theories  of  this  class  have  essentially  the  same  limitations  and  de- 
fects as  Coulomb's  and  Weyrauch's. 

545.  Applicability  of  Theoeetical  Formulas.  It  is  generally 
conceded  that  the  ordinary  theories — Coulomb's,  Weyrauch's,  and 
Eankine's, — types  of  the  only  ones  for  which  there  is  any  consider- 
able show  of  reasonableness, — are  safe  ;  but  "  to  assume  upon  theo- 
retical grounds  a  lateral  thrust  which  practice  shows  to  be  excessive, 
and  then  compensate  for  it  by  giving  no  factor  of  safety  to  the  wall, 
although  the  common  way,  is  not  a  scientific  mode  of  procedure." 
This  is  only  another  reason  for  the  statement,  already  made,  that 
theoretical  investigations  are  of  but  little  value  in  designing  re- 
taining walls.  The  problem  of  the  retaining  wall  is  not  one  that 
admits  of  an  exact  mathematical  solution;  the  conditions  can  not  be 
expressed  in  algebraic  formulas.  Something  must  be  assumed  in 
any  event,  and  it  is  far  more  simple  and  direct  to  assume  the  thick- 
ness of  the  wall  at  once  than  to  derive  the  latter  from  equations 
based  upon  a  number  of  uncertain  assumptions. 

Bear  in  mind  that  none  of  the  above  formulas  apply  if  the  back 
of  the  wall  inclines  towards  the  earth  to  be  supported,  or  if  the 
wall  has  a  curved  profile,  or  if  the  upper  surface  is  irregular.  It 
seems  to  be  conceded  that  in  these  cases  the  surface  of  rupture  is 
not  a  plane,  and  hence  no  theory  yet  proposed  will  apply. 

In  this  connection  it  seems  necessary  to  warn  the  student  that 
not  all  theories  for  retaining  walls  are  as  nearly  correct  as  those 
referred  to  above.  Some  of  them,  although  having  all  the  prestige 
of  antiquity  and  offering  the  advantages  of  extended  tables  for  their 
application,  are  totally  valueless,  being  based  upon  unwarranted 
assumptions,  and  violating  the  fundamental  principles  of  mechanics. 

546.  Theoretical  investigations  of  many  engineering  problems 
which  in  every-day  practice  need  not  be  solved  with  extreme  accu- 

*  Civil  Engineering,  pp.  401-07. 


ART.  2.]  EMPIRICAL   RULES.  349 


racy,  are  useful  in  determining  the  relations  of  the  various  elements 
involved,  and  thus  serve  as  a  skeleton  about  which  to  group  the 
results  of  experience  ;  but  the  preceding  discussion  shows  that  the 
present  theories  of  the  stability  of  retaining  walls  are  not  sufficiently 
exact  to  serve  even  as  a  guide  for  future  investigations.  Further- 
more, the  stability  of  a  retaining  wall  is  not  a  purely  mathematical 
problem.  Often  the  wall  is  designed  and  built  before  the  nature  of 
the  backing  is  known;  and  the  variation  of  the  backing,  due  to  rain, 
frost,  shock,  extraneous  loads,  etc.,  can  not  be  included  in  any 
formula. 

Art.  2.  Empirical  Eules. 

547.  English  Rules.  The  eminent  English  engineer  Benjamin 
Baker,  who  has  had  large  experience  in  this  line  in  the  construc- 
tion of  the  underground  railroads  of  London,  says,  "Experience 
has  shown  that  a  wall  [to  sustain  earth  having  a  level  top  surface], 
whose  thickness  is  one  fourth  of  its  height,  and  which  batters  1  or 
2  inches  per  foot  on  the  face,  possesses  sufficient  stability  when  the 
backing  and  foundation  are  both  favorable.  This  allows  a  factor  of 
safety  of  about  two  to  cover  contingencies.  It  has  also  been  proved 
by  experience  that  under  no  ordinary  conditions  of  surcharge  or 
heavy  backing  is  it  necessary  to  make  a  retaining  wall  on  a  solid 
foundation  more  than  double  the  above,  or  one  half  of  the  height  in 
thickness.  Within  these  limits  the  engineer  must  vary  the  strength 
according  to  the  conditions  affecting  the  particular  case.  Outside 
of  these  limits,  the  structure  ceases  to  be  a  retaining  wall  in  the 
ordinary  acceptation  of  the  term.  As  a  result  of  his  own  experi- 
ence, the  author  [Benj.  Baker]  makes  the  thickness  of  retaining 
walls  in  ground  of  an  average  character  equal  to  one  third  of  the 
height  from  the  top  of  the  footings. 

"'Hundreds  of  revetments  have  been  built  by  royal  engineer 
officers  in  accordance  with  Gen.  Fansliawe's  rule  of  some  fifty  years 
ago,  which  was  to  make  the  thickness  of  a  rectangular  brick  wall, 
retaining  ordinary  material,  24  per  cent,  of  the  height  for  a  batter 
of  \,  25  per  cent,  for  ^,  26  per  cent,  for  ^,  27  per  cent,  for  r^,  28  per 
cent,  for  yV.  30  per  cent,  for  ^\,  and  32  per  cent,  for  a  vertical  wall.' "  * 

548.  Trautwine's  Rule.     Trautwinef  recommends  that  "  the 

*  Van  Nostrand's  Engineering  Magazine,  vol.  xxv.  p.  370,  from  Proc.  Inst,  of 
C.  E. 

t  Engineer's  Pocket-Book  (Ed.  1885),  p.  683. 


350  RETAINING  WALLS.  [CHAP.  XIV. 


tliickness  of  tlie  top  of  the  footing  course  of  a  vertical  or  nearly 
vertical  wall  which  is  to  sustain  a  backing  of  sand,  gravel,  or  earth, 
level  top  surface,  when  the  backing  is  deposited  loosely  (as  when 
dumped  from  cars,  carts,  etc.),  for  railroad  practice,  should  not  be 
less  than  the  following  : 

Wall  of  cut-stone,  or  of  first-class  large-ranged  rubble  in  mortar,   35  per  cent. 

"     "  good  common  scabbled  mortar-rubble,  or  brick 40  per  cent. 

"     "  well  scabbled  dry  rubble 50  per  cent. 

When  the  backing  is  somewhat  consolidated  in  horizontal  layers, 
each  of  these  thicknesses  may  be  reduced;  but  no  rule  can  be  given 
for  this.  Since  sand  or  gravel  has  no  cohesion,  the  full  dimensions 
as  above  should  be  used,  even  though  the  backing  be  deposited  in 
layers.  A  mixture  of  sand,  or  earth  with  pebbles,  paving  stones, 
bowlders,  etc.,  will  exert  a  greater  pressure  against  the  wall  than 
the  materials  ordinarily  used  for  backing ;  and  hence  when  such 
backing  has  to  be  used,  the  above  thicknesses  should  be  increased, 
say,  about  ^  to  |  part." 

549.  Details  of  Construction.  The  arrangement  of  the  foun- 
dation of  a  retaining  wall  is  an  important  matter,  but  has  already 
been  sufficiently  discussed  (see  Part  III,  and  also  §§  491  and  551). 
It  is  universally  admitted  that  a  large  majority — by  some  put  at 
nine  out  of  ten,  and  by  others  at  ninety-nine  out  of  a  hundred — of 
failures  of  retaining  walls  are  due  to  defects  in  the  foundation. 

Retaining  walls  are  constructed  of  ashlar  or  brick,  or  of  either 
ashlar  or  brick  backed  with  rubble,  or  of  rubble  either  with  mortar 
or  dry.  As  the  pressure  at  each  bed-joint  is  concentrated  towards 
the  face  of  the  wall,  the  larger  and  most  regular  stones  should  be 
placed  on  the  front.  Occasional  stones  or  even  courses  should 
project  beyond  the  back  of  the  wall,  so  that  the  backing  can  rest 
upon  them,  thus  increasing  the  resistance  of  the  wall  to  overturn- 
ing. This  object  is  also  promoted  by  building  the  back  in  steps. 
The  coping  should  consist  of  large  flat  stones  extending  clear  across 
the  wall. 

As  a  rule,  the  greatest  thrust  comes  against  retaining  walls  when 
the  mortar  is  green  and  least  able  to  resist  it,  which  is  a  reason  for 
preferring  cement  to  lime  mortar.  If  the  backing  is  to  be  filled  in 
before  the  mortar  hardens,  it  should  be  deposited  in  thin,  horizon- 
tal layers,  or  the  wall  should  be  supported  temporarily  by  shores. 

550.  Drainage.     Next  to  a  faulty  foundation,  water  behind  the 


AKT.  2.]  EMPIRICAL   RULES.  351 

wall  is  the  most  frequent  cause  of  the  failure  of  retaining  walls. 
The  water  not  only  adds  to  the  weight  of  the  backing  material,  but 
also  softens  the  material  and  changes  the  angle  of  repose  so  as  to 
greatly  increase  its  lateral  thrust.  With  clayey  soil,  or  any  material 
resting  upon  a  stratum  of  clay,  this  action  becomes  of  the  greatest 
importance.  To  guard  against  the  possibility  of  the  backing's  be- 
coming saturated  with  water,  holes,  called  weepers,  are  left  through 
the  wall.  One  weep-hole,  three  or  four  inches  wide  and  the  depth 
of  a  course  of  masonr}^,  is  generally  sufficient  for  every  three  or 
four  square  yards  of  front  of  the  wall.  When  the  backing  is  clean 
sand,  the  weep-holes  will  allow  all  the  water  to  escape  ;  but  if  the 
backing  is  retentive  of  water,  a  vertical  layer  of  stones  or  coarse 
gravel  should  be  placed  next  to  the  wall  to  act  as  a  drain.  An 
ordinary  drain  at  the  back  of  the  wall  is  often  useful. 

Waen  the  backing  is  liable  to  be  reduced  to  quicksand  or  mud 
by  saturation  with  water,  and  when  this  liability  can  not  be  removed 
by  efficient  drainage,  one  way  of  making  provision  to  resist  the 
additional  pressure  which  may  arise  from  such  saturation  is  to  cal- 
culate the  requisite  thickness  of  wall  as  if  the  earth  were  a  fluid. 
A  puddle-wall  is  sometimes  built  against  the  back  of  dock-walls  to 
keeji  out  the  water. 

The  resistance  of  the  wall  to  sliding  is  materially  increased  by 
laying  the  lower  courses  of  masonry  with  an  inclination  inward. 
An  objection  to  inclining  the  joints,  particularly  in  dry  masonry, 
is  that  the  water  will  enter  them  and  be  carried  to  the  backing. 
This  objection  is  sometimes  met  by  building  the  face  with  horizon- 
tal courses,  and  inclining  the  courses  in  the  back  of  the  wall.  The 
back  of  the  wall  for  2  or  3  feet  from  the  top  should  have  a  batter 
of  at  least  1  inch  in  1  foot,  in  order  that  the  frost  may  lift  the 
earth  and  not  break  the  joints  of  the  masonry. 

Walls  are  sometimes  built  with  both  faces  inclined  toward  the 
material  to  be  supported,  and  sometimes  with  a  curved  profile  ;  but 
it  is  generally  considered  unwise  to  do  either,  owing  to  the  extra 
expense  and  trouble  in  construction. 

551.  Land  Ties.  Retaining  walls  may  have  their  stability  in- 
creased by  being  tied  or  anchored  by  iron  rods  to  vertical  plates  of 
iron  or  blocks  of  stones  imbedded  in  a  firm  stratum  of  earth  at  a 
distance  behind  the  wall.  "  The  holding  power,  per  foot  of  breadth, 
of  a  rectangular  vertical  anchoring  plate,  the  depth  of  whose  upper 


352  RETAINING   WALLS.  [CHAP.  XIY. 

and  lower  edges  below  the  surface  are  respectively  a:,  and  x^,  may 
be  approximately  calculated  from  the  following  formula  : 

„          a;„'  —  a-,'  4  sin  0  .^^. 

H=tv-^— — '- -^, (14) 

2         cos  2  0  ^     ' 

in  which  H  is  the  holding-power  of  the  plate  in  pounds  per  foot  of 
breadth,  w  is  the  weight  in  pounds  of  a  cubic  foot  of  the  earth, 
and  0  its  angle  of  repose.  The  center  of  pressure  of  the  plate  is 
about  two  thirds  of  its  height  below  its  upper  edge, — at  which  point 
the  tie-rod  should  be  attached. 

''If  the  retaining  wall  depends  on  the  tie-rods  alone  for  its 
security  against  sliding  forward,  they  should  be  fastened  to  plates 
on  the  face  of  the  wall  in  the  line  of  the  resultant  pressure  of  the 
earth  behind  the  wall,  that  is,  at  one  third  [see  §  530]  of  the  height 
of  the  wall  above  its  base.  But  if  the  resistance  to  sliding  forward 
is  to  be  distributed  between  the  foundation  and  the  tie-rods,  the 
latter  should  be  placed  at  a  higher  level.  For  example,  if  half  the 
horizontal  thrust  is  to  be  borne  by  the  foundation  and  half  by  the 
tie-rods,  the  latter  should  be  fixed  to  the  wall  at  two  thirds  of  its 
height  above  the  base."  * 

652.  Relieving  Arches.     In  extreme  cases,  the  pressure  of  the 

earth  may  be  sustained  by  relieving-arches.     These  consist  of  a  row 

Ejjroa-v.-.' ■— -•     of  arches  having  their  axes  and  the  faces  of  their 


piers  at  right  angles  to  the  face  of  a  bank  of  earth. 
There  may  be  either  a  single  row  of  them  or  several 
^%JM^MM^\  tiers;  and  their  front  ends  may  be  closed  by  a  ver- 
^^~^:V:!  tical  wall, — which  then  presents  the  appearance  of 
a  retaining  wall,  although  the  length  of  the  arch- 
ways is  such  as  to  prevent  the  earth  from  abutting 


Fig.  77.  against  it.     Fig.  77  represents  a  vertical  transverse 

section   of   such   a  wall,   with   two   tiers   of   relieving   arches    be- 
Jiind  it. 

To  determine  the  conditions  of  stability  of  such  a  structure  as  a 
whole,  the  horizontal  pressure  against  the  vertical  plane  OD  may  be 
determined,  and  compounded  with  the  weight  of  the  combined 
mass  of  masonry  and  earth  OAED,  to  find  the  resultant  pressure 
on  the  foundation. 

*Raakine's  Civil  Ens^ineering,  p.  411. 


CHAPTER  XV. 


BRIDGE  ABUTMENTS. 


653.  General  Forms.  There  are  four  forms  of  abutments  in 
more  or  less  general  use.  1.  A  plain  wall  parallel  to  the  current, 
shown  in  elevation  at  Fig.  78,  with  or  without  the  wings  ^  1)  F  and 
BEG.  The  slopes  may  be  finished  with  an  inclined  coping,  as 
A  D,  or  offset  at  each  course,  as  B  E — usually  the  latter.  This  form 
may  appropriately  be  called  the  straight  abutment.  2.  The  wings 
may  be  swung  around  into  the  bank  at  any  angle,  as  shown  (in  plan) 
in  Fig.  79.     The  angle  0  is  usually  about  30°.     This  form  is  known 


A 


B 


B 


4^^^^:~a: 


D         E 

Fig.  79. 


D         E 

Fig.  80. 


Fig.  81. 


as  the  loing  abutment.  3.  When  0  of  Fig.  79  becomes  90°,  we  have 
Fig.  80,  which  is  called  the  U  ahutment.  4.  If  the  wings  of  Fig. 
80  are  moved  to  the  center  of  the  head-wall,  we  get  Fig.  81,  which 
is  known  as  the  T  abutment. 

The  abutment  of  an  ordinary  bridge  has  two  offices  to  perform, 
viz.,  (1)  to  support  one  end  of  the  bridge,  and  (2)  to  keep  the  earth 
embankment  from  sliding  into  the  water.  In  Fig.  78,  the  portion 
D  E  G  F  serves  both  these  purposes,  while  the  wings  A  D  F  and 
BEG  act  only  as  retaining  walls.  In  Figs.  79  and  80,  the  portion 
D  E  performs  both  offices,  while  the  wings  A  D  and  B  E  are  merely 
retaining  walls.  In  Fig.  81  the  "  head  "  D  E  supports  the  bridge, 
and  the  ''tail,"  or  ''stem,"  A  B  carries  the  train;  hence  the  whole 
structure  acts  as  a  retaining  wall  and  also  supports  the  load.  The 
abutment  proper  may  fail  (1)  by  sliding  forward,  (2)  by  bulging,  or 
(3)  by  crushing;  however,  it  is  improbable  that  it  will  fail  by  sliding 
forward.  Its  dimensions  are  to  be  determined  as  for  a  retaining 
wall  (Chap.  XIV);  but  the   mathematical  theory  of    the  lateral 

353 


354  BRIDGE    ABUTMENTS.  [CHAP.   XV. 

pressure  of  earth  is  a  much  less  perfect  guide  for  designing  bridge 
abutments  than  it  is  for  simple  retaining  Avails.  The  weight  of  the 
bridge  helps  the  abutment  to  resist  the  thrust  of  the  earth;  but,  on 
the  other  hand,  the  weight  of  the  train  on  the  embankment  in- 
creases the  lateral  pressure  against  the  abutment. 

554.  The  form  of  the  abutment  to  be  adopted  for  any  particular 
case  will  depend  upon  the  locality, — whether  the  banks  are  low  and 
flat,  or  steep  and  rocky;  whether  the  current  is  swift  or  slow;  and 
also  upon  the  relative  cost  of  earthwork  and  masonry.  If  the  shore 
is  flat,  and  not  liable  to  be  cut  away  by  the  current,  an  abutment 
like  Fig.  78  will  be  sufficient  and  most  economical.  However,  this 
form  is  seldom  used,  owing  to  the  danger  of  the  water's  flowing 
along  immediately  behind  the  wall. 

The  form  of  Fig.  79  may  be  adopted  when  there  is  a  contraction 
of  the  waterway  at  the  bridge  site,  since  deflecting  the  wing  walls, 
above  and  below,  slightly  increases  the  amount  of  Avater  that  can 
pass.  This  advantage  can  be  obtained,  to  some  degree,  with  the 
straight  abutment  (Fig.  78)  by  thinning  the  wings  on  the  front  and 
leaving  the  back  of  the  wings  and  abutments  in  one  straight  line. 
There  is  not  only  no  hydraulic  advantage,  but  there  is  a  positive 
disadvantage,  in  increasing  the  deflection  of  the  wings  beyond,  say, 
10°  or  15°.  The  more  the  wing  departs  from  the  face  line  as  it 
SAvings  round  into  the  embankment,  the  greater  its  length  and  also 
the  greater  is  the  thrust  upon  it.  The  wings  are  not  usually  ex- 
tended to  the  toe,  B,  of  the  embankment  slope,  but  stop  at  a  height, 
depending  upon  the  angle  of  deflection  and  the  slope,  such  that  the 
earth  flowing  around  the  end  of  the  wall  will  not  get  into  the  chan- 
nel of  the  stream.  It  can  be  shoAvn  mathematically  that,  if  the  toe 
of  the  earth  Avhich  flows  around  the  end  of  the  Aving  is  to  be  kept 
three  or  four  feet  back  from  the  straight  line  through  the  face  of 
the  abutment,  an  angle  of  25°  to  35°  is  best  for  economy  of  the 
material  in  the  wing  walls.  This  angle  varies  slightly  with  the  pro- 
portions adopted  for  the  Aving  wall  and  with  the  details  of  the 
masonry.  This  form  of  construction  is  objectionable,  since  the 
foot  of  the  slope  in  front  of  the  wing  is  liable  to  be  washed  away  j 
but  this  could  be  remedied  somewhat  by  rii^rapping  the  slope,  or, 
better,  by  making  the  wings  longer. 

Fig.  78  is  one  extreme  of  Fig.  79,  and  Fig.  80  is  the  other.  As 
the  wing  swings  back  into  the  embankment  the  thrust  upon  it  in- 


WING   ABUTMENT.  355 


creases,  reaching  its  maximum  at  an  angle  of  about  45°;  when  the 
wing  is  thrown  farther  back  the  outward  thrust  decreases,  owing  to 
the  filling  up  of  the  slope  in  front  of  the  wing.  Bringing  the  wings 
perpendicular  to  the  face  of  the  abutment,  as  in  Fig.  80,  also  de- 
creases the  lateral  pressure  of  the  earth,  owing  to  the  intersection  of 
the  surfaces  of  rupture  for  the  two  sides,  which  is  equivalent  to  re- 
moving part  of  the  ''prism  of  maximum  thrust."  If  the  banks  of 
the  stream  are  steep,  the  base  of  the  wing  walls  of  Fig.  80  may  be 
stepped  to  fit  the  ground,  thereby  saving  masonry.  Under  these 
conditions,  also  the  wing  abutment.  Fig.  79,  can  be  treated  in  the 
same  way;  but  the  saving  ig  considerably  less.  When  the  masonry 
is  stepped  off  in  this  way,  the  angle  thus  formed  becomes  the  weak- 
est part  of  the  masonry;  but,  as  the  masonry  has  a  large  excess  of 
strength,  there  is  not  much  probability  of  danger  from  this  cause, 
provided  the  work  is  executed  with  reasonable  care. 

655.  Fig.  81  is  the  most  common  form  of  abutment.  For  equal 
amounts  of  masonry,  Aving  abutments  give  better  protection  to  the 
embankments  than  T  abutments.  The  latter  are  more  stable,  be- 
cause the  center  of  gravity  of  the  masonry  is  farther  back  from  the 
line  of  the  face  of  the  abutment,  about  which  line  the  abutment 
must  turn  or  along  which  it  will  first  crush.  The  amount  of  ma- 
sonry in  tall  T  abutments  can  be  decreased  by  building  the  tail  wall 
hollow,  or  by  introducing  arches  under  it.  The  more  massive  the 
masonry,  the  cheaper  it  can  be  constructed;  and,  for  this  reason,  it 
is  probable  that  the  simple  T  abutment  is  cheaper  than  the  U  abut- 
ment, although  the  latter  may  have  less  masonry  in  it.  On  the  other 
hand,  the  opportunities  for  inspecting  the  masonry  during  construc- 
tion are  better  with  the  U  than  with  the  T  abutment,  and  hence  the 
former  is  usually  better  built  than  the  latter.  This  is  an  important 
item,  since  it  is  somewhat  common  for  railroad  masonry  to  fail  by 
being  shaken  to  pieces  by  the  passage  of  trains. 

556.  Wing  Abutment.  Fig.  82  shows  a  common  form  of  the 
wing  abutment.  This  one  is  finished  with  stone  pedestal  blocks — 
marked  B  in  plan,  A  in  elevation,  and  C  in  section, — which  is  not 
always  done.  The  thickness  of  pedestal  blocks  and  the  thickness 
of  the  coping  under  the  pedestal  blocks  vary  slightly  with  the  span 
(see  §  558).  The  height  of  the  parapet  wall,  or  dirt  wall  (the  wall 
which  keeps  back  the  top  of  the  embankment,  marked  F  W  in 
section),  will  vary  with  the  style  of  the  bridge,  but  should  not  have 


356 


BRIDGE   ABUTMENTS. 


[chap.  XV. 


a  thickness  less  than  four  tenths  of  its  height  (see  §§  547  and  548). 
The  bridge  often  rests  directly  upon  the  coping.  The  top  dimen- 
sions of  the  abutment  will  depend  somewhat  upon  the  size  and 
form  of  bridge  :  but  for  railroad  bridges  it  will  usually  not  be  less 
than  5  ft.  wide  by  20  ft.  long,  nor  more  than  6  ft.  by  23  ft. 

u 

Z 

U3 


< 


/u    "■' 


t~-^<-<*       ,    ft"        3jqjrj      ijsxNSO 


The  usual  batter  is  1  in  12;  sometimes  1  in  24.  For  heights 
under  about  20  ft.,  the  top  dimensions  and  the  batter  determine  the 
thickness  at  the  bottom.     For  greater  heights,  the  quite  uniform 


WING    ABUTMENT. 


357 


TABLE  37. 

Quantity  of  ]\Iasonry  in  Wing  Abutments  of  the  General  Form 

SHOWN  IN  Fig.  82.     See  g  557. 


1  g 

Dimensions  of  the 

Area  of  Lowest      i 

Masonry  in  one  Abutment, 

Is 
gg 

Bottom  of  abutment. 

Course 

EXCLUSIVE  of  Footing, 
Copings,  and  Pedestals.* 

"3 

is 

^2 

o 

& 

m 

^ 

^ 

°5 

■^ 

•"  !X 

■6 

tao 

be 

SO 
a  2; 

_M    CO 

o  a 

1/ 

^ 

a 

a 

t 

EP 

U 

O 

3 

o 

c 

p 

3 

o 

H 

J 

o 

H 

H 

O 

H 

(2 

H 

feet. 

/ee^ 

feet. 

feet. 

sqft. 

sq.ft. 

sq.ft. 

cu.  ft. 

cu.  ft. 

cu.ft. 

CM.  yds. 

5 

22.2 

6.8 

18.9 

1.51 

165 

316 

709 

640 

230 

57.7 

6 

22.2 

7.0 

20.6 

155 

184 

339 

863 

821 

230 

70  8 

22.3 

7  2 

22.3 

ICl 

203 

364 

1.021 

1.020 

230 

84.1 

8 

22.3 

7^3 

24.0 

j     163 

223 

386 

l.lS.'i 

1,238 

230 

98.1 

9 

22.4 

7.5 

25  7 

168 

243 

411 

1.348 

1.475 

S30 

113  0 

10 

22.4 

7.7 

27.4 

172 

264 

436 

1,518 

1.732 

230 

128.8 

11 

22.5 

7.8 

29.1 

176 

285 

461 

i.«92 

2.011 

230 

145.6 

12 

22.5 

8.0 

30.8 

180 

306 

486 

1.869 

2.310 

2.30 

162.6 

13 

22.5 

8.2 

32.5 

185 

328 

513 

2  052 

2,632 

230 

182.0 

14 

22.6 

8.3 

34.2 

188 

351 

539 

2.23S 

2.975 

230 

201.6 

15 

22.6 

8.5 

35.9 

192 

374 

566 

2.429 

3.341 

230 

222.2 

16 

22.7 

8.7 

37.6 

197 

398 

595 

2.623 

3,731 

230 

243.8 

ir 

22.7 

8.8 

39.4 

200 

422 

622 

2.822 

4.144 

230 

266.5 

18 

22.7 

9.0 

41.0 

204 

447 

651 

3.024 

4,580 

230 

290.1 

19 

22.8 

9.1 

42.8 

207 

472 

679 

3.232 

5.041 

230 

314.9 

20 

22.8 

9.3 

44.5 

'     212 

497 

709 

3.442 

5.526 

230 

340.6 

21 

22.9 

9.5 

46.2 

,     217 

523 

740     1 

3.657 

6.038 

230 

367.5 

22 

22.9 

9.7 

47.9 

222 

550 

772     ; 

3.876 

6.577 

2.30 

395.6 

23 

23.0 

9.8 

49.6 

225 

577 

802 

4,100 

7.143 

230 

424.9 

24 

23.0 

10.0 

51.3 

230 

604 

834 

4..327 

7.735 

230 

455.3 

25 

23.0 

10.2 

53.0 

2a5 

633 

868 

4.559 

8,354 

230 

486.7 

26 

23.1 

10.3 

.54.7 

238 

661 

899 

4.T96 

9.002 

230 

519.5 

27 

23.1 

10.5 

56.4 

243 

690 

933 

5.036 

9.678 

230 

553.4 

28 

23.2 

10.7 

58.1 

248 

720 

968 

5.281 

10.384 

230 

588.7 

29 

23.2 

10.8 

59.8 

251 

750 

1,001 

5.530 

11.120 

230 

625.1 

30 

23  3 

11.0 

61.5 

i     256 

780 

1,036 

5.784 

11.880 

230 

662.9 

31 

23.3 

11.2 

63.2 

261 

811 

1,072 

6.041 

12.682 

230 

701.9 

32 

Zi.S 

11.3 

64.9 

263 

843 

1,106 

6,303 

13.509 

230 

742.9 

33 

23.4 

11.5 

66.6 

269 

8T5 

1,144 

6,569 

14,309 

230 

784.0 

34 

23.4 

11.7 

68.4 

273 

907 

1,180 

6.841 

15.259 

230 

827.0 

35 

23.5 

11.8 

70.1 

277 

940 

1,217 

7.116 

16.182 

230 

871.4 

36 

23.5 

12.0 

71.8 

282 

973 

1,255 

7,395 

17.139 

230 

917.2 

37 

23.6 

12  2 

73.5 

t     288 

1,007 

1.295 

7,679 

18,127 

230 

964.2 

38 

23.6 

12.3 

75.2 

290 

1.042 

1,332 

7,967 

19,150 

230 

1,012.8 

*  Dimension  stone  in  two  pedestal  blocks =    64  cu.  feet. 

"  "         '•  coping  of  one  abutment  =234    "      " 

Total  dimension  stone  in    "  "         =298    " 

rule  is  to  make  the  thickness  four  tenths  of  the  height.  The  amount 
of  masonry  in  the  abutment  is  computed  in  accordance  witli  this 
rule,  although  the  actual  quantity  is  usually  more  than  that  required 
by  it.     Since  there  is  no  objection  to  the  wall's  being  rough,  no 


358  BRIDGE    ABUTMEXTS.  [CHAP.   XV. 

stones'  are  cut  out  to  secure  the  specified  thickness,  and  hence  the 
actual  quantity  of  masonry  usually  exceeds  the  amount  required. 
The  spread  of  the  footing  courses  and  foundation  will  depend,  of 
course,  upon  the  location. 

The  wings  should  be  proportioned  according  to  the  rules  for 
retaining  walls  (see  §§  547  and  548).  The  wings  are  not  always  pro- 
longed u^ntil  their  outer  ends  intersect  the  foot  of  the  embankment 
slope;  but  are  frequently  stopped  with  an  end  height  of  3  to  5  feet 
above  the  footing.  The  thickness  of  the  wing  wall  decreases  from 
the  body  of  the  abutment  toward  the  tail  in  proportion  to  the  height. 
For  appearance,  the  top  of  the  wing  is  usually  made  unifo^'m  from 
head  to  tail,  being  usually  from  2|  to  3^  feet,  according  to  the  size 
of  the  structure.  The  steps  should  be  capped  with  stones,  not  less 
than  1  foot  thick,  covering  the  entire  step  and  extending  under  the 
step  above  not  less  than  1  foot. 

557.  Contents  of  Wing  Abutments.  The  table  on  page  357 
gives  the  quantities  of  masonry  in  wing  abutments  of  the  form 
shown  in  Fig.  82.  Since  the  outlines  of  such  structures  are  not 
simple  geometrical  figures,  it  is  necessary  to  make  more  or  less  ap- 
proximations in  computing  the  cubical  contents.  For  exariple,  in 
Fig.  82  the  wings  are  stepped  off  to  fit  the  slope  of  the  emba  ikment 
as  shown;  and  hence  the  corner  of  each  course  projects  beyond  the 
earthwork.  The  amount  of  masonry  in  these  projecting  corners 
varies  as  the  thickness  of  the  courses,  and  for  any  particular  abut- 
ment it  could  be  found  accurately;  but,  in  computing  a  table  of 
general  results,  it  is  necessary  to  assume  some  thickness  for  the 
courses.  In  this  case  the  courses  were  assumed  to  be  1  foot  thick. 
The  back  of  the  "  head  "  was  assumed  to  conform  strictly  to  the  batter 
line,  although  in  construction  it  would  be  stepped.  The  dimensions 
of  the  parapet  wall  will  vary  with  the  thickness  of  the  pedestal 
blocks  used,  and  also  with  the  style  of  the  bridge.  The  contents 
of  the  parapet  as  given  in  the  table  are  for  the  dimensions  shown  in 
Fig.  82. 

Footing  courses  were  not  included  in  the  table,  since  they  vary 
with  the  nature  of  the  foundation.  The  area  of  the  lowest  course 
of  masonry  is  given,  from  which  the  areas  of  the  footing  courses  and 
of  the  foundation  pit  may  be  determined.  The  thickness  at  the 
top  and  the  batter,  as  in  Fig.  82,  give,  for  any  height  found  in  the 
table,  a  thickness  of  wall  at  the  bottom  of  at  least  four  tenths  of  its 


U   ABUTMENT.  359 


height  (see  §548);  for  heights  greater  than  in  the  table,  the  back 
of  tlie  wall  must  be  stepped  to  keep  the  thickness  four  tenths  of  the 
height.  * 

558.  U  Abutment.  Fig.  S3  shows  the  standard  plans  of  the 
Atchison,  Topeka  and  Santa  Fe  R.  R.f  for  U  abutments.  This  is 
the  only  form  of  bridge  abutment  used  on  this  road,  except  in 
special  cases.  The  T  abutment  was  once  the  standard,  but  was 
abandoned  about  fifteen  years  ago.  J 

The  specifications  under  which  these  abutments  are  built,  require 
as  follows  :  '"l.  Bed-plate  pedestal  blocks  to  be  2  feet  thick,  and 
placed  symmetrically  with  regard  to  the  plates.  2.  Coping  under 
pedestal  blocks  to  be  18  inches  thick  for  all  spans  exceeding  100 
feet,  16  inches  for  90  feet,  and  14  inches  for  spans  under  90 
feet, — said  coping  to  be  through  stones,  and  spaced  alike  from  both 
sides  of  abutment.  3.  Distances  from  front  of  dirt  wall  to  front 
of  bridge  seat,  and  from  grade  line  to  top  of  bridge  seat,  and 
thickness  of  dirt  wall,  to  vary  for  different  styles  and  lengths  of 
bridges.  4.  Front  walls  to  be  22  feet  wide  under  bridge  seat  for 
all  spans  of  100  to  160  feet  inclusive.  5.  Total  width  of  bridge 
seat  to  be  5^  feet,  for  all  spans.  6.  Steps  on  back  of  walls  to 
be  used  only  when  necessary  to  keep  thickness  ^-^  of  the  height. 
7.  In  case  piling  is  not  used,  footing  courses  may  be  added  to  give 
secure  foundation.  8.  Length  of  wing  walls  to  be  determined  by  a 
slope  of  1|^  to  1  at  the  back  end  of  the  walls — as  shown  by  dotted 
line  in  front  elevation, — thence  by  a  slope  of  1  to  1  down  the. outside 
— as  shown  on  side  elevation — to  the  intersection  of  the  ground  line 
with  face  of  abutment.  This  rule  may  be  modified  in  special  cases. 
9.  Dimensions  not  given  on  the  drawing  are  determined  by  the 
style  and  length  of  bridge,  and  are  to  be  found  on  special  sheet." 

559.  Although  this  road  is  noted  for  the  excellency  of  its 
masonry,  this  design  could  be  improved  by  leaving  a  weep  hole  in 
the  side  walls,  2  or  3  inches  wide  and  the  depth  of  a  course  of 


*  In  computing  the  contents  of  masonry  structures,  it  is  necessary  to  remember 
that  the  volume  of  any  mass  which  is  made  up  of  prisms,  wedges,  and  pyramids — or 
cones — must  be  determined  by  the  prismoidal  formula  ;  but  if  the  mass  is  composed 
wholly  of  prisms  and  wedges,  the  contents  can  be  correctly  found  by  using  the  aver- 
age of  the  end  areas. 

t  Published  by  permission  of  A.  A.  Robinson,  Chief  Engineer. 

X  Compare  with  §  555. 


360 


BEIDGE    ABUTMENTS. 


[chap.  XV. 


ddOdDO 


FRONT   ELEIVXTlOI*. 


Fig.  83.— U  Abotment.-A.  T.  &  S.  F.  R.  R 


U   ABUTilEXT. 


361 


TABLE  38. 

Quantity  of  Masonky  in  U  Abutments  of  the  General  Form 
SHOWN  IN  Fig.  83.     See  8  560. 


So 

Dimensions 

OP  THE 

a 

Quantity  op 
Masonky,  ex- 

Bottom of  the 

z. 

clusive  OF 

I?  « 

oo 

Abutment. 

^ 

Coping.* 

Ed     . 

'J 

|5 

EXABfPLES  OF 

THE  Method  or 

©a 

O  O 

Cm 

USING  THE  Table. 

Hg 

n 

«M 

al 

^ 

s 

a 

1! 

2o 

."2 

o 

eS 

2S 

"§■ 

o§ 

a  H 

r^ 

u 

B  <! 

<o 

^iu 

CE 

'^ 

H 

< 

H 

a 

H 

1 
feet. 

feet. 

feet. 

feet. 

feet. 

CM.  ft. 

cu.  ft. 

1 

1 

22.2 

5.1 

113 

3.1 

Ill 

6.0 

1 

di  —  — 

'''''''       s 

2 

22.3 

5.2 

115 

3.2 

225 

12.3 

e 

S'  '  -  ^  ' 

'  ^  z  ::  i  ^  i       i 

3 

22.5 

5.2 

119 

3.2 

342 

18.8 

5 

OWt-m'^l- 

OO00Or-.X00     1    1-. 

4 

22.7 

5.3 

120 

3.3 

462 

25.2 

1 

o  OS  c;  CO  «o  t'. 

5 

22.8 

5.4 

124 

3.4 

584 

32.0 

^ 

»»' 

CO                     o? 

6 

23.0 

5.5 

126 

3.5 

709 

39.0 

8 

II  II  II  II  II  1 

II  II  II  II  II  II  II    II 

7 

23.2 

5.6 

129 

3.6 

837 

46.0 

S 

8 

23.3 

5.7 

132 

3.7 

968 

53.3 

8 

9 
10 

23.5 
23  7 

5.8 
5.8 

135 
138 

3.8 
4.0 

1,101 
1,238 

60.8 
68.4 

1 

^-XX 

11 

23.8 

5.9 

141 

4.4 

1,377 

76.8 

s 

00 

12 
13 

24.0 
24.2 

6.0 
6.1 

144 
147 

4.8 
5.2 

1,520 
1,665 

86.0 
96.0 

iT 

If 

II  II  II  II 

OB 

•a 

14 

24.3 

6.2 

150 

5.6 

1,814 

106.8 

Os 

x> 

d-  -  d 

>no      ^ 

15 

24.5 

^■l 

153 

6.0 

1,966 

118.4 

Oc 

"^S{? 

16 

24.7 

6.3 

156 

6.4 

2,120 

130.8 

g 

o 

OJtt 

Jl"i'So--    - 

17 
18 

24.8 
25.0 

6.8 

7.2 

169 

180 

6.8 

7.2 

2,288 
2,478 

144.0 
158.0 

1 

"^ 

-X 

+4 

'5         S  Xoo     ^ 

19 
20 

25.2 
25.3 

7.6 
8.0 

191 
203 

7.6 
8.0 

2;688 
2,920 

172.8 

188.4 

S5x 

X> 

o  o 

21 

25.5 

8.4 

214 

8.4 

3,174 

204.8 

i? 

•^o 

CJTT 

-     ■,  a,   o  •'^  c  r      -g 

22 

25.7 

8.8 

226 

8.8 

3,449 

222.0 

s 

X|j 

+4 

23 

25.8 

9.2 

238 

9.2 

3,746 

240.0 

ts 

24 

26.0 

9.6 

250 

9.6 

4,066 

258.8 

Ci 

25 

26.2 

10.0 

262 

10.0 

4,408 

278.4 

s 

Xll^«"' 

Br  eT^    ^ 

26 

26.3 

10.4 

274 

10.4 

4,772 

298.8 

27 

26.5 

10.8 

286 

10.8 

5,160 

320.0 

i        ^ 

—  ,-•  C*          OCO  - 

1 S' '  ^-H  g 

28 

26.7 

11.2 

299 

11.2 

5,570 

342.0 

';^ 

II  —     "ScSio 

01 35        a     be 

29 

26.8 

11.6 

311 

11.6 

6,003 

364.8 

o 

=  5§-S.S 

—  2       "3  c  □ 
■«  o-  -  c  °-S 

30 
31 

27.0 
27.2 

12.0 
12.4 

324 
337 

12.0 
12.4 

6,460 
6,941 

388.4 
412.8 

.« 

fjs'  '    C  taoo- 

.a             »•« 

32 

27.3 

12.8 

350 

12.8 

7,445 

438.0 

5   . 

^               §^S 

33 

27.5 

13.2 

363  1 

13.2 

7,973 

464.0 

»  g 

CM,.     ..     ..     «     « 

o-  -   -  -  - 

:               s  r  : 

34 

27.7 

13.6 

376 

13.6 

8,526 

490.8 

^1 

B 

35 

27.8 

14.0 

390 

14.0 

9,103 

518.4 

Q 

Szmz 

a 

o 

s                —  - 

*F( 

jr  dimen 

sions 

of  copin 

J  and 

pedestal 

blocks. 

see  86 

cond  pa 

ragrai 

3h  of  §  55 

«. 

« 

362 


BRIDGE   ABUTMENTS. 


LCHAP.  XV. 


masonry,  for  each  4  or  5  square  yards  of  wing  wall.  Cinders,  or 
sand  and  gravel  are  sometimes  used  to  fill  in  between  the  wing  walls 
to  give  a  better  drainage,  and  also  to  decrease  the  lateral  thrust  of 
the  earth. 

560.  Contents  of  U   Abutments.     The  table  on  page  361  gives 
the  contents  of  U  abutments  of  the  form  shown  in  Fig.  83.     The 


u 

u 

u 

L  1 

i-J 

u 

U 

l.J 

n 

n 

p 

r-i 

m 

r-j 

r-> 

n 

V 

Kj 

1 1 

\j 

'J 

l_  J 

0 

'J 

V^!9  Sietjdiii 

on^ 

J  U  U  iJ  i-J  i-Jl 

n  n  n  n  r.  ,,,. 
Ij  'J  u  'J  'J  ^-^l''  • 


T    ABUTMENT 


quantities  were  computed  on  the  basis  that  the  thickness  of  the 
walls  was  four  tenths  the  height,  except  that  no  wall  was  taken  of  a 
less  thickness  than  that  given  by  the  thickness  at  the  top  and  the 
batter  as  in  the  drawing. 

561.  T  Abutment.     Fig.  84  shows  the  ordinary  form  of  T  abut- 


T  ABUTMENT. 


363 


TABLE   39. 

Quantity  op  Masonry  ix  T  Abutments  of  the  General  Form 

SHOWN  IN  Fig.  84.     See  §562. 


1 

C  a.  ' 

*=  o 
go 

Dimensions  op  the 

Quantity  of  Masonry, 

BOTTOii    Ui    THE  HeAD. 

Exclusive  of 

Coping. 

h 

^  o 

o 

Example  of  the  Method  of 

' 

o 

using  the  Table. 

§5 

si' 

•53 

a 
o 

i 

i 

=1 

feet. 

feet. 

feet. 

feet. 

cu.  ft. 

cu.ft. 

CU.  ft. 

1 

5 

22.8 

5.8 

133 

607 

12.5 

60 

5    W                          m     ai^ 

oT 

6 

23.0 

6.0 

138 

743 

18.0 

73 

Os       II  II  II  II   II  II  II   II   II  II   II   II  II       II 

7 

23.2 

6.2 

143 

883 

24.5 

84 

C         C<  CO  00  0 

■■    "2 

8 

23  3 

6.3 

148 

1,029 

33.0 

96 

^>        XaOGOO 

•^j" 

9  1 

23.5 

6.5 

153 

1,179 

40.5 

108 

•"         y-XXX 

QO  OC  ■^ 

0 

;  1 

10 

11  ! 

23.7 
23.8 

6.7 
6.8 

158 
163 

1,334 
1,495 

50.0 
60.5 

130 
133 

XXX 

13  i 

24.0 

7.0 

168 

1,660 

72.0 

144 

§     jxxx 

II  II  II 

13  j 

14  1 

24.3 
24.3 

7.2 
7.3 

173 

178 

1,831 
2,006 

84.5 
98.0 

156 

168 

^      «_ooo 

■-r  ^    0 

IT 

CO 

:  II 
•     b 

15 

24.5 

7.5 

184 

2,188 

113.5 

180 

00  ^    0       II   II 

X 

OClOO 

a 

16 
17 

34.7 
24.8 

7.7 
7.8 

189 
195 

2,374 
2,566 

138.0 
144.5 

193 
304 

1^  S^ 

QCad-*' 
II  II  II 

.a 

\  i 

:    S 

18 

25.0 

8.0 

200  1 

2,763 

163.0 

316 

19 
20  i 
21 

25.2 
25.3 
25.5 

8.3 
8.3 
8.5 

206 

311   1 
317 

2,966 
3,174 
3,388 

180.5 
300.0 
330.5 

338 
340 
252 

"S-s  oi-  -  X"2  II  lldiiif  ^>^ 

22 

35.7 

8.7 

323 

3,608 

343.0 

264 

ilH  ^ifrF"S 

23 

25.8 

8.8 

328 

3,833 

264.5 

276 

24 

26.0 

9.0 

234 

4,064 

388.0 

288 

25 

26.2 

9.2 

240 

4,301 

313.5 

300 

26 

26.3 

9.3 

246 

4,544 

338.0 

312 

27 

26.5 

9.5 

252 

4,793 

364.5 

324 

1-3  1^0         ^i,^     =,  ^    g:= 

28 

26.7 

9.7 

258 

5,047 

393.0 

336  ' 

29 

26.8 

9.8 

264 

5,308 

430.5 

348 

30 

27.0 

10.0 

270 

5,575 

450.0 

360 

31 
32 
33 

27.2 
27.3 
27.5 

10.2 
10.3 
10.5 

276 

282 
289 

5,848 
6,127 
6,413 

480.5 
513.0 
544.5 

372 
384 
396 

S-:  81^^  Is 2 lists 

34 

27.7 

10.7 

295 

6,705 

578 . 0 

408 

"-^  ^ ... 

35 

27.8 

10.8 

301 

7,003 

613.5 

420 

•S3  » 

.fc—   a 

Area 

5f  coping 

J  on  2  wi 

ngs,  per  f 

t.  of  len{ 

rth=    5 

sq.  ft. 

Area 

of  copinj 

J  on  brid 

geseat 

=  138 

e 

364  BRIDGE   ABUTMENTS.  [CHAP.  XT- 

ment.  For  railroad  bridges  the  head  is  usually  not  less  than  5  ft- 
X  20  ft.,  nor  more  than  6  ft.  X  22  ft.,  under  the  coping,  according 
to  the  size  of  the  bridge.  The  tail  wall  is  usually  10  or  12  ft.  wide, 
and  of  such  length  that  the  foot  of  the  slope  of  the  embankment 
will  just  reach  to  the  back  of  the  head  wall.  The  batter  on  the 
head  wall  is  1  to  12  or  1  to  24  all  around.  The  tail  wall  is  generally 
built  vertical  on  the  sides  and  the  end.  Notice  the  batter  at  the 
top  of  the  free  end  of  the  tail  wall.  This  is  known  as  the  "frost 
batter,"  and  is  to  prevent  the  frost  from  dislocating  the  corner  of 
the  masonry.  The  drainage  of  the  ballast  pocket  should  be  pro- 
vided for  by  leaving  a  space  between  the  ends  of  two  stones. 
Formerly  the  tail  wall  was  sometimes  only  7  or  8  feet  wide,  in  which 
case  the  ties  were  laid  directly  upon  the  masonry  without  the  inter- 
vention of  ballast ;  but  this  practice  has  been  abandoned,  as  being 
very  destructive  of  both  rolling  stock  and  masonry. 

According  to  the  common  theories  for  retaining  walls,  T  abut- 
ments with  dimensions  as  above  have  very  large  factors  of  stability 
against  sliding,  and  overturning,  and  crushing. 

562.  Contents  of  T  Abutments.  The  table  on  page  363  gives  the 
contents  of  the  abutments  of  the  form  shown  in  Fig.  84.  The 
height  of  the  tail  above  the  under  side  of  the  bridge-seat  coping  will 
vary  with  the  thickness  of  the  pedestal  blocks,  and  with  the  style  of 
the  bridge  ;  and  hence  the  table  gives  the  quantities  in  the  abutment 
below  the  bridge-seat  coping  and  above  the  footing.  The  quantity 
of  masonry  above  this  line  will  vary  also  with  the  amount  of  ballast 
used.  The  term  "wedge"  in  the  table  is  used  to  designate  that 
part  of  the  tail  included  between  the  head  and  a  vertical  plane 
through  the  lower  edge  of  the  back  face  of  the  head. 

563.  Foundation.  Usually  but  little  difficulty  is  encountered 
in  securing  a  foundation  for  bridge  abutments.  Frequently  the 
foundation  is  shallow,  and  can  be  put  down  without  a  coffer-dam, 
or  at  most  with  only  a  light  curb  (see  §§  316-20).  Where  the  ground 
is  soft  or  liable  to  scour,  a  pile  foundation  and  grillage  is  generally 
employed.  For  the  method  of  doing  this,  see  Art.  3,  Chapter  XI  ; 
and  for  examples  of  this  kind  of  foundation,  see  Fig.  84  (page  362), 
Fig.  86  (page  380),  and  Fig.  90  (page  386). 

Where  there  is  no  danger  of  underwashing,  and  where  the  foun- 
dation will  at  all  times  be  under  water,  the  masonry  may  be  started 
upon  a  timber  platform  consisting  of  timbers  from,  say,  8  to  13 


QUALITY   OF    MASONKT.  365 

mches  thick,  laid  side  by  side  upon  sills,  and  covered  by  one  or 
more  layers  of  timbers  or  thick  planks,  according  to  the  depth  of 
the  foundation  and  the  magnitude  of  the  structure.  For  an  exam- 
ple of  a  foundation  of  this  class,  see  Plate  II.  For  a  discussion  of 
the  method  of  failure  by  sliding  on  the  foundation,  see  §  491. 

564.  Quality  of  Masonry.— Bridge  abutments  are  built  of 
first-class  masonry  (§  307)  or  of  second-class  (§§  209  and  312),  ac- 
cording to  the  importance  of  the  structure.  See  also  the  specifica- 
tions for  bridge  pier  masonry  (§§  591-600).  The  coping  should  be 
composed  of  as  large  stones  as  practicable — not  less  than  13  inches 
thick,  and  15  or  18  inches  thick  is  better  and  more  frequently  used. 

Sometimes,  the  bed  plates  of  the  bridge  rest  directly  upon  the 
coping,  but  usually  upon  a  stone  pedestal  block  (see  Figs.  83  and 
83),  in  which  case  small  pedestals,  upon  which  the  rail  stringers 
rest  (see  Fig.  90,  page  386),  are  also  generally  used. 

565.  Cost.     For  data  on  the  cost  of  masonry,  see  §§  232-38. 


CHAPTER  XVI. 
BRIDGE     PIERS. 

566.  The  selection  of  the  site  of  the  bridge  and  the  ai-rangement 
of  the  spans,  altliough  important  in  themselves,  do  not  properly  be- 
long to  the  part  of  the  ^^I'oblem  here  considered  ;  therefore  they 
will  be  discussed  only  briefly.  The  location  of  the  bridge  is  usually 
a  compromise  between  the  interests  of  the  railroad  or  highway,  and 
of  the  river.  On  navigable  streams,  the  location  of  a  bridge,  its 
height,  position  of  piers,  etc.,  are  subject  to  the  approval  of  engi- 
neers appointed  for  the  purpose  by  the  United  States  Government. 
The  law  requires  that  the  bridge  shall  cross  the  main  channel  nearly 
at  right  angles,  and  that  the  abutments  shall  not  contract  nor  the 
piers  obstruct  the  water  way.  For  the  regulations  governing  the 
various  streams,  and  also  reports  made  on  special  cases,  see  the 
various  annual  reports  of  the  Chief  of  Engineers,  U.  S.  A.,  particu- 
larly Appendix  X3 ,  of  the  Report  for  1878. 

The  arrangement  of  the  spans  is  determined  mainly  by  the  rela- 
tive expense  for  foundations,  and  the  increased  expense  per  linear 
foot  of  long  spans.  Where  the  piers  are  low  and  foundations  easily 
secured,  with  a  correspondingly  light  cost,  short  spans  and  an  in- 
creased number  of  piers  are  generally  economical,  provided  the  piers 
do  not  dangerously  obstruct  the  current  or  the  stream  is  not  navi- 
gable. On  the  other  hand,  where  the  cost  of  securing  proper  foun- 
dations is  great  and  much  difficulty  is  likely  to  be  encountered,  long 
spans  and  the  minimum  number  of  piers  is  best.  Sound  judgment 
and  large  experience  are  required  in  comparing  and  deciding  upon 
the  plan  best  adapted  to  the  varying  local  conditions. 

Within  a  few  years  it  has  become  necessary  to  build  bridge  piers 
of  very  great  height,  and  for  economical  considerations  iron  has 
been  substituted  for  stone.  The  determination  of  the  stability  of 
such  piers  is  wholly  a  question  of  finding  the  stress  in  frame  struc- 
tures,— the  consideration  of  which  is  foreign  to  our  subject. 

366 


ART.   1.]  THEORY   OF    STABILITY.  367 


Art.  1.  Theory  of  Stability. 

567.  Method  of  Failure.  A  bridge  pier  may  fail  in  any  one 
of  three  ways  :  (1)  by  sliding  on  any  section  on  account  of  the  ac- 
tion of  the  wind  against  the  train,  bridge,  and  exposed  part  of  the 
pier,  and  of  the  current  of  the  stream  against  the  immersed  part  of 
the  pier ;  or  (2)  by  overturning  at  any  section  when  the  moment  of 
the'  horizontal  forces  above  the  section  exceeds  the  moment  of  the 
weight  on  the  section  ;  or  (3)  by  crushing  at  any  section  under  the 
combined  weight  of  the  pier,  the  bridge,  and  the  train.  The 
dimensions  of  piers  are  seldom  determined  by  the  preceding  condi- 
tions ;  the  dimensions  required  at  the  top  (§  58-1)  for  the  bridge 
seat,  together  with  a  slight  batter  for  appearance,  generally  give 
sufficient  stability  agamst  sliding,  overturning,  and  crushing.  How- 
ever, the  method  of  determining  the  stability  will  be  briefly  out- 
lined and  illustrated  by  an  example. 

568.  Stability  against  Sliding.  Effect  of  the  "Wind.  The 
pressure  of  the  wind  against  the  truss  alone  is  usually  taken  at  50 
lbs.  per  sq.  ft.  against  twice  the  vertical  projection  of  one  truss, 
which  for  well-proportioned  iron  trusses  will  average  about  10  sq.  ft. 
per  linear  foot  of  span.  The  pressure  of  the  wind  against  the  truss 
and  train  together  is  usually  taken  at  30  lbs.  per  sq.  ft.  of  truss  and 
train.  The  train  exposes  abgut  10  sq.  ft.  of  surface  per  linear  foot. 
The  pressure  of  the  wind  against  any  other  than  a  flat  surface  is 
not  known  with  any  certainty  ;  for  a  cylinder,  it  is  usually  assumed 
that  the  pressure  is  two  thirds  of  that  against  its  vertical  projection. 

569.  Effect  of  Current.  For  the  pressure  of  the  current  of 
water  against  an  obstruction,  Weisbach's  Mechanics  of  Engineering 
(page  1,030  of  Coxe's  edition)  gives  the  formula, 

P  =  swhf^,     .......     (1) 

in  which  P  is  the  pressure  in  pounds,  s  the  exposed  surface  in 
sq.  ft.,  k  a  co-efficient  depending  upon  the  ratio  of  width  to  length 
of  the  pier,  iv  the  weight  of  a  cubic  foot  of  water,  v  the  velocity  in 
ft.  per  sec,  and  g  the  acceleration  of  gravity.  For  piers  with 
rectangular  cross  section,  Tc  varies  between  1.47  and  1.33,  the  first 
being  for  square  piers  and  the  latter  for  those  3  times  as  long  a& 


368  BRIDGE    PIERS.  [CHAP.   XVI. 

Wide ;  for  cylinders,  h  —  about  0.T3.  Tlie  law  of  the  variation  of 
the  velocity  with  depth  is  not  certainly  known;  but  it  is  probable 
that  the  velocity  varies  as  the  ordinates  of  an  ellipse,  the  greatest 
velocity  being  a  little  below  the  surface.  Of  course,  the  water  has 
its  maxinnim  effect  when  at  its  highest  stage. 

570.  Effect  of  Ice.  The  pier  is  also  liable  to  a  horizontal  press- 
ure due  to  floating  ice.  The  formulas  for  impact  are  not  applica- 
ble to  this  case.  The  assumption  is  sometimes  made  that  the  field 
of  ice  which  may  rest  against  the  pier,  will  simply  increase  the  sur- 
face exposed  to  the  pressure  of  the  current.  The  greatest  pressure 
possible  will  occur  when  a  field  of  ice,  so  large  that  it  is  not  stopped 
by  the  impact,  strikes  the  pier  and  plows  past,  crushing  a  channel 
through  it  equal  to  the  greatest  width  of  the  pier.  The  resulting 
horizontal  pressure  is  equal  to  the  area  crushed  multiplied  by  the 
crushing  strength  of  the  ice.  The  latter  varies  with  the  tempera- 
ture; but  since  ice  will  move  down  stream  in  fields  only  when 
melting,  we  desire  its  minimum  strength.  The  crushing  strength 
of  floating  ice  is  sometimes  put  at  20  tons  per  sq.  ft.  (300  lbs.  per 
sq.  inch);  but  in  computing  the  stability  of  the  piers  of  the  St. 
Louis  steel-arch  bridge,  it  was  taken  at  600  lbs.  per  sq.  inch  (43 
tons  per  sq.  ft.).  According  to  experiments  made  under  the 
author's  direction,*  the  crushing  strength  of  ice  at  23°  F.,  varies 
between  370  and  760  lbs.  per  sq.  in. 

Occasionally  a  gorge  of  ice  may  form  between  the  piers,  and 
dam  the  water  back.  The  resulting  horizontal  pressure  on  a  pier 
will  then  be  equal  to  the  hydrostatic  pressure  on  the  width  of  the 
pier  and  half  the  span  on  either  side,  due  to  the  difference  between 
the  level  of  the  water  immediately  above  and  below  the  bridge 
opening.  A  pier  is  also  liable  to  blows  from  rafts,  boats,  etc. ;  but 
as  these  can  not  occur  simultaneously  with  a  field  of  ice,  and  will 
probably  be  smaller  than  that,  it  will  not  generally  be  necessary  to 
consider  them. 

A  lateral  pressure  on  the  pier  is  possible,  due  to  the  earth's  be- 
ing washed  away  from  one  side  and  not  from  the  opposite.  It  will 
be  on  the  safe  side,  and  near  enough  for  this  purpose,  to  assume 
that  this  effect  is  equal  to  the  pressure  of  a  liquid  whose  density  is 
the  difference  between  that  of  the  water  and  the  saturated  soil  dis- 
placed.    Under  these  conditions,   the  actual  tendency  to  slide  is 

<=  TuE  Technogkapp,  University  of  Illinois,  No.  9  (1894-95),  pp.  38-48. 


AET.   1.]  THEOKY    OF    STABILITY.  369 

equal  to  the  square  root  of  the  sum  of  the  down  stream  forces  and 
the  lateral  thrust.  However,  this  refinement  is  unnecessary,  par- 
ticularly since  a  pier  which  is  reasonably  safe  against  overturning 
and  crushing  will  be  amply  safe  against  sliding. 

571.  Resisting  Forces.  The  resisting  force  is  the  friction  due  to 
the  combined  ^Yeigllt  of  the  train,  bridge,  and  the  part  of  the  pier 
above  the  section  considered.  For  the  greatest  refinement,  it  would 
be  necessary  to  compute  the  forces  tending  to  slide  the  pier  for  two 
conditions  :  viz.,  (1)  with  a  wdnd  of  50  lbs.  per  sq.  ft.  on  truss  and 
pier,  in  which  case  the  weight  of  the  train  should  be  omitted  from 
the  resisting  forces  ;  and  (2)  with  a  wind  of  30  lbs.  per  sq.  ft.  on 
truss,  train,  and  pier,  in  which  case  the  weight  of  a  train  of  empty 
box  cars  should  be  included  in  the  resisting  forces.  For  a  table  of 
weights  of  masonry,  see  page  200.  If  the  water  can  find  its  way 
under  the  foundation  in  thin  sheets,  the  weight  of  the  part  of  the 
pier  that  is  immersed  in  the  water  will  be  diminished  by  62^  lbs. 
per  cu.  ft.  by  buoyancy  ;  but  if  it  finds  its  way  under  any  section 
by  absorption  only,  then  no  allowance  need  be  made  for  buoyancy. 

The  resisting  force  is  equal  to  the  product  of  the  total  weight 
and  the  co-efiicient  of  friction.  For  values  of  the  co-efficient  of 
friction,  see  the  table  on  page  315.  The  tenacity  of  the  mortar  is 
usually  neglected,  although  it  is  a  very  considerable  element  of 
strength  (see  §  137). 

572.  Stability  against  Overturning.  The  forces  which  tend 
to  produce  sliding  also  tend  to  produce  overturning,  and  the  forces 
which  resist  sliding  also  resist  overturning  ;  hence,  there  remains  to 
determine  only  their  points  of  application.  The  stability  can  be 
determined  either  by  moments  or  by  resolution,  as  was  explained  for 
dams  ;  but  in  this  case,  it  is  easier  by  moments,  since  there  are  sev- 
eral horizontal  forces,  and  it  requires  considerable  work  to  find  their 
resultant  as  demanded  by  the  method  by  resolution  of  forces. 

573.  A.  By  Moments.  By  this  method,  it  is  necessary  to  find 
the  arm  of  the  forces,  i.  e.,  the  perpendicular  distance  from  the  line 
of  action  of  the  forces  to  a  point  about  which  the  pier  tends  to  turn. 
This  is  the  same  method  as  that  used  in  §§  493-98,  which  see. 

The  center  of  pressure  of  the  wind  on  the  truss  is  practically  at 
the  middle  of  its  height ;  that  of  the  wind  on  the  train  is  7  to  9 
feet  above  the  top  of  the  rail  ;  and  that  of  the  wind  on  the  pier  is 
at  the  middle  of  the  exposed  part.     The  arm  for  the  pressure  of  the 


370  BRIDGE   PIERS.  [CHAP.  XVI. 

ice  should  be  measured  from  high  water.  The  center  of  pressure 
of  the  current  is  not  easily  determined,  since  the  law  of  the  varia- 
tion of  the  velocity  with  the  depth  is  not  known  ;  but  it  will  j)roba- 
bly  be  safe  to  take  it  at  one  third  the  depth.  Finally,  the  downward 
forces  will  usually  act  vertically  through  the  center  of  the  pier. 

From  these  data  the  overturning  and  resisting  moments  can 
easily  be  computed.  For  equilibrium,  the  summation  of  the  former 
must  be  less  than  the  latter.  The  above  principles  will  be  furtlier 
elucidated  in  §§  579-80  by  an  example. 

574.  B.  By  Resolution  of  Forces,  This  is  the  method  explained 
in  §  499  (page  320).  In  that  case  the  problem  was  very  sim- 
ple, since  there  were  but  two  forces  ;  but  in  the  present  case  there 
are  several  horizontal  forces  and  also  several  vertical  ones.  Tlie  first 
step  is  to  find  a  single  force  which  is  equivalent  in  every  respect  to 
the  combined  effect  of  all  the  horizontal  forces ;  the  second  is  to 
fiud  an  equivalent  for  all  of  the  vertical  forces  ;  and  the  third  is  to 
find  the  resultant  of  these  two  forces. 

The  horizontal  distance,  :c,  of  the  point  of  application  of  the  re- 
sultant of  all  the  vertical  forces,  back  from  the  toe  of  the  pier,  is 
found  by  the  equation, 

sum  of  the  moments  of  the  vertical  forces  .„. 

2;  ^z   - '■ - ,      ,       ,        [liiY 

sum  of  the  vertical  forces 

The  weight  of  the  train  and  bridge  act  vertically  through  the  center 
of  the  pier  ;  and  if  the  pier  is  symmetrical,  as  it  usually  is,  the 
weight  of  the  pier  will  also  act  through  its  center.  Therefore,  x  in 
equation  (2)  will  usually  be  half  the  length  of  the  pier. 

The  vertical  distance,  y,  of  the  point  of  application  of  the  re- 
sultant of  all  the  horizontal  forces  above  any  horizontal  joint  is 
found  by  the  equation, 

sum  of  the  moments  of  the  horizontal  forces  ,  , 

sum  of  the  horizontal  forces 

Having  found  x  and  y,  as  above,  draw  a  vertical  line  at  a  distance 
X  back  from  the  down  stream  end  of  the  pier ;  on  this  line  lay  off  a 
distance  y  above  the  horizontal  joint  under  consideration.  The 
point  thus  determined  corresponds  to  a  of  Fig.  70  (page  320).  Con- 
struct the  parallelogram  of  forces  by  laying  off,  to  any  convenient 


ART.   1.1  THEORY    OF   STABILITY.  371 

scale,  (1)  a  horizontal  line  equal  to  the  sum  of  all  the  horizontal 
forces  acting  on  the  pier,  and  (2)  a  vertical  line  equal  to  the  sum  of 
all  the  vertical  forces  ;  and  complete  the  diagram  by  drawing  the 
resultant.  The  stability  of  the  pier  is  determined  by  the  ratio  of 
^  C  to  N  C,  Fig.  70. 

575.  Stability  against  Crushing.  Represent  the  maximum 
pressure  by  P,  the  total  weight  on  the  section  by  W,  the  area  of  the 
section  by  S,  the  moment  of  inertia  of  the  section  by  /,  the  length 
of  the  section  by  /,  and  the  overturning  moment  by  M ;  then  from 
equation  (1),  page  205,  we  have 

--|  +  # <^) 

Tor  the  particular  case  in  which  the  pier  has  a  rectangular  horizon- 
tal cross  section,  the  above  formula  becomes  the  same  as  equation 
(18),  (page  3,22,)  as  deduced  for  an  element  of  a  masonry  dam. 

The  method  of  applying  the  above  equation  will  be  explained  in 
§  581  by  an  example. 

576.  Example  of  Method  of  Computing  Stability.  Fig.  85 
shows  the  dimensions  of  the  channel  pier  of  the  Illinois  Central  E. 
R.  bridge  over  the  Ohio  River  at  Cairo,  111.     This  pier  stands  be- 

'  tween  two  523-foot  spans.     Its  stability  will  now  be  tested  by  the 
preceding  principles. 

577.  Stability  against  Sliding.  We  will  examine  the  stabil- 
ity against  sliding  on  the  top  footing  course.  The  wind  surface  of 
the  truss  =  10  sq.  ft.  X  523  =  5,230  sq.  ft.  The  wind  pressure 
against  the  truss  at  30  lbs.  per  sq.  ft.  =  30  lbs.  X  5,230  =156,900 
lbs.  —  78  tons ;  and  the  Avind  pressure  on  the  truss  at  50  lbs.  = 
50  lbs.  X  5,230  =  261,500  lbs.  =  131  tons. 

The  wind  pressure  on  train  at  30  lbs.  per  sq.  ft.  =  30  lbs.  X 
523  X  10  =  156,900  lbs  =  78  tons. 

The  pressure  of  the  wind  against  a  section  of  the  pier  52  ft. 
long,  is  20  lbs.  X  52  X  11  =  14,560  lbs.  =  7  tons. 

The  pressure  due  to  the  ice  is  found  as  follows:  Assume  the 
thickness  to  be  1  foot ,  and  also  assume  the  crushing  strength  of 
ice  to  be  200  lbs.  per  sq.  m.  =,  sa}-,  15  tons  per  sq.  ft.  The  pier  is 
16  ft.  wide  at  the  high-water  line.  Hence  the  resistance  required  in 
the  pier  to  crush  its  way  through  a  field  of  ice  is  15  tons  X  16  X  1 
=  240  tons. 


372 


BRIDGE    PIERS. 


[chap.   XVI. 


e 


TT^^ if 


k.^ 


At' ->l 


SCALE 


Fig.  85.— Channel  Pier,  Cairo  Bridok. 


ART.   1.]  THEORY    OF   STABILITY.  373 

The  pressure   due  to  the  current  is  found  as  follows:    From 

§  569,  F  =  swk  ^— .     s  represents  the  exposed  surface  =  70  ft.  X 

19  ft.  =  1,330  sq.  ft.,  which  value  is  equivalent  to  assuming  that 
the  river  may  scour  to  the  top  of  the  footing  courses,  k  represents 
a  co-efficient,  which,  if  the  pier  were  rectangular,  would  be  about 
1.4,  and  if  the  pier  were  cylindrical  would  equal  about  0.73.  We 
will  assume  it  to  be  1.1, — a  trifle  more  than  the  mean  of  these  two 
values,  w  =  62.5  lbs.  per  cu.  ft.  The  surface  velocity  at  the 
bridge  site  was  measured*  "  when  the  Mississippi  and  the  Ohio 
were  at  about  the  same  stage,"  and  found  to  be  4  miles  per  hour 
(=6  ft.  per  second);  but  as  high  water  may  occur  in  the  Ohio  at 
the  time  of  moderately  low  water  in  the  Mississippi,  the  possible 
maximum  velocity  is  greater  than  the  above,  and  hence  we  will  as- 
sume that  it  is  10  ft.  per  second.  The  velocity  of  the  water  at 
various  depths  below  the  surface  of  a  stream  varies  as  the  ordinate 
of  an  ellipse;  but  the  effect  of  the  mean  velocity  is  approximated 
with  sufficient  accuracy  for  this  purpose  by  assuming  that  the  mean 
pressure  is  half  of  that  due  to  the  surface  velocity.  Substituting 
these  numbers,  the  above  equation  becomes  P  —  1,330  X  1-1  X 
62. 5  X  VV  —  *^0-  5  ^0^^  —  '^^  ^ons  with  sufficient  accuracy.  Divid- 
ing this  by  2  to  get  the  pressure  corresponding  to  the  mean  velocity, 
we  have  the  pressure  of  the  current  equal  to  35  tons. 
Collecting  the  preceding  results,  we  have: 

Wind  on  truss, 78  tons. 

"  train, 78     " 

"      "  pier, 7    " 

Pressure  of  ice,     ...  240     " 

"       "  water, 35     " 


Total  force  tending  to  slide  the  pier  on  the  foot- 
ing      =    438  tons. 

578.  The  weight  of  the  bridge  will  be  assumed  at  2  tons  per 
lineal  foot;  and  hence  the  total  weight  is  2  tons  X  523  =  1,046 
tons. 

The  weight  of  a  train  of  empty  cars  is  about  0. 5  ton  per  lineal 

*  Third  Annual  Report  of  the  Illinois  Society  of  Engineers,  p.  78. 


374  BRIDGE    PIEES.      .  [CHAP.  XVI. 

foot;  and  hence  the  total  weight  of  the  train  is  0.5  tons  X  523  = 
261  tons. 

The  amount  of  masonry  below  the  higli- water  line  =  67,946  cu. 
ft.;  the  amount  above  the  high  water  line  =  24,534  cu.  ft.;  and 
hence  the  total  masonry  =  92,480  cu.  ft.  We  will  assume  the 
weight  of  the  masonry  to  be  150  lbs.  per  cubic  foot.  Then  the 
weight  of  the  masonry  is  150  lbs.  x  92,480  =  6,936  tons. 

Collecting  these  results,  we  have: 

Weight  of  the  bridge, 1,046  tons. 

"       "    "    train  of  empty  cars, 361     " 

"       "    "    masonry, 6,936     " 

Total  weight  to  resist  sliding =     8,343  tons. 

Sliding  cannot  take  place,  if  the  co-efficient  of  friction  is  equal 
to  or  greater  than  438  ~  8,243  =  0.053.  For  values  of  the  co-ef- 
ficients of  friction,  see  the  table  on  page  315.  In  the  above  ex- 
ample, the  factor  of  safety  against  sliding  is  at  least  12  to  15. 

579.  Stability  against  Overturning.  We  will  consider  the 
stability  against  overturning  about  the  top  of  tlie  upper  footing 
course.  The  wind  on  the  truss  =  78  tons;  the  arm  of  this  force  = 
heiy/ht  of  the  pier  (123  ft.)  +  ludf  the  depth  of  the  tniss  (30  ft.)  = 
153  ft.;  and  therefore  the  moment  of  this  force  =  78  tons  X  153 
ft.  =  11,934  foot-tons. 

The  pressure  of  the  wind  on  the  train  =  78  tons;  and  the  arm 
of  this  pressure  =  distance  from  footing  to  top  of  pier  (123  ft.)  + 
distance  from  top  of  pier  to  top  of  rail  (8  ft.)  +  distance  from  top 
df  rail  to  center  of  train  (8  ft.)  =  139  ft.  Therefore  the  moment 
oi  this  pressure  is  78  tons  X  139  ft.  =  10,842  foot-tons. 

The  pressure  of  the  wind  against  the  pier  is  7  tons  (§  577);  the 
arm  of  this  force  =  ^  (202  +  150)  —  79  =  97  ft. ;  and  the  moment  of 
this  force  =  679  foot-tons. 

The  pressure  of  the  ice  is  240  tons,  the  arm  is  70  ft.,  and  the 
moment  is  16,800  foot-tons. 

The  pressure  of  the  water  is  35  tons.  The  center  of  pressure 
lies  somewhere  between  one  third  and  one  half  of  the  depth  from 
the  top;  and  as  the  increased  area  at  the  base  of  the  pier  compen- 
sates in  part  for  the  decrease  of  velocity  with  the  depth,  we  will  as- 
sume that  it  is  at  half  the  depth.  The  arm  then  is  36  ft.,  and  the 
moment  is  35  tons  x  36  ft.  =  1,260  foot-tons. 


AKT.   1.]  THEORY   OF   STABILITY.  375 


Collecting  these  results,  we  haver 

Moment  of  the  wind  on  the  truss,     . 

U.984  foot-tons. 

"         "     "       "      "     "    train,      .     . 

.     10,842 

"        "     "       "      "     "    pier,  .     .     . 

679 

"        "     "   pressure  of  the  ice,    .     . 

.     16,800 

"        "     "         "        "    <<    current. 

.       1,260 

Total  overturning  moment 

=   41,515  foot-tons. 

580.  The  total  weight  above  the  joint  considered  is  (§  578) 
8,243  tons.  This  force  acts  vertically  down  through  the  center  of 
the  pier;  hence  the  arm  is  31.5  ft.,  and  the  total  moment  resisting 
overturning  is  8,243  X  31.5  =  259,654  foot-tons.  The  factor  of 
safety  against  overturning  about  the  top  of  the  upper  footing 
course  is  259,654  -f-  41,515  =  6.3. 

Assuming  the  train  to  be  off  the  bridge,  and  that  the  wind 
pressure  on  the  truss  is  50  lbs.  per  sq.  ft.,  and  following  the  method 
pursued  above,  it  is  found  tliat  the  factor  of  safety  against  over- 
turning these  conditions  is  0.4. 

581.  Stability  against  Crushing.  The  maximum  pressure  on 
the  section  will  occur  when  the  loaded  train  is  on  the  bridge  and 
all  the  horizontal  forces  are  acting  with  their  full  intensity.  The 
load  when  an  emi)ty  train  is  on  the  bridge  is  (§  578)  8,243  tons. 
Assuming  that  a  loaded  train  will  weigh  \\  tons  per  lineal  foot,  we 
must  add  (0.75  tons  X  523  =)  392  tons  to  the  above  for  the 
difference  between  a  loaded  and  an  unloaded  train.  Then  the  total 
direct  pressure  is  8,243  +  392  —  8,635  tons.  The  area  of  the  sec- 
tion at  the  top  of  the  footing  course  is  1,160  sq.  ft.  Hence,  the 
maximum  direct  pressure  is  8,635  -h  1,160  =  7.4  tons  per  sq.  ft. 

The  moment  to  overturn,  M,  =  41,515  foot-tons.  The  greatest 
length  of  the  section  =  63  ft.  The  moment  of  inertia  of  the  sec- 
tion about  an  axis  through  its  center  and  perpendicular  to  its 
length  =  287,917  (ft.).     From  §  575.  the  maximum  pressure 

P_JL  ,  1^ 

Substituting  the  above  quantities  in  this  equation  gives 

^  =  ^-^+  fx287^917  =  ^'^  +  ^-^  "  ^^-^  *^'''  P^""  '^-  ^*- 
Since  it  is  highly  improbable  that  all  the  forces  will  act  at  the 
same  time  with  the  intensity  assumed  in  the  preceding  computa- 


376  BKIDGE   PIERS.  [CHAP.  XVI. 

tions,  we  may  conclude  that  the  pressure  will  never  exceed  11.9 
tons  per  sq.  ft.  A  comparison  of  this  with  the  values  of  the  com- 
pressive strength  of  masonry  as  given  in  §  ^22  (page  149)  shows 
that  this  pressure  is  entirely  safe. 

Since  this  is  an  unusually  high  pier  under  an  unusually  long 
span,  and  since  the  overturning  and  resisting  moments  and  also  the 
top  dimensions  of  the  pier  vary  with  the  span,  we  may  draw  the 
conclusion  that  any  pier  which  has  sufficient  room  on  top  for  the 
bridge  seat  (§  584)  and  ivhich  has  a  batter  of  1  in  12,  or  1  in  24,  is 
safe  against  any  mode  of  failure. 

582.  Pressure  on  the  Bed  of  the  Foundation.  The  caisson 
is  70  feet  long,  30  feet  wide,  and  50  feet  high.  The  load 
on  the  base  is  equal  to  the  weight  on  the  top  of  the  footing  j^lus 
the  weight  of  the  footings  plus  the  weight  of  the  caisson. 
The  weight  above  the  footing  =  8,635  tons  (§  581).  Tiie  weight 
of  the  footings  =  1,300  sq.  ft.  x  4  ft.  X  150  lbs.  -  390  tons.  The 
weight  of  the  caisson  =  70  ft.  X  30  ft.  X  50  ft.  X  100  lbs.  =  5,250 
tons.  The  total  weight  on  the  bed  =  8,635  +  390  +  5,250  =  14,- 
275  tons.  The  area  =  70  ft.  X  30  ft.  =  2,100  sq.  ft.  The  direct 
pressure  per  unit  of  area  —  14,275  -h  2,100  =  6.8  tons  per  sq.  ft. 

The  overturning  moment,  M,  is  equal  to  the  moment  about  the 
top  of  the  footing  (§  581)  plus  the  product  of  the  sum  of  the  hori- 
zontal forces  and  tlie  distance  from  the  footing  to  the  base  of  the 
caisson;  or,  the  moment  about  the  base  =  41,515  foot-tons  -|-  438 
tons  X  54  ft.  ==  65,167  foot-tons.  The  moment  of  inertia,  /,  = 
^  30  (70)'  =  857,500  (ft.).  I  =  70  ft.  The  concentrated  pressure 
caused  by  the  tendency  to  overturn  is 
i^/_65A67^xTO_ 
2  /       2  X  857,500 

The  caisson  was  sunk  all  the  way  through,  and  rests,  on  sand  ; 
consequently  the  water  will  find  its  way  freely  under  the  entire 
foundation,  thus  causing  buoyancy  to  act  with  its  full  force.  This 
upward  force  of  the  water  will  be  equal  to  the  volume  of  the  im- 
mersed masonry  multiplied  by  the  weight  of  a  cubic  foot  of  water; 
or  the  buoyancy  =  (67,946  +  5,200  +  105,000)  X  62.4  =  5,558  tons. 
The  lifting  efEect  of  buoyancy  is  (5,558  h-  2,100  =)  2.62  tons  per 
sq.  ft. 

Therefore,  the  total  pressure  is  not  greater  than  6.8  +  2.7  —  2.6 
=  6.9  tons  per  sq.  ft. 


ART.   2.]  DETAILS    OF    COXSTKUCTIO^'.  377 

The  pressure  would  never  be  so  much,  for  the  following  reasons  : 
1.  There  is  no  probability  that  both  spans  will  be  covered  by  a  train 
of  maximum  weight  at  the  same  time  that  the  maximum  effects  of 
the  wind,  of  the  current,  and  of  the  ice  occur.  2.  The  friction  on 
the  sides  of  the  caisson  will  sustain  part  of  the  load.  A  friction  of 
600  lbs.  per  sq.  ft.,  which  was  about  the  amount  experienced  in 
sinking  these  piei's  (see  §  455),  would  decrease  this  pressure  about 
1^  tons  per  sq.  ft. 

Therefore,  we  conclude  that  the  pressure  on  the  sand  will  be  at 
least  as  much  as  6.8  —  1.5  —  2.6  =  2.7  tons  per  sq.  ft,;  and  that  it 
may  possibly,  but  not  probably,  amount  to  6.8  -|-  2.7  —  2.6  —  1.5  = 
5.4  tons  per  sq.  ft.  The  larger  value  was  taken  at  the  gi-eatest  pos- 
sible one  for  the  sake  of  establishing  the  conclusion  stated  in  the 
last  paragraph  of  §  581. 

583.  Oihej-  BxanqjJes.  At  the  St.  Louis  steel-arch  bridge 
the  greatest  pressure  possible  on  the  deepest  foundation  (bed- 
rock) is  19  tons  per  sq.  ft.  The  pressure  at  the  base  of  the 
New  York  tower  of  the  East  Eiver  suspension  bridge  is  about 
7:^  tons  per  sq.  ft,  upon  a  stratum  of  sand  2  feet  thick  overlying 
bed-rock  ;  and  at  the  base  of  the  masonry  the  pressure  is  about  11^ 
tons  per  sq.  ft.*  The  corresponding  quantities  for  the  Brooklyn 
tower  were  a  little  over  a  ton  less  in  each  case.  At  the  Plattsmouth 
bridge  f  the  maximum  pressure  caused  by  the  weight  of  train,  bridge, 
and  pier  is  3  tons  per  sq.  ft.  At  the  Bismarck  bridge  f  the  pressure 
due  to  the  direct  weight  is  3  tons  per  sq.  ft.  on  clay. 

Art.  2.  Details  of  Construction. 

584.  Top  Dimensions.  The  dimensions  on  the  top  will  depend 
somewhat  upon  the  form  of  the  cross  section  of  the  pier,  and  also 
upon  the  style  and  span  of  the  bridge;  but,  in  a  general  way,  it  may 
be  stated  that,  for  trussed  spans  of  100  ft.  or  over,  the  dimensions 
under  the  coping  will  not  be  less  than  5  ft.  X  20  ft. ;  for  250-ft. 
spans,  8  ft.  X  30  ft.:  and  for  500-ft.  spans,  10  ft.  X  40  ft.  Appar- 
ently 6  ft,  X  22  ft.  under  the  coping  is  the  favorite  size  for  spans  of 
100  to  200  ft. 

*  F.  CoUingwood,  assistant  engineer,  in  Van  Nostrand's  Engin'g  Mag.,  vol  xyL 
p.  431. 

t  Report  of  Geo.  S.  Morison,  chief  engineer. 


378  BRIDGE   PIERS.  [CHAP.   XVI. 

685.  Bottom  Dimensions.  Theoretically  the  dimensions  at  the 
"bottom  are  determined  by  the  area  necessary  for  stability;  but  the 
top  dimensions  required  for  the  bridge  seat,  together  with  a  slight 
batter  for  the  sake  of  appearance,  gives  suflBcient  stability  (§  581). 
Only  high  piers  for  short  spans — a  combination  not  likely  to  occur 
in  practice — are  liable  to  fail  by  overturning  or  crushing. 

586.  Battee.  The  usual  batter  is  1  inch  to  a  foot,  although  ^ 
an  inch  to  a  foot  is  very  common.  In  high  piers  it  is  customary  to 
use  a  batter  of  1  to  24,  and  offset  the  masonry  and  introduce  a  water- 
table  at  the  high-water  line,  so  as  to  give  an  average  batter  of  about 
1  to  12.  This  construction  very  much  improves  the  appearance, 
and  does  not  add  materially  to  the  cost. 

A  corbel  course,  or  "belt  course,"  is  sometimes  introduced  im- 
mediately under  the  coping  for  appearance's  sake.  For  an  exam- 
ple, see  Fig.  85  (page  372),  Fig.  87  (page  383),  and  Fig.  88  (page 
384). 

587.  Ceoss  Section.  The  up-stream  end  of  a  pier,  and  to  a 
considerable  extent  the  down-stream  end  also,  should  be  rounded 
or  pointed  to  serve  as  a  cut-water  to  turn  the  current  aside  and  to 
prevent  the  formation  of  whirls  which  act  uj)on  the  bed  of  the 
stream  around  the  foundation,  and  also  to  prevent  shock  from  ice, 
logs,  boats,  etc.  In  some  respects  the  semi-ellipse  is  the  best  form 
for  the  ends  ;  but  as  it  is  more  expensive  to  form,  the  ends  are 
usually  finished  to  intersecting  arcs  of  circles  (see  Figs.  85,  87,  and 
Sd — pages  372,  383,  and  385,  respectively),  or  with  semi-circular 
ends.  Above  the  high-water  line  a  rectangular  cross  section  is  as 
good  as  a  curved  outline,  except  possibly  for  appearance. 

A  cheaper,  but  not  quite  as  efficient,  construction  is  to  form  the 
two  ends,  called  starlings,  of  two  inclined  planes.  As  seen  in 
plan,  the  sides  of  the  starlings  usually  make  an  angle  of  about  45° 
with  the  sides  of  the  pier  (see  Fig.  90,  page  386).  A  still  cheaper 
construction,  and  the  one  most  common  for  the  smaller  piers,  is  to 
finish  the  up-stream  end,  below  the  high-water  line,  with  two  in- 
clined planes  which  intersect  each  other  in  a  line  having  a  batter  of 
from  3  to  9  inches  per  foot,  and  build  the  other  three  sides  and  the 
part  of  the  up-stream  face  above  the  high- water  line  with  a  batter 
of  1  in  12  or  1  in  24.  Of  course  the  simplest  construction  is  to 
make  the  pier  rectangular  in  horizontal  cross  sections  and  give  it  the 
same  batter  on  ill  faces. 


ART.   2.]  DETAILS   OF    COXSTRUCTION.  379 

Occasionally,  for  economy,  piers,  particularly  pivot  piers,  are 
built  hollow — sometimes  with  and  sometimes  without  interior  cross 
walls  (see  Fig.  86,  page  380).  The  piers  of  the  bridge  across  the 
Missouri  River  at  Glasgow,  Mo.,  are  solid  up  to  the  high- water  line, 
and  above  that  each  pier  consists  of  two  stone  columns.  The  piers 
of  the  bridge  over  the  Missouri  at  St.  Charles,  Mo.,  have  a  somewhat 
similar  construction,  except  that  the  secondary  piers  are  connected 
by  a  comparatively  thin  wall. 

With  piers  subjected  to  a  severe  pressure  from  ice,  it  is  customary 
to  protect  the  edge  of  the  nose  with  an  angle-iron  or  a  railroad  rail. 

588.  Pivot  Piees.  These  differ  from  the  ordinary  piers  only 
in  that  they  are  circular,  are  larger  on  top,  and  have  plumb  sides. 
Pivot  piers  are  about  25  to  30  feet  in  diameter,  under  the  coping, 
for  spans  of  250  to  350  feet,  respectively. 

Fig.  86  shows  the  pivot  pier  for  the  Northern  Pacific  R.  R, 
bridge  over  the  Red  River  at  Grand  Forks,  Dakota.  The  specifica- 
tions for  the  grillage  were  as  follows:  '*^ Fasten  the  first  course  of 
timbers  together  with  |-inch  X  20-inch  drift  bolts,  18  inches  apart; 
fasten  second  course  to  first  course  with  drift  bolts  of  same  size  at 
every  other  intersection.  Timbers  to  be  laid  with  broken  joints. 
Put  on  top  course  of  4-inch  X  12-inch  plank,  nailed  every  2  feet 
with  ^''g^-inch  X  8-inch  boat  spikes.  The  last  course  is  to  be  thor- 
oughly calked  with  oakum." 

Pivot  piers  are  protected  from  the  pressure  of  ice  and  from 
shock  by  boats,  etc.,  by  an  ice  breaker  which  is  entirely  distinct 
from  the  pier.  The  ice  breaker  is  usually  constructed  by  driving  a 
group  of  60  or  70  piles  in  the  form  of  a  V  (the  sharp  end  up  stream), 
at  a  short  distance  above  the  pier.  On  and  above  these  piles  a 
strong  timber  crib-work  is  framed  so  as  to  form  an  inclined  ridge 
up  which  the  cakes  of  ice  slide  and  break  in  two  of  their  own  weight. 
Between  the  ice  breaker  and  the  pier  two  rows  of  piles  are  driven, 
on  which  a  comparatively  light  crib  is  constructed  for  the  greater 
security  of  the  pier  and  also  for  the  protection  of  the  river  craft. 

589.  Quality  of  Masonry.  Bridge  piers  are  usually  quarry- 
faced  ashlar,  /.  e.,  first-class  masonry  (see  §  207)  backed  with  rubble. 
Good  concrete,  if  made  with  reasonable  care,  is  equally  as  good  as 
ordinary  rubble  masonry,  and  is  sometimes  cheaper, — since  it  affords 
an  opportunity  to  use  up  the  refuse  from  the  quarry. 


380 


BKIDGE    PIERS. 


[chap.  XYI. 


ART.   2.]  DETAILS   OF   CONSTRUCTION.  381 


For  an  illustrated  description  of  the  method  of  building  concrete 
bridge  piers,  see  Engineering  News,  vol.  xix.  pp.  443-44. 

590.  Specifications.  The  following  specifications  for  the  ma- 
sonry of  the  railroad  bridge  over  the  Missouri  River  near  Sibley,  Mo., 
(Octave  Chanute,  engineer)  may  be  taken  as  an  example  of  the  best 
practice.  * 

591.  General  Requirements.  "  The  stone  to  be  used  in  these  piers  must  be 
of  what  is  known  as  the  best  quality  of  Cottonwood  limestone,  or  other  stone 
which,  in  the  opinion  of  the  engineer,  is  of  equally  good  quality  and  in  every 
way  suitable  for  the  purpose  for  which  it  is  to  be  used.  It  must  be  sound  and 
durable,  free  from  all  drys,  shakes,  or  flaws  of  any  kind  whatever,  and  must 
be  of  such  a  character  as  will,  in  the  opinion  of  the  engineer,  withstand  the 
action  of  the  weather.  No  stone  of  an  inferior  quality  will  be  accepted  or 
even  permitted  to  be  delivered  upon  the  ground.  The  masonry  in  the  bridge 
piers  must  be  of  the  best  and  largest  stones  that  the  quarry  will  afford,  and 
must  be  quarried  in  time  to  season  against  frost  before  being  used. 

"  The  face  stones  composing  the  starling,  and  the  ends  and  sides  of  the  river 
piers  from  the  neat  line  about  low  water  up  for  a  distance  of  twelve  (12;  feet, 
and  also  the  pedestal  blocks  of  the  main  piers  will  be  of  Minnesota  granite, 
or  a  granite  of  equal  quality  approved  by  the  engineer. 

"  All  masonry  of  the  main  piers  shall  be  regular  coursed  ashlar  of  the  best 
iescription,  and  must  be  laid  in  mortar  of  the  proportions  of  sand  and  cement 
hereinafter  specified. 

"  All  stones  must  be  so  shaped  that  the  bearing  beds  shall  be  parallel  to  the 
natural  beds,  and  be  prepared  by  dressing  and  hammering  before  they  are 
brought  on  the  walls,  as  tooling  and  hammering  will  not  be  allowed  after  the 
stones  are  in  place.  They  are  to  be  laid  to  a  firm  bearing  on  their  natural  beds 
in  a  full  bed  of  mortar,  without  the  use  of  chips,  pinners,  or  levelers.  No 
shelving  projections  will  be  allowed  to  extend  beyond  the  under  bed  on  either 
side.  The  stone  and  work  are  to  be  kept  free  from  all  dirt  that  will  interfere 
with  the  adhesion  of  mortar.  Stones  must  be  sprinkled  with  water  before 
being  placed  in  position  on  the  wall.  In  laying  stone  in  mortar,  their  beds  are 
to  be  so  prepared  that  when  settled  down  they  may  rest  close  and  full  on  the 
mortar.  In  handling  the  stones  care  must  be  used  not  to  injure  the  joints  of 
those  already  laid;  and  in  case  a  stone  is  moved  after  being  set  and  the  joint 
broken,  it  must  be  taken  out,  the  mortar  thoroughly  cleaned  from  the  beds, 
and  then  reset. 

"  Wherever  the  engineer  shall  so  require,  stones  shall  have  one  or  two  IJ- 
inch  iron  dowels  passing  through  them  and  into  the  stones  below.  The  holes 
for  the  dowels  shall  be  drilled  through  such  stones  before  they  are  put  in 
position  on  the  walls.  After  the  stones  are  in  place  the  holes  shall  be  con- 
tinued down  into  the  under  stones  at  least  six  (6)  inches  ;  the  dowel  pins  will 
then  be  set  in  and  the  holes  filled  with  neat  cement  grout.     Cramps  binding 

*  For  specifications  for  first-class  masonry,  see  §  207 ;  see  also  Appendix  I. 


382  BRIDGE    PIERS.  [CHAP.  XVI. 

the  several  stones  of  a  course  together  may  be  inserted  when  required  by  the 
engineer  ;  in  such  case  they  will  be  counter-sunk  into  the  stones  which  they 
fasten  together. 

592.  Face  Stones.  "  The  face  stones  must  be  accurately  squared,  jointed, 
and  dressed  on  their  beds  and  builds  ;  and  the  joints  must  be  dressed  back  at 
least  twelve  inches  (12)  from  the  face.  Face  stones  are  to  be  brought  to  a  joint, 
when  laid,  of  not  more  than  three  quarters  (f )  of  an  inch  nor  less  than  one 
half  (i)  inch.  The  courses  shall  not  be  less  than  eighteen  (18)  inches  in  thick- 
ness, decreasing  from  bottom  to  top  of  the  wall.  Courses  to  be  well  bonded. 
The  face  stones  shall  break  joints  at  least  twelve  (12)  inches.  The  face  stones 
may  be  left  rough,  except  the  stones  forming  the  starling,  which  must  be  care- 
fully dressed  to  a  uniform  surface.  The  edges  of  face  stones  shall  be  pitched 
true  and  full  to  line,  and  on  corners  of  all  piers  a  chisel  draft  one  and  a  half 
(1+)  inches  must  be  carried  up  from  base  to  the  under  side  of  the  coping.  No 
projection  of  more  than  three  (3)  inches  from  the  edge  of  face  stones  will  be 
allowed.     No  stone  with  a  hollow  face  will  be  allowed  in  the  work. 

593.  Stretchers.  "Each  stretcher  shall  have  at  least  twenty  (20)  inches 
width  of  bed  for  all  courses  of  from  eighteen  (18)  to  twenty  (20)  inches  rise, 
and  for  all  thicker  courses  at  least  as  much  bed  as  rise ;  and  shall  have  an 
average  length  of  at  least  three  and  one  half  (3*)  feet,  and  no  stretcher  shall  be 
less  than  three  (3)  feet  in  length. 

594.  Headers.  "  Each  header  shall  have  a  width  of  not  less  than  eighteen 
inches  (18)  and  shall  hold,  back  into  the  heart  of  the  wall,  the  size  that  it  shows 
on  the  face.  The  headers  shall  occupy  at  least  one  fifth  (^)  of  the  whole  face 
of  the  wall,  and  be,  as  nearly  as  practicable,  evenly  distributed  over  it,  and  be 
so  placed  that  the  headers  in  each  course  shall  divide  equally,  or  nearly  so,  the 
spaces  between  the  headers  in  the  course  directly  below.  In  walls  over  six 
feet  (6)  in  thickness,  the  headers  shall  in  no  case  be  less  than  three  and  one  half 
feet  (3i)  long;  and  in  walls  over  nine  (9)  feet  thick,  the  headers  shall  be  equal 
in  length  to  one  third  the  thickness  of  the  wall,  except  when  this  length  of 
header  exceeds  six  (6)  feet, — no  header  over  six  (6)  feet  long  being  required. 

595.  Backing.  "  The  headers  must  alternate  front  and  back,  and  their 
binding  effect  be  carried  through  the  wall  by  intermediate  stones — not  less  in 
length  and  thickness  than  the  headers  of  the  same  course — laid  crosswise  in 
the  interior  of  the  wall.  The  stretchers  and  all  stones  in  the  heart  of  the  wall 
shall  be  of  the  same  general  dimensions  and  proportions  as  the  face  stones, 
and  shall  have  equally  good  bed  and  bond,  but  may  have  less  nice  vertical 
joints, — although  no  space  greater  than  five  (5)  inches  in  width  shall  be  left  be- 
tween stones.  All  stones  in  the  backing  must  be  well  fitted  to  their  places, 
and  carry  the  course  evenly  quite  through  the  wall. 

596.  Coping.  "  The  tops  of  the  bridge  piers,  cap  stones  of  the  pedestals, 
and  such  other  parts  of  the  masonry  as  the  engineer  shall  direct,  shall  be  cov- 
ered with  coping  of  such  dimensions  as  prescribed.  All  coping  stones  shall 
be  neatly  bush-hammer  dressed  on  the  face,  bed,  top,  and  joints;  and  shall  be 
well  and  carefully  set  on  the  walls,  brought  to  one  quarter  (i)  inch  joints,  and,. 


ART.   3.]  DETAILS   OF   CONSTRUCTIO]^-. 


383 


Fig.  87.— Shore  Pier,  Blair  Bridgb. 


384 


BRIDGE    PIERS. 


[chap.   XVI. 


if  required,  be  doweled,  tlie  dowels  being  well  secured  in  and  to  the  coping 
with  grout.     No  coping  stone  shall  be  less  than  nine  (9)  square  feet  on  top. 

597.  Pointing.  "  All  masonry  is  to  be  pointed  .so  as  to  till  the  joints  solid. 
The  surface  of  the  wall  is  to  be  scraped  clean  and  the  joints  freed  of  all  loose 
rnortar  and  refilled  solid  by  using  proper  rauamiug  tools.  Joints  must  be  well 
wet  before  being  pointed.  Mortar  used  in  pointing  is  to  be  composed  of  one 
part  Portland  cement  and  one  part  sand. 

598.  Cement.  "  The  cement  used  in  the  work  shall  be  equal  in  quality  to 
the  best  brands  of  Milwaukee  or  Louisville  cement,  and  shall  be  ground  so 
that  at  least  90  per  cent,  in  weight  will  pass  a  standard  sieve  of  2,500  meshes 
to  the  square  inch,  and  shall  have  a  tensile  strength — after  being  exposed  one 
hour,  or  until  set,  in  air,  and  the  balance  of  the  twenty- four  hours  in  water  not 


jS'cale 


J^ee-ir 


(f--  -  //'  O"-  -^ 


^ 


5:::±_ 


-^ 


30'  O" J  e  5' 5'^ 


/3'   o" -J 

— 7 


-/<?'   6'- 


FiG.  88.— Top  OF  Pier,  Henderson  Bridge. 

below  60^  F. — of  at  least  40  pounds  per  square  inch;  and,  after  being  exposed 
one  day  in  air  and  six  days  in  water,  from  60  to  100  pounds  per  square  inch. 

"  All  cements  shall  be  furnished  by  the  contractor  subject  to  approval  by 
the  engineer.  The  contractor  shall  provide  a  .suitable  building  for  .storing  the 
cement,  in  which  the  same  must  be  placed  before  being  tested.  The  engineer 
shall  be  notified  of  the  receipt  of  cement  at  least  three  days  before  it  is  required 
for  use,  and  the  inspector  may  take  a  sample  from  each  package  for  testing. 

599.  Mortar.  "  The  mortar  shall  be  composed  of  the  above  cement  and 
■clean,  dry,  sharp  sand  in  the  proportion  of  one  part  cement  to  two  parts  of 
;sand  by  weight.*  The  sand  and  cement  shall  be  thoroughly  mixed  dry,  and, 
after  adding  sufficient  water  to  render  the  mass  plastic,  shall  be  mixed  and 
worked  until  of  uniform  consistency  throughout. 

"  Mortar  remaining  unused  so  long  as  to  have  taken  an  initial  set  shall  not 
1)6  tised  in  the  work. 


*  This  is  an  unusual,  but  exact,  method  of  specifying  proportions ;    they  are 
usually  stated  in  volumes. 


.KT.   U.] 


DETAILS   OF   COXSTKUCTIOX. 


385 


600.  Pedestal  Masonry.  "  The  pedestals  shall  be  founded  upon  a  bed  of 
concrete  or  upon  piles,  as  may  be  directed  by  the  engineer.  The  masonry  in 
the  pedestals  shall  be  of  the  best  de- 
scription of  coursed  ashlar  composed  of 
the  limestone  and  the  mortar  described 
above,  the  stones  to  be  not  less  than 
twelve  (12)  inches  thick,  and  to  have 
horizontal  beds  and  vertical  joints  on 
the  face.  When  the  walls  do  not  ex- 
ceed three  and  one  half  (3i)  feet  in 
thickness,  the  headers  shall  run  entirely 
through,  or  a  single  stone — square  and 
of  the  proper  thickness — may  be  used. 
In  walls  over  three  and  one  half  (3i) 
feet  in  thickness,  and  not  over  seven 
(7)  feet  in  thickness,  headers  and 
stretchers  shall  alternate,  and  there 
shall  be  as  many  headers  as  stretchers. 
The  space  in  the  interior  of  the  walls 
shall  be  filled  with  a  single  stone  cut 
to  fit  such  space,  and  said  stone  shall 
be  of  the  same  height  as  the  headers 
and  stretchers  of  the  course.  In  all  the 
masonry  of  these  pedestals  the  slope 
must  be  carried  up  by  steps  and  in  ac- 
cordance with  the  plans  of  the  engineer. 
All  the  quoins  must  have  hammer- 
dressed  beds,  builds,  and  joints,  and 
draft  corners." 

601.  Examples  of  Bridge 
PlEES.  Fig.  85  (page  372)  shows 
the  channel  pier  of  the  Illinois 
Central  K.  E.  bridge  over  the 
Ohio  at  Cairo,  111. 

Fig.  86  (page  380)  shows  the 
pi'-ot  pier  of  the  Northern  Pacific 
R.  R.  bridge  over  the  Red  River 
at  Grand  Forks,  Dakota. 

Fig.   87  (page  383)  shows  one 
of    the   two   shore   j^iers   of     the 
bridge  over   the   Missouri   River, 
near  Blair,  Neb.*     This  pier   stands  between  two   330-ft.   spans. 
*  From  the  Report  of  Geo.  8.  Morison,  chief  engineer  of  the  bridge. 


386 


BRIDGE    PIERS. 


[chap,  xvi* 


Seise   of  Jiail. 


wmm 


;* . 


n  I  I  H  I  I  I  I  I  I  I  I  I  I  I  I  I  I  ITTTnTTT 


n  n  1 1 1 1 1 1  iTTTi  M  1 1 1 1 1 1 1 1  n 


Fio.  80.— Pier  of  St.  Croix  River  Bridge. 


ART.   2.] 


DETAILS   OF    CONSTRUCTIOX. 


387 


"  The  vertical  joints  are  shown  as  they  actually  are  in  the  struct- 
ure."    The  masonry  is  145  i't.  from  top  to  bottom. 

Fig.  88  (page  384)  shows  the  top  of  the  pier  between  two  o25-ft. 
channel  spans  of  the  Louisville  and  Nashville  R.  R.  bridge  across 
the  Ohio  River  at  Henderson,  Ky. 

Fig.  89  (page  385)  shows  the  actual  arrangement  of  the  stones  in 
one  of  the  courses  of  one  of  the  channel  piers  (Fig.  85)  of  the  Illinois 
Central  R.  R.  bridge  over  the  Ohio  River,  at  Cairo,  111. 

602.  Fig.  90  (page  386)  shows  the  river  pier  of  the  Chicago,  Bur- 
1 '.ngton  and  Northern  R.  R.  bridge  across  the  St.  Croix  River.  This 
pier  stands  between  a  draw  of  370  feet  and  a  fixed  span  of  153  feet. 
The  thickness  of  the  courses  is  as  follows,  in  order  from  the  bottom 
up  :  Two  courses,  including  the  footing,  28  inches;  two  26  inches  ; 
one  each  24,  22,  21,  19,  and  17  inches  ;  two  15  inches ;  four  14 
inches  ;  one  13  inches  ;  one  12  inches  ;  and  the  coping  18  inches. 

The  following  table  gives  tiie  quantity  of  masonry  in  the  pier  and 
illustrates  the  manner  of  computing  the  contents  of  such  structures. 
Notice  tha^  the  order  in  the  table  is  the  same  as  that  in  the  pier  ; 
i.  e.,  the  top  line  of  the  table  relates  to  the  uppermost  masonry,  etc. 

TABLE  40. 
Contexts  of  the  Pier  shown  in  Fig.  90  (page  386). 


Description. 


Strinc-er  Rests. 
Bridge  Seats. . . 
Copiug 

Keat  Work 


Ice  Breaker. . . . 
Footing  Course. 


Dimensions. 


3X  2.75  X  3.0'  X  3.13' 
3  X  2.75'  X  3.0'  X  1.46'. 
7.0'  X  24.0'  X  1.5.' 


i (3  X  6.5'  +  8.6)  23'  +  (2  X  8.6'  +  6.5') 25.1' | 


25.17' 


18.17' 


(3x8.6' +  7.1')  3.8'  X 

(3.6'X3.6)(^-  +  l.o) 

^(3.6'  X  3.6'-f  4.3'  X  4.3)  18.17' 

9.6'  X  29.4'  X  2.33' 

4.8'  X4  8'  X2.33' 

Total  =  230.39  cubic  yards : 


Cubic 
Feet. 


51.6 

24.1 
270.0 

,579.7 
279.6 

17.3 

285.8 

658.5 

53.8 

,220.4 


603.  Iron  Tubular  Piers.  For  a  description  of  an  iron  tubular 
pier,  see  §  415  ;  and  for  a  description  of  a  pier  founded  upon  screw 
piles,  see  Engineering  News,  vol.  xiii.  pp.  210-12. 


388  BRIDGE   PIERS.  [CHAP.    XVI. 


604.  Timber  Barrel  Piers.  The  Chicago,  Burlington  and  Quincy 
R.  R.  has  constructed  a  few  "  barrel  piers"  as  an  experiment,  the 
object  being  to  reduce  the  cost  of  foundations,  and  also  to  find  some 
cheap  substitute  for  masonry.  The  barrels  are  cylindrical,  8  feet  in 
diameter,  and  20  to  30  feet  in  length.  The  staves  are  10  inches 
thick,  8  inches  wide  on  the  outside,  and  are  dressed  to  fit  together 
to  form  a  cylinder.  The  staves  are  bolted  at  the  top  and  bottom 
to  two  inside  rings  made  of  I-beams,  and  are  further  held  in  place 
by  strong  outside  hoops  of  iron.  These  caissons  or  barrels  are  sunk 
by  excavating  the  soil  from  the  inside.  The  bottom  and  top  por- 
tions of  the  caisson  are  filled  with  concrete,  and  the  intermediate 
portion  with  sand.  On  top  of  the  wooden  barrel,  an  iron  frame  is 
placed,  upon  which  the  truss  rests.  Two  barrels  constitute  a  pier. 
The  advantages  claimed  for  the  wooden  caissons  are  that  they  can 
be  put  in  without  interfering  with  traffic,  or  without  loss  of  time  in 
sinking  by  the  passage  of  trains.  Tlie  objection  to  them  is  that  they 
are  not  durable. 

605.  Contents  of  Bridge  Piers.  The  table  on  page  380  gives 
the  quantity  of  masonry  in  bridge  piers  having  rectangular  cross 
sections  and  such  dimensions  on  top  and  batters  as  occur  most 
frequently  (see  §§  584-8T).  The  quantities  in  the  first  four  columns 
cover  most  of  the  cases  for  highway  and  single  track  railway  bridges; 
and  the  quantities  in  the  last  two  columns  are  applicable  to  double 
track  railway  bridges.  Since  that  portion  of  the  pier  below  the 
water  should  have  more  or  less  pointed  ends,  and  since  there  is 
likely  to  be  an  offset  in  the  profile — particularly  of  high  piers, — 
the  quantities  in  the  table  (being  for  a  rectangular  cross  section)  are 
mainly  useful  in  making  preliminary  estimat'^s. 

The  contents  of  piers  of  other  dimensionsL  than  those  in  the  table 
may  be  computed  by  the  following  formula  :  * 

contents  =  thl+h{l  +  t)  Ir  +  1^  h^¥, 
in  which  /  =  the  length  on  top  under  the  coping, 

/;  =    ''    thickness  on  top  under  the  coping, 
A  =    ''    height  to  the  under  side  of  the  coping, 
Jj  —    "    batter—/,  v.,  h  =  ^^  ^^  ii- 
The  length  on  the  bottom  =  I  -\~2bh;  and  the  thickness  on  the 
bottom  =  t  -\-  )lb]i.     To  illustrate  the  method  of  applying  this  for- 

*  See  foot-note,  page  3S9. 


ART.   2.] 


DETAILS   OF   CONSTRUCTION. 


3S9 


TABLE  41. 

Contents  of  Bridge  Piers  having  Rectangular  Cross  Section  and 
THE  same  Batter  on  all  Faces. 


Height- 

Dimension  oi 

'  THE  Pier  ox  top  under 

THE  Coping. 

top  OF 
Footing 
TO  bottom 

5  ft.  X 

20  ft. 

6  ft.  X 

22  ft. 

6  ft.  X 

S4ft. 

OF 

Coping. 

Batter  1 :  12 

Batter  1  ;  24 

Batter  1  :  13 

Batter  1  :  24 

Batter  1 :  12 

Batter  1  :  24 

fe€t. 

cu.  yds. 

cu.  yds. 

cu.  yds. 

cu.  yds. 

cu.  yds. 

cu.  yds. 

5 

20.49 

19.49 

26.64 

25.53 

40.90 

39.33 

6 

25.07 

23.63 

32.51 

30.90 

49.84 

47.57 

7 

29.83 

27.85 

38.57 

36.36 

59.05 

55.98 

8 

34.74 

32.13    . 

44.81 

41.90 

68.52 

64.43 

9 

39.84 

36.52 

51.34 

47.55 

78  23 

73.04 

10 

45.09 

40.97 

57.86 

53.28 

88.23 

81.80 

11 

50.53 

45.51 

64.67 

59.09 

98.50 

90.68 

12 

56.14 

50.14 

71.69 

65.02 

109.02 

99.68 

13 

61.93 

54.85 

78.89 

71.02 

119. !<3 

108.83 

14 

67.91 

59.64 

86.31 

77.13 

130.90 

118.09 

15 

74.07 

64.52 

93.93 

83.33 

142.25 

127.49 

16 

80.40 

69.48 

101.72 

89.61 

!     153.88 

137.03 

17 

86.93 

74.53 

109.75 

96.00 

105.79 

146.69 

18 

93.65 

79.66     , 

117.98 

103.49 

177.99 

156.49 

19 

100.56 

84.87     1 

126.43 

109.06 

190.45 

166.40 

20 

107.66 

90.18     j 

135.07 

115.73 

203.22 

176.47 

21 

114.96 

95.57     1 

143.94 

132.49 

216.28 

186.67 

22 

122.46 

101.06      : 

153.01 

129.36 

229.60 

196.98 

23 

130.15 

106.63     i 

163.33 

136.34 

243.24 

207.45 

24 

138.04 

112.27     ! 

171.84 

143.39 

257.17 

218.05 

25 

146.14 

118.03 

181 . 56 

150.53 

271.39 

228.79 

26 

154.45 

123.86 

191.53 

157.79 

i     285.91 

239.65 

27 

162.96 

129.79 

201.74 

165.17 

300.74 

250.67 

28 

171.69 

135.81 

212.16 

173.63 

315.87 

261.82 

29 

180.62 

141.93 

233.79 

180.18 

i     331.27 

273.09 

30 

189.77 

148.12 

233.68 

187.85 

1     347.01 

284.51 

32 

208.72 

160.81 

256.15 

203.47 

379.43 

307.78 

34 

228.54 

173.86 

279.58 

219.52 

413.06 

331 . 59 

36 

249.26 

187.30     i 

303.98 

235.98 

447.99 

355.99 

38 

270.91 

201.12 

329.36 

252.84 

1     484.17 

480.92 

40 

293.47 

215.32 

355.74 

270.13 

'     521.66 

406.43 

42 

316.98 

229.92 

383.17 

287.88 

560.51 

432.57 

44 

341.46 

214.91 

411.59 

306.02 

1     600.64 

459.22 

46 

366.90 

260.29 

441.05 

324.60 

643.15 

486.47 

48 

393.36 

276.09 

471.66 

343.66 

1     684.99 

514.33 

50 

420.82 

292.29 

503.32 

363.13 

739.24 

542.78 

52 

449.33 

308.90 

536.07 

383.08 

1     774.88 

571.80 

54 

478.86 

325.93 

569.96 

403.45 

821.98 

601.47 

56 

509.45 

343.38 

604.96 

424.39 

870.45 

631.71 

58 

541.13 

361.24 

641.11 

445.57 

1     920.41 

662.57 

60 

573.85 

379.52 

678.48 

467.42 

971.78 

694.02 

390  BRIDGE    PIERS.  [CHAP.   XVI. 

mula,  assume  that  it  is  required  to  find  the  contents  of  a  pier  4  feet 
thick,  20  feet  long  on  top,  and  30  feet  high,  having  a  batter  on 
all  four  faces  of  1  inch  per  foot.  Then  I  =  20,  t  =  4:,  b  =  y^,  and 
the  preceding  formula  becomes 

contents  =  4  X  20  x   30  +  I'g  (20  +  4)   (30)=  +  f  X  ji?  X  (30)' 
=4^450  cubic  feet. 

606.  Cost.  For  a  general  discussion  of  the  cost  of  masonry,  see 
§§  226-38  (pp.  153-60) ;  and  for  data  on  the  cost  of  bridge  pier 
masonry,  see  §  235  (p.  157). 


CHAPTER  XVII. 
CULVERTS. 

Art.  1.    Water  Way  Eequired. 

607.  The  determination  of  the  amount  of  water  way  required  in 
any  given  case  is  a  problem  that  does  not  admit  of  an  exact  mathe- 
matical solution.  Although  the  proportioning  of  culverts  is  in  a 
measure  indeterminate,  it  demands  an  intelligent  treatment.  If 
the  culvert  is  too  small,  it  is  liable  to  cause  a  washout,  entailing 
possibly  loss  of  life,  interruptions  of  traffic,  and  cost  of  repairs. 
On  the  other  hand,  if  the  culvert  is  made  unnecessarily  large,  the 
cost  of  construction  is  needlessly  increased.  Any  one  can  make  a 
culvert  large  enough  ;  but  it  is  the  province  of  the  engineer  tc 
design  one  of  sufficient  but  not  extravagant  size. 

608.  The  Factors.  The  area  of  water  way  required  depends 
upon  (1)  the  rate  of  rain-fall,  (2)  the  kind  and  condition  of  the 
soil,  (3)  the  character  and  inclination  of  the  surface,  (4)  the  condi- 
tion and  inclination  of  the  bed  of  the  stream,  (5)  the  shape  of  the 
area  to  be  drained  and  the  position  of  the  branches  of  the  stream, 
(6)  the  form  of  the  mouth  and  the  inclination  of  the  bed  of  the 
culvert,  and  (7)  whether  it  is  j)ermissible  to  back  the  water  up  above 
the  culvert,  thereby  causing  it  to  discharge  under  a  head. 

1.  It  is  the  maximum  rate  of  rain-fall  during  the  severest  storms 
which  is  required  in  this  connection.  This  certainly  varies  greatly 
in  different  sections  ;  but  there  are  almost  no  data  to  show  what  it  is 
for  any  particular  locality,  since  records  generally  give  the  amount 
per  day,  and  rarely  per  hour,  while  the  duration  of  the  storm 
is  seldom  recorded.  Further,  probably  the  longer  the  series  of 
observations,  the  larger  will  be  the  maximum  rate  recorded,  since 
the  heavier  the  storm  the  less  frequent  its  occurrence  ;  and  hence  a 
record  for  a  short  period,  however  complete,  is  of  but  little  value 
in  this  connection.  Further,  the  severest  rain-falls  are  of  compara- 
tively limited  extent,  and  hence  the  smaller  the  area,  the  larger  the 

391 


392  CULVERTS.  [chap.   XVII. 

possible  maximum  precipitation.  Finally,  the  effect  of  the  rain-fall 
in  melting  snow  would  have  to  be  considered  in  determining  the 
maximum  amount  of  water  for  a  given  area. 

2.  The  amount  of  water  to  be  drained  off  will  depend  upon  the 
permeability  of  the  surface  of  the  ground,  which  will  vary  greatly 
with  the  kind  of  soil,  the  degree  of  saturation,  the  condition  of 
cultivation,  the  amount  of  vegetation,  etc. 

3.  The  rapidity  with  which  the  water  will  reach  the  water 
courses  depends  upon  whether  the  surface  is  rough  or  smooth,  steep 
or  flat,  barren  or  covered  with  vegetation,  etc. 

4.  The  rapidity  with  which  the  water  will  reach  the  culvert 
uepends  upon  whether  there  is  a  well-defined  and  unobstructed 
channel,  or  whether  the  water  finds  its  way  in  a  broad  thin  sheet. 
If  the  water  course  is  unobstructed  and  has  a  considerable  inclina- 
tion, the  water  may  arrive  at  the  culvert  nearly  as  rapidly  as  it 
falls  ;  but  if  the  channel  is  obstructed,  the  water  may  be  much 
longer  in  passing  the  culvert  than  in  falling. 

5.  Of  course,  the  water  way  depends  upon  the  amount  of  area 
to  be  drained ;  but  in  many  cases  the  shape  of  this  area  and  the 
position  of  the  branches  of  the  stream  are  of  more  importance  than 
the  amount  of  the  territory.  For  example,  if  the  area  is  long  and 
narrow,  the  water  from  the  lower  portion  may  pass  through  the 
culvert  before  that  from  the  upper  end  arrives  ;  or,  on  the  other 
hand,  if  the  upper  end  of  the  area  is  steeper  than  the  loAver,  the 
water  from  the  former  may  arrive  simultaneously  with  that  from 
the  latter.  Again,  if  the  lower p)art  of  the  area  is  better  supplied 
with  branches  than  the  upper  portion,  the  water  from  the  former 
will  be  carried  past  the  culvert  before  the  arrival  of  that  from  tlie 
latter;  or,  on  the  other  hand,  if  the  upper  portion  is  better  supplied 
with  branch  water  courses  than  the  lower,  the  water  from  tlie 
whole  area  may  arrive  at  the  culvert  at  nearly  the  same  time.  In 
large  areas  the  shape  of  the  area  and  the  position  of  the  water 
-*,ourses  are  very  important  considerations. 

6.  The  efficiency  of  a  culvert  may  be  materially  increased  by  sc 
arranging  the  upper  end  that  the  water  may  enter  it  without  being 
retarded  (see  §  639).  The  discharging  capacity  of  a  culvert  can 
also  be  increased  by  increasing  the  inclination  of  its  bed,  provided 
the  channel  below  will  allow  the  water  to  flow  away  freely  after 


ART.    1.]  WATER   WAY    REQUIRED.  di)'6 

having  passed  the  culvert.  The  last,  although  very  important,  is 
frequently  overlooked. 

7.  The  discharging  capacity  of  a  culvert  can  be  greatly  increased 
by  allowing  the  water  to  dam  up  above  it.  A  culvert  will  discharge 
twice  as  much  under  a  head  of  4  feet  as  under  a  head  of  1  foot. 
This  can  only  safely  be  done  with  a  well-constructed  culvert. 

609.  Formulas.  The  determination  of  the  values  of  the  differ- 
ent factors  entering  into  the  problem  is  almost  wholly  a  matter  of 
judgment.  An  estimate  for  any  one  of  the  above  factors  is  liable 
to  be  in  error  from  100  to  200  per  cent.,  or  even  more,  and  of 
coui'se  any  result  deduced  from  such  data  must  be  very  uncertain. 
Fortunately,  mathematical  exactness  is  not  required  by  the  jiroblem 
nor  warranted  by  the  data.  The  question  is  not  one  of  10  or  20 
per  cent,  of  increase;  for  if  a  3-foot  pipe  is  insufficient,  a  13-foot 
pipe  will  probably  be  the  next  size — an  increase  of  225  per  cent., — 
and  if  aG-foot  arch  culvert  is  too  small,  an  8-foot  will  be  used — 
an  increase  of  180  per  cent.  The  real  question  is  whether  a  2-foot 
pipe  or  an  8-foot  arch  culvert  is  needed. 

Numerous  empirical  formulas  have  been  i^roposed  for  this  and 
similar  problems  ;  *  but  at  best  they  are  all  only  approximate,  since 
no  formula  can  give  accurate  results  with  inaccurate  data.  The 
several  formulas,  when  applied  to  the  same  problem,  give  very 
discordant  results,  owing  (1)  to  the  sources  of  error  already  re- 
ferred to  and  (2)  to  the  formulas'  having  been  deduced  for  localities 
differing  widely  in  the  essential  characteristics  upon  which  the 
results  depend.  For  example,  a  formula  deduced  for  adi-y  climate, 
as  India,  is  wholly  inapplicable  to  a  humid  and  swampy  region,  as 
Florida  ;  and  a  formula  deduced  from  an  agricultural  region  is 
inapplicable  in  a  city. 

However,  an  approximate  formula,  if  simple  and  easily  applied, 
may  be  valuable  as  a  nucleus  about  which  to  group  the  results  of 
personal  experience.  Such  a  formula  is  to  be  employed  more  as  a 
guide  to  the  judgment  than  as  a  working  rule;  and  its  form,  and 
also  the  value  of  the  constants  in  it,  should  be  changed  as  subse- 
quent experience  seems  to  indicate.  With  this  use  in  view,  a  few 
formulas  will  be  referred  to  briefly. 

There  are  two  classes  of  these  formulas,  one  of  which  purports 

*  For  a  general  note  on  empirical  formulas,  see  §  .S&i. 


394  CULVERTS.  [chap.  xtii. 

to  give  the  quantity  of  water  to  be  discharged  per  unit  of  drainage 
area  and  the  other  the  area  of  the  water  way  in  terms  of  the  area  of 
the  territory  to  be  drained.  The  former  gives  the  amount  of  water 
supposed  to  reach  the  culvert;  and  the  area,  slope,  form,  etc.,  of 
the  culvert  must  be  adjusted  to  allow  this  amount  of  water  to  pass. 
There  are  no  reliable  data  by  which  to  determine  the  discharging 
capacity  of  a  culvert  of  any  given  form,  and  hence  the  use  of  the 
formulas  of  the  first  class  adds  complication  without  securing  any 
compensating  reliability.  Most  of  the  formulas  in  common  use  for 
proportioning  water  ways  belong  to  this  class.  Such  formulas  will 
not  be  considered  here. 

The  two  following  formulas  belong  to  the  second  class. 

610.  Myer's  Formula.  Of  the  formulas  giving  a  relation  be- 
tween the  area  of  water  way  and  the  area  to  be  drained,  Myer's  is 
the  one  most  frequently  used.     It  is 


A7'ea  of  water  way,  in  square  feet  =  C  \^ Drainage  area,  in  acres, 

in  which  C  is  a  variable  co-eflficient  to  be  assigned.  For  slightly 
rolling  prairie,  C  is  usually  taken  at  1;  for  hilly  ground  at  1.5;  and 
for  mountainous  and  rocky  ground  at  4.  For  most  localities,  at 
least,  this  formula  gives  too  large  results  for  small  drainage  areas. 
For  example,  according  to  the  formula,  a  culvert  having  a  watei 
way  of  one  square  foot  will  carry  the  water  from  a  single  acre  only. 
Further,  if  the  preponderance  of  the  testimony  of  the  formulas  for 
the  quantity  of  water  reaching  the  culvert  from  a  given  area  can 
be  relied  upon,  the  area  of  water  way  increases  more  rapidly  than 
the  square  root  of  the  drainage  area  as  required  by  this  formula. 
Hence,  it  appears  that  neither  the  constants  nor  the  form  of  this 
formula  were  correctly  chosen;  and,  consequently,  for  small  drainage 
areas  it  gives  the  area  of  waterway  too  great,  and  for  large  drain- 
age areas  too  small. 

611.  Talbot's  Formula.  Prof.  A.  N.  Talbot  proposed  the  fol- 
lowing formula,  '^more  as  a  guide  to  the  judgment  than  as  a  work- 
ing rule  :"  * 


Area  of  water  way,  in  square  feet  =  C  \/ {Drainage  area,  in  axyresf, 

in  which  C  is  a  variable  co-efRcient.    Data  from  various  States  gave 
values  for  C  as  follows:    ''For  steep  and  rocky  ground,  C varies 

*  Selected  Papers  of  the  Civil  Engineers'  Club  of  the  University  of  Illinois.  No,  2, 
pp.  14-17. 


AKT.  1.]  "WATER   WAY    REQUIRED.  SS'o 


from  f  to  1.  For  rolling  agricultural  country  subject  to  floods  at 
times  of  melting  of  snow,  and  with  the  length  of  valley  three  or 
four  times  its  width,  C  is  about -^;  and  if  the  stream  is  longer  in 
proportion  to  the  area,  decrease  C.  In  districts  not  affected  by 
accumulated  snow,  and  where  the  length  of  the  valley  is  several 
times  the  width,  \  or  \,  or  even  less,  may  be  used.  C  should  be 
increased  for  steep  side  slopes,  especially  if  the  upper  part  of  the 
valley  has  a  much  greater  fall  than  the  channel  at  the  culvert." 

The  author  has  tested  the  above  formula  by  numerous  culverts 
and  small  bridges  in  a  small  city  and  also  by  culverts  under  high- 
ways in  the  country  (all  slightly  rolling  prairie),  and  finds  that 
it  agrees  fairly  well  with  the  exjDcrience  of  fifteen  to  twenty  years. 
In  these  tests,  it  was  found  that  water  ways  proportioned  by  this 
formula  will  probably  be  slightly  flooded,  and  consequently  be  com- 
pelled to  discharge  under  a  small  head,  once  every  four  or  five 
years. 

612.  In  both  of  the  preceding  formulas  it  will  be  noticed  that 
the  large  range  of  the  "■  constant "  C  affords  ample  opportunity  for 
the  exercise  of  good  judgment,  and  makes  the  results  obtained  by 
the  formulas  almost  wholly  a  matter  of  opinion. 

613.  Practical  Method.  Valuable  data  on  the  proper  size  of 
any  particular  culvert  may  be  obtained  (1)  by  observing  the  existing 
openings  on  the  same  stream,  (2)  by  measuring — preferably  at  time 
of  high  water — a  cross  section  of  the  stream  at  some  narrow  place, 
and  (3)  by  determining  the  height  of  high  water  as  indicated  by 
drift  and  the  evidence  of  the  inliabitants  of  the  neighborhood. 
With  these  data  and  a  careful  consideration  of  the  various  matters 
referred  to  in  §  608,  it  is  possible  to  determine  the  proper  area  of 
water  way  with  a  reasonable  degree  of  accuracy. 

Ordinarily  it  is  wise  to  take  into  account  a  probable  increase  of 
flow  as  the  country  becomes  better  improved.  However,  in  con- 
structing any  structure,  it  is  not  wise  to  make  it  absolutely  safe 
against  every  possible  contingency  that  may  arise,  for  the  expen- 
diture necessitated  by  such  a  course  would  be  a  ruinous  and  un- 
justifiable extravagance.  Washouts  can  not  be  jorevented  altogether, 
nor  their  liability  reduced  to  a  minimum,  without  an  unreasonable 
expenditure.  It  has  been  said — and  within  reasonable  limits  it  is 
true — that  if  some  of  a  number  of  culverts  are  not  carried  away 


396  CULYERTS.  [chap.   XVII. 

each  year,  tliey  are  not  '^\'ell  designed;  that  is  to  say,  it  is  only  a 
question  of  time  when  a  properly  proportioned  culvert  will  perish 
in  some  excessive  flood.  It  is  easy  to  make  a  culvert  large  enough 
to  be  safe  under  all  circumstances,  but  the  difference  in  cost  be- 
tween such  a  structure  and  one  that  would  be  reasonably  safe  would 
probably  much  more  than  overbalance  the  losses  from  the  washing 
out  of  an  occasional  culvert.  It  is  seldom  justifiable  to  provide  for 
all  that  may  possibly  happen  in  the  course  of  fifty  or  one  hundred 
years.  One  dollar  at  5  per  cent,  compound  interest  will  amount  to 
$1147  in  50  years  and  to  $131.50  in  100  years.  Of  course,  the 
question  is  not  purely  one  of  finance,  but  also  one  of  safety  to  human 
life;  but  even  then  it  logically  follows  that,  unless  the  engineer  is 
prej^ared  to  s^^end  $131.50  to  avoid  a  given  danger  now,  he  is  not 
justified  in  spending  $1  to  avoid  a  similar  danger  100  years  hence. 
This  phase  of  the  problem  is  very  important,  but  is  foreign  to  the 
subject  of  this  volume. 

614.  In  the  construction  of  a  new  railroad,  considerations  of 
first  cost,  time,  and  a  lack  of  knowledge  of  the  amount  of  future 
traffic  as  well  as  ignorance  of  the  physical  features  of  the  country, 
usually  require  that  temjDorary  structures  be  first  put  in,  to  be  re- 
placed by  permanent  ones  later.  In  the  mean  time  an  incidental 
but  very  important  duty  of  the  engineer  is  to  make  a  careful  study 
of  the  requirement  of  the  permanent  structures  which  will  ulti- 
mately replace  the  temporary  ones.  The  high-water  mark  of  streams 
and  the  effect  of  floods,  even  in  water  courses  ordinarily  dry,  should 
be  recorded.  With  these  data  the  proper  j^^'oportioning  of  the 
water  way  of  the  permanent  structures  becomes  a  comparatively  easy 
task.  Upon  the  judgment  and  ability  displayed  in  this  depends 
most  of  the  economical  value  of  the  improvements;  for,  as  the  road 
will  have  fixed  or  standard  plans  for  culverts,  abutments,  piers, 
etc.,  the  supervision  of  the  construction  will  not  be  difficult. 

Art.  2.    Box  axd  Pipe  Culverts. 

615.  Stone  Box  Culvert.  This  culvert  consists  of  vertical  side 
walls  of  masonry  with  flag  stones  on  top  from  one  wall  to  the  other. 
Masonry  box  culverts  were  constructed  much  more  frequently  for- 
merly than  at  the  present  time.  The  lack  of  suitable  stone  in  many 
parts  of  the  West  led  to  the  adoption  of  vitrified  pipes  (§  627)  and 
iron  pipes  (§  631)  instead  of  masonry  box  culverts.     However,  in 


ART.   2.]  STOXE    BOX   CULVERTS.  397 

many  localities  they  are  built  frequently  enough  to  warrant  a  brief 
discussion  here. 

616.  Foundation.  A  common  foundation  for  masonry  box  cul- 
verts is  a  stone  pavement  (§  219)  under  the  entire  culvert,  upon 
which  the  side  walls  rest  (see  Fig.  91«).  This  is  not  good  practice  ; 
for,  since  the  paving  is  liable  to  be  washed  out,  it  endangers  the 


Fig.  91a.  Fig.  916. 

wall.  The  tendency  of  the  pavement  to  undermine  may  be  dimin- 
ished (1)  by  driving  sheet  piling  or  by  setting  deep  curb-stones  at 
both  ends,  or  (2)  by  extending  the  paving  to  a  considerable  distance 
beyond  bo+h  ends.  The  first  is  the  better  method  ;  but  usually 
these  devices  only  postpone,  and  do  not  prevent,  final  failure.  The 
water  is  nearly  certain  to  carry  the  soil  away  from  under  the  pave- 
ment, even  if  the  curb-vstones  or  sheet  piles  remain  intact. 

Sometimes  culvert  foundations  are  paved  by  laying  large  stones 
flatwise.  This  practice  is  no  better  than  ordinary  stone  paving,  un- 
less the  flags  are  large  enough  to  extend  under  both  walls ;  but 
stones  large  enough  for  this  can  seldom  be  obtained. 

A  much  better  method  is  to  give  each  side  wall  an  independen/ 
foundation  and  to  pave  between  the  walls  only  (see  Fig.  915).  Ak 
important  advantage  of  this  method  is  that  each  wall  can  be  placed 
separately,  which  facilitates  the  keeping  of  the  water  away  from  the 
foundation  pit.  Indeed,  if  the  foundations  are  deep,  or  if  there  is 
not  much  current,  the  paving  may  be  entirely  omitted.  If  the  cur- 
rent is  only  moderate,  it  is  sufficient  to  build  in,  at  each  end  of  the 
culvert,  between  the  ends  of  the  side  walls  with  solid  masonry  up 
to  the  bed  of  the  stream  ;  but  if  the  culvert  is  long,  it  is  wtse  to 
build  one  or  more  intermediate  cross  walls  also.  If  the  current  is 
strong,  the  cross  walls  at  the  ends  should  be  carried  down  deep, 
and  the  space  between  the  side  walls  should  be  paved  with  large 
stones  closely  set  and  deeply  bedded.  The  best  job  possible  is  se- 
cured by  setting  the  paving  in  cement  mortar.  In  this  connection, 
see  Figs.  94,  95,  and  96  (pages  403,  404,  and  406). 


398 


CULVERTS. 


[CHAP.  xvn» 


The  side  walls  and  the  cross  walls  (particularly  at  the  end  of  the 
culvert)  should  have  their  foundations  below  the  effect  of  frost. 

617.  End  Walls.  The  ends  of  box  culverts  are  usually  finished 
either  with  a  plane  wall  perpendicular  to  the  axis  of  the  culvert  as 
shown  in  Fig.  95  (page  404),  or  by  stepping  the  ends  off  as  shown 
in  Fig.  92.  Either  form  is  liable  to  become  clogged  and  to  have 
its  effectiveness  greatly  decreased,  and  probably  its  own  existence 
endangered,  by  drift  collected  at  its  upper  end.     This  danger  is 


jELevaiion 


'^^-"■■'^>ai  '  ^_  -1^. i^'tf-'-.^.^.i-M-^i 

'       Lon^itudinaL 
'  Section. 


Lion^itadinai 
Section 


JPlan 


Fig.  92. 


Fig.  93. 


considerably  decreased  by  extending  the  side  walls  at  the  upper  end 
as  shown  in  Fig.  93  and  in  Fig.  94  (page  403).  If  the  mouth  of 
the  culvert  should  become  stopped  with  drift,  the  open  top  is  a  well 
into  which  the  water  may  fall.  In  this  way  the  full  discharging 
capacity  of  the  culvert  can  be  maintained.  The  lower  end  may  be 
stepped  as  shown  in  Fig.  92. 

The  wing  walls  may  be  made  thinner  at  the  outer  end,  thus  pro- 
ducing to  a  small  degree  the  same  effect  as  is  obtained  in  splaying 
the  wings  of  arch  culverts  (see  §§  638-39). 

In  this  connection,  see  also  Fig.  96  (page  406). 

618.  Cover  Stones.  To  deduce  a  relationship  between  the  thick- 
ness of  the  cover  stones  and  the  load  to  be  supported,  let 


T  =  the  thickness,  in  inches  ; 
S  =  the  span,  in  feet ; 

H  =  the  height  of  bank,  in  feet,  above  the  top  of  the  culvert ; 
R  =  the  modulus  of  rupture,  in  pounds  per  sqiiare  inch  ; 
C  =  the  co-eflficient  of  transverse  strength  (§  15)  ; 
W  =  the  total  weight  of  the  earth  over  the  cover  stone,  iti  pounds. 


ART.  2.]  STONE    BOX   CULVERTS.  39& 

For  simplicity,  consider  a  section  of  the  culvert  only  a  foot  long. 

The  cover  stones  are  in  the  condition  of  a  beam  supported  at  the 
ends  and  loaded  uniformly.  By  the  principles  of  the  resistance  of 
materials,  one  eighth  of  the  uniform  load  mnltijMed  hy  the  span  is 
equal  to  one  sixth  of  the  continued  product  of  the  modulus  of  rup- 
ture, the  breadth,  and  the  square  of  the  thickness.  Expressing 
this  in  symbols  as  above,  and  reducing,  gives 


T^VV-^ (1) 


8     R 

Ordinarily,  earth  weighs  from  80  to  100  lbs.  per  cu.  ft.,  but  for 
convenience  we  will  assume  it  at  100  lbs.  per  cu.  ft.,  which  is  on 
the  safe  side  ;  then  W  =  100  US.  The  maximum  moving  load  for 
railroad  bridges  may  be  taken  at,  say,  2  tons  per  foot  of  track.* 
This  is  distributed  over  at  least  8  square  feet ;  and  hence  the  live 
load  is  equal  to  one  quarter  of  a  ton,  or  500  pounds,  per  square  foot, 
i.  e.  the  live  load  is  equal  to  an  embankment  5  feet  high.  Therefore, 
the  maximum  live  load — a  locomotive — is  provided  for  by  adding  5 
feet  to  the  actual  height  of  the  embankment.  The  table  on  page 
12  shows  that  for  limestone  R  =  1,500.  Substituting  these  values 
in  equation  (1),  above,  gives  for  limestone 

T  =  0.20  S  VH+T, (2) 

By  substituting  the  corresponding  value  of  R  from  the  table  on 
page  12,  we  have  for  sandstone 


T=0.2bSVH+b, (3) 

For  highways,  it  is  sufficiently  exact  to  drop  the  5  under  the 
radical,  i.  e.,  to  neglect  the  live  load  ;  and  equation  (1)  then  becomes 
for  limestone 


and  for  sandstone 


T=0.20SVH, (4) 

T=  0.25  SVH. (5) 


The  preceding  formulas  give  the  thickness  which  a  stone  of 
average  quality  must  have  to  be  on  the  point  of  breaking;  and  hence 


400  CULVERTS,  [chap.  XVIL 

in  applying  them  it  will  be  necessary  to  allow  a  margin  for  safety, 
either  by  selecting  the  stone  or  by  increasing  the  computed  thick- 
ness. If  reasonable  care  is  used  in  selecting  the  stones,  it  is  probably 
safe  to  double  the  thickness  found  as  above.  To  allow  for  any  given 
factor  of  safety,  multiply  the  thickness  found  by  applying  the  above 
formulas  by  the  square  root  of  the  factor  of  safety.  Thus,  to  allow 
for  a  factor  of  4,  multiply  the  thickness  found  as  above  by  2  ;  for  a 
factor  of  G,  multiply  by  2|  ;  and  for  a  factor  of  9,  multiply  by  3. 

-619.  The  thickness  of  the  cover  stones  does  not,  however,  de- 
pend alone  upon  the  depth  of  the  earth,  the  live  load,  and  the  span. 

In  the  first  place,  the  pressure  on  the  cover  stone  does  not  vary 
directly  as  the  depth  of  the  earth  above  it.  (a)  The  earth  itself 
acts  more  or  less  as  a  beam  to  support  part,  at  least,  of  the  weight 
over  the  opening.  That  earth  may  act  thus  is  proven  by  the  fact 
that  an  excavation  can  be  carried  horizontally  into  an  embankment 
or  side  hill  without  supporting  the  roof.  The  beam  strength  of  the 
earth  increases  with  the  compactness  and  the  tenacity  of  the  soil 
and  with  the  square  of  the  height  of  the  embankment  above  the 
roof.  This  effect  would  be  zero  with  clean  sand  ;  but,  owing  to  the 
nature  of  that  material,  it  would  seldom  be  employed  for  filling  over 
a  culvert.  Hence,  under  ordinary  conditions,  part  of  the  load  is 
supported  by  the  beam  strength  of  the  earth  itself.  Therefore,  a 
low  embankment  may  produce  a  greater  strain  in  the  cover  than  a 
much  higher  one.  (b)  The  prism  of  earth  directly  over  the  culvert 
will  be  partially  supported  by  the  adjacent  soil  ;  that  is  to  say,  the 
particles  of  earth  directly  above  the  culvert  will  act  more  or  less  as 
arches  resting  upon  the  earth  at  the  sides  of  the  culvert,  thus  par- 
tially relieving  the  cover  stones.  This  effect  would  be  greater  with 
sharp  sand  than  with  clay,  but  would  be  entirely  destroyed  by  shock, 
as  of  passing  trains,  (c)  The  stones  at  the  center  of  the  culvert 
would  be  relieved  of  part  of  their  load  by  an  action  similar  to  that 
mentioned  above,  whereby  the  weight  over  the  center  of  the  culvert 
is  transferred  towards  its  ends.  However,  the  relief  caused  by  this 
-action  is  but  slight. 

In  the  second  place,  the  pressure  due  to  the  live  load  is  trans- 
nnitted  downward  in  diverging  lines,  thus  distributing  the  weight 
over  a  considerably  larger  area  than  that  assumed  in  deducing  equa- 
tions (2)  and  (3)  above. 

In  the  third  place,  the  cover  must  be  thick  enough  to  resist  the 


ART.  2. J  STONE   BOX   CULVEETS.  iOl 

effect  of  frost,  as  well  as  to  support  the  earth  and  live  load  above  it. 
The  freezing,  and  consequent  expansion,  of  the  earth  is  a  force 
tending  directly  to  break  the  cover  stones.  That  this  is  an  impor- 
tant consideration  is  proved  by  the  fact  that  these  stones  break 
near  the  ends  of  culverts  as  frequently  as  near  the  middle,  although 
the  weight  to  be  supported  is  greater  at  the  latter  place. 

620.  It  is  impossible  to  compute,  even  approximately,  the  effect 
of  the  preceding  factors  ;  but  experience  shows  that  the  thickness 
is  indejDcudent  of  the  height  of  the  embankment,  provided  there  is 
sufficient  earth  over  the  cover  stones  to  prevent  serious  shock, — say 
3  feet  for  railroads  and  1  to  2  feet  for  higliways. 

The  thickness  employed  on  the  raih'oads  in  States  along  the 
fortieth  parallel  of  latitude  is  generally  about  as  follows,  irrespec- 
tive of  the  height  of  the  bank  or  of  whether  the  cover  is  limestone 
or  sandstone : 

Span  of  Culvert.  Thickness  of  Coyer. 

2  feet 10  inches. 

3  feet, 13  inches. 

4  feet 15  inches. 

On  the  Canadian  Pacific  R.  R.,  the  minimum  thickness  of  cover 
stones  for  spans  of  3  feet  is  16  inches,  and  under  3  feet,  14  inches. 

621.  Quality  of  Masonry.  Box  culverts  are  usually  built  of 
rubble  masonry  (§  213)  laid  in  cement  mortar.  Formerly  they  were 
often  built  of  dry  rubble,  except  for  3  or  4  feet  at  each  end,  which 
was  laid  in  mortar.  It  is  now  generally  held  that  box  culverts 
should  be  so  built  that  they  may  discharge  under  a  head  without 
damage.  It  is  usually  specified  that  the  cover  stones  must  have  a 
solid,  well-leveled  bearing  on  the  side  walls  of  not  less  than  15 
inches.  The  most  careful  constructors  close  the  joints  between  the 
cover  stones  by  bedding  spalls  in  mortar  over  them. 

622.  Specifications.*  All  stone  box  culverts  shall  have  a  water  way  at  least 
24  X  3  feet.  The  side  walls  shall  not  be  less  than  two  feet  (2')  thick,  and 
shall  be  built  of  sound,  durable  stones  not  less  than  six  inches  (6")  thick,  laid 
in  cement  mortar  [usually  1  part  Rosendale  cement  to  2  parts  sand].  The 
walls  must  be  laid  in  true  horizontal  courses,  but  in  case  the  thickness  of  the 
course  is  greater  than  12  inches  (12"),  occasionally  two  stones  may  be  used  to 
make  up  the  thickness.  The  walls  must  be  laid  so  as  to  be  thoroughly  bonded, 
and  at  least  one  fourth  of  the  area  of  each  course  must  be  headers  going  en- 

*  Pennsylvania  Kaikoad. 


40;<5  CULVERTS.  [CHAP,  XVII. 

tirely  through  the  wall.  The  top  course  must  have  one  half  its  area  of  through 
stones,  and  the  remainder  of  this  course  must  consist  of  stone  going  at  least 
one  half  of  the  way  across  the  wall  from  the  Inside  face.  The  face  stones  of 
each  course  must  be  dressed  to  a  straight  edge,  and  pitched  off  to  a  true  line. 
All  of  the  coping  stones  of  head  walls  must  be  throughs,  and  must  have  the 
upper  surface  hammer-dressed  to  a  straight  edge,  and  the  face  pitched  off 
to  a  line  with  margin  draft.  Cover  stones  shall  have  a  thickness  of  at  least 
twelve  inches  (13")  for  opening  of  three  feet  (3),  and  at  least  14  inches  (14")  for 
opening  of  four  feet  (4) ;  and  must  be  carefully  selected,  and  must  be  of  such 
length  as  to  have  a  bearing  of  at  least  one  foot  (1')  on  either  wall. 

The  beds  and  vertical  joints  of  the  face  stones  for  a  distance  of  S:ix 
inches  (6")  from  the  face  of  the  wall  shall  be  so  dressed  as  to  require  a  mortar 
joint  not  thicker  than  three  fourths  of  an  inch  (f ").  Joints  between  the  cov- 
ering stones  must  be  not  wider  than  three  fourths  of  an  inch  (f ';,  and  the 
bearing  surface  of  cover  stones  upon  side  walls  must  be  so  dressed  as  to 
require  not  more  than  a  one-inch  (1")  mortar  joint. 

The  paving  shall  consist  of  flat  stones,  set  on  edge,  at  right  angles  with  the 
line  of  the  culvert,  not  less  than  twelve  inches  (13")  deep,  and  shall  be  laid  in 
cement  mortar  and  grouted. 

623.  Examples.  The  box  culvert  shown  in  Fig.  94  (page  403), 
is  presented  as  being  on  the  whole  the  best  (see  §  617).  The  table 
accompanying  the  diagram  gives  the  various  dimensions  of,  and  also 
quantities  of  masonry  in,  box  culverts  for  different  openings.  The 
former  data  and  the  diagrams  are  ample  for  the  construction  of  any 
box  culvert ;  while  the  latter  data  will  be  useful  in  making  esti- 
mates of  cost  (§  626).  In  the  headings  of  the  colums  under  "  Size 
of  the  Openings,"  the  first  number  is  the  span  of  the  culvert,  and 
the  second  is  the  clear  height  of  water  way.  The  quantities  of 
masonry  in  the  table  were  computed  for  a  cross  wall  at  each  end  of 
the  culverts,  of  the  section  shown  in  Fig.  94 ;  but  in  many  cases, 
this  should  be  3  feet  deep  instead  of  2,  as  shown.  In  using  the 
table  this  correction  is  easily  applied. 

624.  The  box  culvert  shown  in  Fig.  95  is  the  one  employed  in 
the  construction  of  the  ''West  Shore  R.  R."— New  York  City  to 
Buffalo.  The  data  in  the  table  accompanying  the  diagram  give  tlie 
dimensions  and  quantities  of  masonry  of  various  sizes.  In  the  head- 
ings under  "  Size  of  the  Openings,"  the  first  number  is  the  span  of 
the  opening  and  the  second  is  its  height. 

Box  culverts  of  the  general  form  shown  in  Fig.  95  are  sometimes 
built  double ;  i.  e.,  two  culverts  are  built  side  by  side  in  such  a 
manner  as  to  have  one  side  wall  in  common.     The  following  table 


iRT.   2.] 


STOKE    BOX   CULVERTS. 


403 


"-^.O  ,J -* 


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5j  I-  e<  Si  Tf 

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Jb  w  5j  5j  Tp 

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bib 

a    ■  - 

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D.    ■  O     •  O 

o      o    ;  u 

o    .<„    .^ 

D    •  o   :  o 

5    ^   :  <u 

3  :S 

o 

3  :o 

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a>    ■' 

<c 

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a    -to 

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of  tO] 

"  bo 
covet 

n  two 
yards 
n  the 
yards 
the  t; 
yards 

'^  --^    Cfl         (M    '/ 

TUNTS: 

Masonry  i 
cubic  . 

Masonry  i 
cubic 

Paving  in 
cubic 

a 

Zi 

o 

Q 

o 

404 


CULVERTS. 


[chap.  XYII. 


*s 

c 

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ART.  2.] 


STONE   BOX   CULVERTS. 


405 


gives  the  dimensions  and  quantities  for  such  box  culverts.  The 
dimensions  not  given  in  the  following  table  are  the  same  as  in  the 
table  accompanying  Fig.  95. 

TABLE  44. 
Dimensions  and  Contents  of  Double  Box  Culverts. 


Items. 


Dimensions: 

End  wall,  length  of 

Center  wall,  thickness  of 

Contents: 

Masonry  in  two  end  walls,  in  cu.  yds 

Masonry  in  trunk,  per  font  of  lengtli  from  in- 
side to  inside  of  end  walls,  in  cii.  yds. 
Paving  in  trunk,  per  foot  of  lengtli  from  inside 
to  inside  of  end  walls,  in  cu.  yds 


Size  oi'  the  Opening. 


2X2J 
feet. 


IC  6" 
•.>'  0" 


13.16 

0.864 
0.407 


ajx3 

feet. 


20'  0" 
2'  0" 


10.18 
0.982 
0.444 


2J  X  3J 
feet. 


20'  0" 
2'  0'^ 


21.50 
1 .222 

0.481 


3X4 
feet. 


23'  0'' 

2'  u" 


32.18 
1.778 
0.592 


4X5 
feet. 


30'  3' 
3'  0- 


53.25 
2.565 

0,703 


The  standard  double  box  culvert  employed  in  the  construction 
of  the  Canadian  Pacific  E.  E.  differed  from  the  form  described 
above  in  having  (1)  shorter  end  walls,  and  wiugs  at  an  angle  of  30° 
with  the  axis  of  the  culvert,  and  (2)  a  triangular  cut-water  at  the 
upper  end  of  the  division  wall. 

625.  The  culvert  shown  in  Fig.  96  is  the  standard  on  the  Inter- 
colonial Eailway  of  Canada,  and  is  very  substantially  constructed. 

626.  Cost.  With  the  data  accompanying  Figs.  94  and  95  (pages 
403  and  404),  and  the  table  of  cost  of  masonry  on  page  160,  it  is 
an  easy  matter  to  make  an  estimate  of  the  cost  of  a  box  culvert. 
For  example,  assume  that  it  is  proposed  to  build  a  culvert  30  feet 
long — out  to  out  of  culvert  proper — having  a  water  way  3  feet  wide 
and  4  feet  high,  and  that  estimates  of  the  cost  of  the  general  forms 
shown  in  Fig.  94  and  also  of  that  of  Fig.  95  are  desired. 

Estimates  for  a  3  X  4/i!.  Box  Culvert  oftJie  General  Form,  shown  in  Fi^.  94, 

Masonry  in    2  end  walls— 16.88  cu.  yds ®  S3.50  per  cu.  yd.  =    $59.08 

"  25  feet  of  trunk  (1  444x25=)  36.10  cu.  yds.  @  S3.50       "        "     =     126  35 
Paving      "25    "     "       "        (0.111x25=)   2.78   "      "     @  $2.00       "         "     =         5.55 

Total  cost $190.98 

Estimates  for  a  3  X  4/if.  Box  Culvert  of  the  General  Form  shown  in  Fig.  95. 

Masonry  in    2  end  walls— 24.20  cu.  yds @  $3  ,50  per  cu.  yd.  =    $84.70 

"  24  feet  of  trunk  (24  x  1.148=)  27..55  cu.  yds.  @,  $3  50       "        "     =       96.43 
Paving      "24    "      "      "       (24x0.370=)    8.88   "      "     (^  $2.00        "        "      =       17.76 

Total  cost $198.89 


406 


CULVERTS. 


[chap.  XVIL 


U 4JtJ M 


A.KT.  2.]  VITRIFIED    PIPE    CULVERTS.  407 

If  the  price  for  the  masonry  does  not  include  the  expense  for 
th,e  necessary  excavation,  the  above  estimates  should  be  increased 
by  tJie  cost  of  excavation,  which  will  vary  with  the  situation  of  the 
culvert. 

To  make  a  comparison  of  the  relative  cost  of  the  two  types 
of  culverts  just  mentioned,  we  may  proceed  as  follows :  The  cost 
per  foot  of  the  trunk  of  a  3  X  4  culvert  of  the  form  shown  in 
Fig.  94  is  (1.444  cu.  yds.  of  masonry  @  $3.50  j^^us  0.111  cu.  yds. 
of  paving  @  $2.00)  $5.28;  and  the  corresponding  cost  for  Fig. 
95  is  (1.148  cu.  yds.  of  masonry  @  $3.50  ji^/ms  0.370  cu.  yds. 
of  paving  @  $2.00)  $4.76.  The  difference  in  cost  per  foot 
is  ($5.28 -$4.76)  $0.52  in  favor  of  Fig.  95.  The  cost  of  the 
end  walls  for  Fig.  94  is  (16.88  cu.  yds.  @  $3.50)  $59.08;  and  the 
corresponding  cost  for  Fig.  95  is  (24.20  cu.  yds.  @  $3.50)  $84.70. 
The  difference  is  $25.62  in  favor  of  Fig.  94.  Since  in  the  former 
the  cross  wall  extends  but  2  feet  below  the  floor  of  the  culvert, 
while  in  the  latter  the  end  walls  extend  3  feet,  the  difference  in  cost 
should  be  decreased  by  the  cost  of  the  difference  of  the  foundations. 
If  the  cross  Avails  of  Fig.  94  be  carried  down  another  foot,  the 
amount  of  masonry  will  be  increased  2  cu.  yds.  and  the  cost  $7.00; 
and  the  difference  in  cost  of  the  end  walls  will  be  ($25.62  —  $7.00) 
$18.62  in  favor  of  Fig.  94.  Under  these  conditions,  for  a  culvert 
40  feet  long,  the  two  types  will  cost  the  same;  for  lengths  less  than 
40  feet  Fig.  94  is  the  cheaper,  and  for  lengths  greater  than  40  feet 
Fig.  95  is  the  cheaper.  If  the  end  walls  of  Fig.  95  are  carried 
down  only  2  feet,  the  amount  of  masonry  will  be  decreased  by  3.4 
cu.  yds.  and  the  cost  by  $11.90;  and  then  the  difference  of  cost  will 
be  ($25.62  —  $11.90)  $13.72.  Under  this  condition,  for  a  culvert 
30  feet  long,  the  two  types  will  cost  the  same;  for  lengths  less  than 
30  feet  Fig.  94  is  the  cheaper,  and  for  lengths  greater  than  30  feet 
Fig.  95  is  the  cheaper.  We  may  conclude,  therefore,  that  for 
lengths  under  35  or  40  feet  the  type  shown  in  Fig.  94  is  a  little 
cheaper,  while  for  greater  lengths  than  35  or  40  feet  that  in  Fig. 
95  is  slightly  cheaper.  For  the  smallest  size  the  length  of  equal 
cost  is  about  10  feet. 

There  is  no  material  difference  in  the  first  cost  of  the  two  types; 
but  the  culvert  shown  in  Fig.  94  is  the  more  efficient. 

627.  Vitrified  Pipe  Culverts.  During  the  past  lew  years 
vitrified  sewer  pipes  have  been  extensively  employed  for  small  cul- 


408  CULVERTS.  [chap.  XYIl 


verts  under  both  highways  and  railroads.  The  pipe  generally 
employed  for  this  purpose  is  that  known  to  the  trade  as  culvert 
pipe  or  "extra  heavy"  or  "double  strength"  sewer  pipe,  which  is 
20  to  40  per  cent,  (varying  with  the  maker  and  the  size)  heavier  than 
the  quality  ordinarily  employed  for  sewers. 

Apparently  the  heavier  pipe  is  used  on  the  supposition  that  the 
lighter  is  not  strong  enough  for  culverts.  In  most  cases,  at  least, 
tills  is  an  erroneous  assumption.  1.  AVith  the  same  depth  of  earth 
over  the  pipe,  there  is  but  little  more  pressure  on  the  pipe  when 
used  as  a  culvert  than  when  employed  in  a  sewer.  At  most,  tlie 
difference  of  pressure  is  that  due  to  the  live  load,  which  can  not 
exceed  the  weight  of  an  additional  5  feet  of  earth  (see  §  618),  and 
will  generally  be  much  less  (see  the  second  paragraph  of  §  619). 
2.  Experience  demonstrates  that  the  lighter  pipes  are  not  deficient 
in  strength  when  used  in  sewers,  however  deep  they  are  laid. 
According  to  experiments  made  by  bedding  the  lower  half  of  the 
pipe  in  sand  and  applying  a  pressure  along  a  comparcdivcly  narrow 
urea,  the  average  crushing  strength  of  ordinary  sewer  pipe  was 
2,400  lbs.  per  sq.  ft.  of  horizontal  section,  and  for  culvert  pipe 
12,000  lbs.  per  sq.  ft.*  If  the  pressure  had  been  applied  more 
.nearly  as  in  actual  practice,  the  pipes  would  have  borne  consider- 
ably more.  The  first  of  the  above  results  is  equal  to  the  weight  of 
24  feet  of  earth,  and  the  second  to  that  of  120  feet,  although  actual 
embankments  of  these  heights  would  not  give  anything  like  the 
above  pressures  (see  §  619). 

There  is  a  little  difference  between  culverts  and  sewers  in  the 
exposure  to  frost;  but  no  danger  need  be  apprehended  from  this 
cause,  provided  the  culverts  are  so  constructed  that  the  water  is 
carried  away  from  the  lower  end,  since  ordinary  soft  drain  tile  are 
not  in  the  least  injured  by  the  expansion  of  the  frost  in  the  earth 
around  them. 

628.  Construction.  In  laying  the  pipe,  the  bottom  of  the  trench 
should  be  rounded  out  to  fit  the  lower  half  of  the  body  of  the  pipe, 
with  proper  depressions  for  the  sockets.  If  the  ground  is  soft  or 
sandy,  the  earth  should  be  rammed  carefully,  but  solidly,  io.  and 
around  the  lower  part  of  the  pipe.  On  railways,  three  feet  of  earth 
between  the  top  of  the  pipe  and  the  bottom  of  the  tie  has  been 
found  sufficient.  On  highways  pipes  have  stood  from  10  to  15 
years  under  heavy  loads  with  only  8  to  12  inches  of  earth  over 
*  For  additional  data,  see  Note  7,  page  547. 


ART.  2.] 


VITRIFIED    PIPE   CULVERTS. 


409 


them;  but  as  a  rule  it  is  not  wise  to  lay  them  with  less  than  12  to 
18  inches  of  earth  covering. 

In  many  cases — perhaps  in  most — the  joints  are  not  calked.  If 
this  is  not  done,  there  is  liability  of  the  water's  being  forced  out  at 
the  Joints  and  washing  away  the  soil  from  around  the  pipe.  Even 
if  the  danger  is  not  very  imminent,  the  joints  of  the  larger  pipes, 
at  least,  should  be  calked  with  hydraulic  cement,  since  the  cost  is 
very  small  compared  with  the  insurance  of  safety  thereby  secured. 
Sometimes  the  joints  are  calked  with  clay.  Every  culvert  should 
be  built  so  that  it  can  discharge  water  under  a  head  Vi^ithout  damage 
to  itself. 

The  end  sections  should  be  protected  with  a  timber  or  masonry 
bulkhead,  although  it  is  often  omitted.  Of  course  a  parapet  wall 
of  rubble  masonry  or  brick-work  laid  in  cement  is  best  (see  Fig.  97). 


Fig.  97. 


Fig. 


The  foundation  of  the  bulkhead  should  be  deep  enough  not  to  be 
disturbed  by  frost.  In  constructing  the  end  wall,  it  is  well  to  in- 
crease the  fall  near  the  outlet  to  allow  for  a  possible  settlement  of 
the  interior  sections.  When  stone  and  brick  abutments  are  too 
expensive,  a  fair  substitute  can  be  made  by  setting  posts  in  the 
ground  and  spiking  plank  on  as  shown  in  Fig.  98.  When  planks 
are  used,  it  is  best  to  set  them  with  considerable  inclination  towards 
the  road  bed  to  prevent  their  being  crowded  outward  by  the  pressure 
of  the  embankment.  The  upper  end  of  the  culvert  should  be  so 
protected  that  the  water  will  not  readily  find  its  way  along  tlie  out- 
side of  the  pipes,  in  case  the  mouth  of  the  culvert  should  become 
submerged. 

The  freezing  of  water  in  the  pipe,  particularly  if  more  than 
half  full,  is  liable  to  burst  it;  consequently  the  pipe  should  have  a 
sufficient  fall  to  drain  itself,  and  the  outlet  should  be  so  low  that 


410 


CULVEKTS. 


[chap.  XVII. 


there  is  no  danger  of  back-water's  reaching  the  pijDe.     If  properly 
drained,  there  is  no  danger  from  frost. 

When  the  capacity  of  one  pipe  is  not  sufficient,  two  or  more 
may  be  laid  side  by  side.  Although  two  small  pipes  do  not  have 
as  much  discharging  capacity  as  a  single  large  one  of  equal  cross 
section,  yet  there  is  an  advantage  in  laying  two  small  ones  side  by 
side,  since  then  the  water  need  not  rise  so  high  to  utilize  the  full 
capacity  of  the  two  pipes  as  would  be  necessary  to  discharge  itself 
through  a  single  one  of  larger  size. 

629.  Examples.  Fig.  99  (page  411)  shows  the  standard  vitri- 
fied pipe  culverts  employed  on  the  Kansas  City  and  Omaha  E.  E. 
This  construction  gives  a  strong,  durable  culvert  which  passes  water 
freely.  The  dimensions  of  the  masonry  end  walls  and  of  the  con- 
crete bed  for  the  intermediate  sizes  are  nearly  j)ro23ortional  to  those 
shown  in  Fig.  99.  Table  46  (page  411)  shows  the  quantities  of 
masonry  required  for  the  principal  sizes. 

630.  Cost.  Prices  of  vitrified  pipe  vary  greatly  with  the  con- 
ditions of  trade,  and  with  competition  and  freight.  Current  (1888), 
non-competitive  prices  for  ordinary  sewer  pipe,  in  car-load  lots 
/.  0.  h.  at  the  factory,  are  about  as  in  the  table  below.  * 

TABLE  45. 
Cost  and  Weight  of  Vitrified  Sewer  Pipe. 


Inside  Diameter. 

Price  per  Foot. 

Area. 

Weight  per 
Foot. 

Amount  in  a 
Car  Load. 

12  inclies. 

15  cents. 

.  78  sq.   ft. 

45  lbs. 

500  feet. 

14      " 

23       " 

1.07    "     " 

55    " 

400     " 

16      " 

30       " 

1.40   "     " 

65    " 

350     " 

18      " 

38       " 

1.76    "     " 

75    " 

300     " 

20      " 

53       " 

2.18    "     " 

90   " 

260     " 

23      " 

57       " 

2.64  "     " 

110   " 

230     " 

24      " 

87      " 

3.14   "     " 

140   " 

200     " 

Culveift  pipe  costs  about  20  to  25  per  cent,  more  than  as  above, 
and  second  quality  sewer  pipe  about  20  to  25  per  cent.  less.  The 
latter  differs  from  first  quality  in  being  less  perfectly  glazed,  less 
perfectly  burned,  or  not  perfectly  round,  or  in  having  fire  cracks  in 
the  glazing,  blisters  on  either  surface,  excrescences  or  pimples  on 
the  inside,  or  a  piece  broken  out  of  the  end.  Frequently  such 
pipe  is  as  good  for  culverts  as  first  quality  sewer  pipe. 


ART.  y.J 


VITEIFIED    PIPE   CULVERTS. 


ni 


<--Z'9"-- 


1        *-? 

^^ 

1 

^-Z'6- 

5f 

1 

% 



\ 



»^ 

CONCRETE 

M, 

1 

1 

Fig.  99.— Standard  Vitrified  Pipe  Culvert.— K.  C.  «&  O.  R.  R. 


TABLE  46. 

ll.j*ONRY  Required  for  Vitrified  Pipe   Culverts   of   the   General 
Form  shown  above. 


Items. 

Diameter 

OF  PlP3. 

14  inches. 

16  inches. 

20  inches. 

24  inches. 

iJoping,  two  ends 

cu.  yds. 
0.54 
2.93 

CM.  yds. 
0.71 
4.45 

cu.  yds. 
0.97 
6.98 

cu.  yds. 
1  07 

I  arapets,  two  ends 

8.47 

Total  Masonry 

3.47 

5.16 

7.95 

S.54 

Concrete,  per  lineal  foot.. 

0.070 

0.102 

0.136 

0.180 

412 


CULVERTS. 


[chap.  XVII. 


631.  Iron  Pipe  Culverts.  In  recent  years,  iron  pipes  have 
been  much  used  for  culverts.  In  many  localities  good  stone  is  not 
available,  and  hence  stone  box  culverts  {§§  615-26)  can  not  be  used. 
In  such  localities  vitrified  stoneware  j^ipes  are  used  ;  but  as  they 
are  not  made  larger  than  2  feet  in  diameter,  iron  or  stone  is  the  only 
material  available  for  permanent  culverts  requiring  a  greater  water 
way  than  that  obtained  by  using  one  or  two  of  the  largest  vitrified 
pipes.  Apparently,  stone  culverts  if  well  built  should  last  forever; 
but,  as  constructed  in  the  past,  they  have  been  found  to  last  rela- 
tively only  a  short  time.  Hence,  with  the  increasing  cheapness  of 
iron,  there  has  been  an  increasing  tendency  to  use  iron  pipe  for  even 
large  culverts.  Cast-iron  pipes  from  12  to  48  inches  in  diameter 
and  12  feet  long  are  in  common  use  by  all  of  the  prominent  roads 
of  the  Mississippi  Valley.  Some  of  the  roads  cast  their  own,  while 
others  buy  ordinary  water  pipe.  The  lightest  water  pipes  made,  or 
even  siich  as  have  been  rejected,  are  sufficiently  strong  for  use  in 
culverts.  The  dimensions  used  on  the  Chicago,  jVIilwaukee  and  St. 
Paul  R.  R.  are  about  as  follows: 


TABLE  47. 
Dimensions  of  Cast-Iron  Culvert  Pipe. 


Inside  Diameter. 

Weight  per  Foot. 

Thickness. 

Weight  per  Lineal  Foot 
PER  SQ.  ft.  of  Area. 

12  inches. 

60  lbs. 

y^^  inch. 

77  lbs. 

16       '• 

88   " 

i     " 

63    " 

20       " 

118   " 

1     " 

59    " 

24      " 

175   " 

i    " 

56    " 

30      " 

240    " 

f    " 

49    " 

36      " 

320   " 

1    " 

46    " 

42       " 

400    " 

7         " 

42    " 

48       " 

510   " 

1        " 

41     " 

632.  Construction.  In  constructing  a  culvert  with  cast  iron, 
the  points  requiring  particular  attention  are  (1)  tamping  the  soil 
tightly  around  the  pipe  to  prevent  the  water  from  forming  a  chan- 
nel along  the  outside,  and  (2)  protecting  the  ends  by  suitable  head 
walls  and,  Avhen  necessary,  laying  riprap  at  the*  lower  end.  The 
amount  of  masonry  required  for  the  end  walls  depends  upon  the 
relative  width  of  the  embankment  and  the  number  of  sections  of. 
pipe  used.  For  example,  if  the  embankment  is,  say,  40  feet  wide 
at  the  base,   the    culvert  may  consist  of   three  12-foot  lengths  of 


ART.  2.]  IRON    PIPE   CULVERTS.  413 

pipe  and  a  light  end  wall  near  the  toe  of  the  bank  ;  but  if  the 
embankment  is,  say,  32  feet  wide,  the  culvert  may  consist  of  two 
12-foot  lengths  of  pipe  and  a  comparatively  heavy  end  wall  well 
back  from  the  toe  of  the  bank.  The  smaller  sizes  of  pij^e  usually 
iome  in  12-foot  lengths,  but  sometimes  a  few  6-foot  lengths  are 
included  for  use  in  adjusting  the  length  of  culvert  to  the  width  of 
bank.     The  larger  sizes  are  generally  6  feet  long. 

Fig.  100  (page  414)  shows  the  method  employed  on  the  Atchi- 
son, Topeka  and  Santa  Fe  K.  R.  in  putting  in  cast-iron  pipe 
culverts.  Table  48  (page  414)  gives  the  dimensions  for  the  end 
walls  for  the  various  sizes.  The  length  of  pipe  is  determined  by 
taking  the  multiple  of  6  feet  next  larger  than  the  length  given  by 
the  position  slope  as  in  Fig.  100.  To  allow  for  settling,  the  pij)e  is 
laid  to  a  vertical  curve  having  a  crown  at  the  center  of  1  inch  for 
each  5  feet  in  vertical  height  from  bottom  of  pipe  to  profile  grade. 

Where  the  soil  is  treacherous,  it  would  be  wise  to  lay  the  pipes 
on  a  bed  of  broken  stone  to  prevent  undue  settling.  In  this  con- 
nection, see  Figs.  96  and  99  (pages  406  and  411). 

633.  Fig.  101  (page  415)  shows  the  method  employed  on  the 
Chicago,  Burlington  and  Quincy  R.  R.  of  putting  in  cast-iron  pipe 
culverts.     This  construction  has  given  entire  satisfaction. 

The  same  road  has  recently  commenced  the  use  of  iron  for  cul- 
verts up  to  12  feet  in  diameter.  For  diameters  greater  than  4  feet, 
the  pipes  are  cast  in  quadrants  2,  4,  6,  and  8  feet  long,  which  are 
afterwards  bolted  together,  through  outside  flanges,  to  form  a 
cylinder  of  any  desired  length.  The  different  segments  are  so  com- 
bined as  to  break  joints  around  and  also  along  the  pipe.  The  body 
of  the  pipe  was  formerly  1|  inches  thick  ;  but  is  now  1^,  stiffened  on 
the  outside  by  ribs.  The  sections  are  put  together  without  any  chip- 
ping, drilling,  or  other  skilled  labor.  Between  the  different  sec- 
tions is  a  recess  in  which  a  tarred  rope  smeared  with  neat  cement 
mortar  is  placed  before  bolting  the  segments  together,  which  makes 
t.e  joints  tight.* 

634.  Cost.  The  cost  of  cast-iron  pipe  varies  greatly  with  com- 
petition and  the  conditions  of  trade.  The  price  ranges  from  §26  to 
$36  per  ton  for  first  quality  water  pipes,  /.  o.  h.  at  the  foundry;  or 
approximately,  say,  1^  cents  per  pound. 

*  For  illustration  of  details,  see  Railroad  Gazette,  vol.  six.  pp.  123-^4. 


414 


CULVERTS. 


[chap.   XVII, 


be 

5 


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5 


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OE^K 


ART.   2,] 


IRON    PIPE    CULVERTS. 


415 


Fie.  101.— Cast-Ihon  Pipe  Culvert.— C,  B.  &  Q.  E.  B. 


416  CULVERTS.  [chap.  XVII: 

Table  47  (page  412)  shows  that  the  average  weight  of  the  pipe 
per  foot  per  square  foot  of  water  way  is  about  60  pounds  ;  anu 
hence  the  cost  of  the  trunk  of  a  cast-iron  pipe,  exclusive  of  trans- 
portation and  labor,  is  about  (60  X  1|-)  90  cents  j)er  lineal  foot  per 
sq.  ft.  of  area.  The  cost  of  sewer  pipes  is,  from  Table  46  (page 
411),  about  22  cents  per  foot  per  square  foot  of  water  way  ;  and  for 
culvert  pipe  about  30  cents. 

Assuming  the  cost  of  rubble  masonry  to  be  $3.50  per  cubic  yard 
and  of  paving  to  be  $2.00  per  cubic  yard,  the  average  cost  of  the 
niahonry  in  the  trunk  of  the  box  culvert  shown  in  Fig.  95  (page 
404)  is  40  cents  per  lineal  foot  for  each  square  foot  of >water  way  ; 
and  the  corresponding  cost  for  the  culvert  of  Fig.  94  (page  403)  is 
46  cents.  The  end  walls  required  for  these  different  forms  of  cul- 
verts are  essentially  the  same ;  and  hence  the  above  comparison 
sliows  approximately  t.ie  relative  cost  of  the  different  forms  of  cul- 
verts. According  to  this  showing,  cast-iron  pipe  is  the  most  ex- 
pensive ;  but  this  difference  is  partly  neutralized  by  the  greater 
ease  with  which  the  iron  pipe  can  ])e  put  into  place  either  in  new 
work  or  in  replacing  a  Avooden  box-culvert. 

635.  The  following  figures  give  the  cost  of  a  7-foot  cast-iron 
culvert  of  the  form  referred  to  in  §  633,  which  see. 

42  ft.  body       @  $26.55  per  foot  (1.55  ceuts  per  pound) |], 114.83 

8  ft.  specials  @  $29.43    "      "       "        "       "        "      235.32 

Bolts  and  washers 29.91 

Unloading 1 7. 52 

Putting  iu  place 14S.95 

Stone  for  end  walls,  70  cu.  yds.,                @  $1..50 105.00 

Stone  for  riprap  f(niudation,  60  cu.  yds.,  @,  $1.00 60.00 

Removing  temporary  bridge 235.63 

Total $1,947. 15 

Excluding  the  cost  of  removing  the  tem.porary  bridge — which 
Is  not  a  jDart  of  the  culvert  proper, — and  of  the  riprap  foundation — 
which  the  unusual  conditions  required, — the  cost  of  the  culvert  was 

■•$33.0.3  per  foot,  or  83  cents  per  lineal  foot  for  each  square  foot  of 

\water  way. 

636.  Timber  Box  Culverts.  Timber  box  culverts  should  be 
nsed  only  where  more  substantial  material  is  not  attainable  at  a 
reasonable  cost.       Many   culvertc   are  constructed    of  timber  an^ 


ART.  2.]  BOX   AND    PIPE    CULVERTS.  417 

periodically  renewed  with  the  same  material,  and  many  are  con- 
structed of  wood  and  replaced  with  stone,  or  sewer  or  iron  pipe. 

The  latter  is  an  example  of  what  may  be  called  the  standard 
practice  in  American  railroad  building;  /.  e.,  constructing  the  road 
as  quickly  and  cheaply  as  possible,  using  temporary  structures,  and 
completing  with  permanent  ones  later  as  the  finances  of  the  company 
will  allow  and  as  the  requirements  of  the  situation  become  better 
understood.  After  the  line  is  open,  the  permanent  structures  can 
be  built  in  a  more  leisurely  manner,  at  ai^j^i'opriate  seasons,  and 
thus  insufe  the  maximum  durability  at  a  minimum  cost. 

There  is  a  great  variety  of  timber  box  culverts  in  common  use, 
but  probably  there  are  none  more  durable  and  eflBcient  than  those 
used  on  the  Chicago,  Milwaukee  and  St.  Paul  E.  E., — shown  in 
Pig.  103  (page  418).*  On  this  road,  it  is  the  custom  to  rejilace  the 
wooden  boxes  with  iron  pipes  before  the  timber  has  seriously  de- 
cayed. If  experience  has  shown  the  size  of  the  wooden  box  to  be 
about  right,  the  timbers  are  cut  out  a  little  and  an  iron  pipe  is 
placed  inside  of  the  box  without  disturbing  the  earth. 

For  timber  box  culverts  of  sizes  larger  than  can  be  made  of 
plank,  the  Atchison,  Topeka  and  Santa  Fe  E.  E.  employs  bridge- 
tie  box  culverts.  These  are  made  by  laying  Q>  X  8  inch  sawed 
bridge  ties  flatwise,  in  contact,  to  form  a  floor.  These  ties  are 
gained  at  the  ends  so  as  to  leave  a  shoulder  1  inch  deep  against 
which  the  inside  of  the  side  walls  bears.  Upon  this  floor,  vertical 
side  walls  are  constructed  by  laying  ties  flatwise,  one  on  top  of  the 
other  ;  the  lowest  timber  in  each  side  wall  is  fastened  to  each  tie  in 
the  floor  by  a  drift-bolt  12  inches  long,  and  each  timber  in  the  side 
wall  is  fastened  to  the  one  below  it  by  a  12-inch  drift-bolt  every  3 
feet.  The  lengths  of  the  ties  employed  in  the  side  Avails  are  so  ad- 
justed as  to  make  the  exposed  ends  conform  closely  to  the  slope  of 
the  embankment.  The  roof  consists  of  6-  X  8-inch  ties  set  edgewise, 
in  close  contact,  with  a  shoulder  1  inch  deep  on  the  inside,  both 
ends  of  each  piece  being  also  drift-bolted  to  the  side  wall. 

637.  Timber  Baerel  Culverts.  For  a  number  of  years  past 
the  Chicago,  Burlington  and  Quincy  E.  E.  has  found  it  desirable, 
in  view  of  the  absence  or  poor  quality  of  the  stone  along  its  lines,  to 
use  a  timber  '^barrel-culvert"  when  the  opening  is  too  large  for  a 

*  From  Railroad  Gazette. 


418 


CULVERTS. 


[CIIAP.   XVIL 


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ART.  3.] 


ARCH    CULVERTS. 


419 


timber  box-culvert.  The  staves  are  10  or  12  inches  thick,  accord- 
ing to  the  size  of  tlie  culvert,  and  8  inches  wide  on  the  outside, 
dressed  to  form  a  circle  4|^  or  6  feet  in  diameter.  Iron  rings — made 
of  old  rails — spaced  about  10  feet  apart,  are  used  as  a  form  upon 
which  to  construct  the  culvert  and  also  to  give  it  strength.  The 
staves  break  joints  and  are  drift-bolted  (§  381)  together.  As  soon 
as  the  timber  is  thoroughly  seasoned,  the  culverts  are  lined  with  a 
single  ring  of  brick,  and  concrete  or  stone  parapet  walls  are  built. 
If,  at  an}^  time,  the  timber  fails,  it  is  the  intention  to  jnit  iron  pipe 
through  the  present  opening. 

The  timber  costs  about  812  per  thousand  feet,  board  measure, 
at  the  Mississippi  Eiver  ;  and  the  cost  of  dressing  at  the  company's 
shops  is  about  $1.50  per  thousand. 


Art.  3.  Arch  Culverts. 

638.  In  this  article  will  be  discussed  what  may  be  called  the 
theory  of  the  arch  culvert  in  contradistinction  to  the  theory  of  the 
arch.     The  latter  will  be  considered  in  the  next  chapter. 

By  the  theory  of  the  arch  culvert  is  meant  an  exposition  of  the 
method  of  disposing  a  given  quantity  of  masonry  so  as  to  secure  (1) 
maximum  discharging  capacity,  (2)  minimum  liability  of  being 
choked  by  drift,  and  (3)  maximum  strength.  Attention  to  a  few 
points,  which  are  often  neglected  in  the  design  of  culverts,  will  se- 
cure these  ends  without  additional  cost. 

639.  General  Form  of  Culvert.  Splay  of  Wings.  There 
are  three  common  ways  of  disposing  the  Aving  walls  for  finishing 
the  ends  of  arch  culverts.  1.  The  culvert  is  finished  with  a  straight 
wall  at  right  angles  to  the  axis  of  the  culvert  (see  Fig.  103).     2.   The 


L 


Fig.  10.3. 


Fig.  104. 

wings  are  placed  at  an  angle  of  30°  with 
(see  Fig.  104).  3.  The  wing  walls  are 
axis    of    the    culvert,  the  back  of    the  wing  and 


Fig.  105, 

the  axis  of  the  culvert 

built    parallel    to    the 

the    abutment 


being  in  a  straight  line  and  the  only  splay  being  derived  from  thiu- 


420 


CULVEETS. 


[chap.  xvn. 


ning  the  wings  at  their  outer  ends  (see  Fig.  105).  The  first  method 
is  shown  on  a  kirger  scale  in  Plate  II,  the  third  in  Plate  III,  and 
the  second  in  Plate  IV. 

The  quantity  of  masonry  required  for  these  three  forms  of  wings 
does  not  differ  materially,  Fig.  105  requiring  the  least  and  Fig.  103 
the  most.  The  most  economical  angle  for  the  wings  of  Fig,  104  is 
about  30°  with  the  axis. 

The  position  of  the  wings  shown  in  Fig.  104  is  much  the  most 
common  and  is  better  than  either  of  the  otliers.  Fig.  103  is  ob- 
jectionable for  hydraulic  considerations  which  will  be  considered 
in  the  next  section,  and  also  because  it  is  more  liable  to  become 
choked  than  either  of  the  others.  Fig.  105  does  not  have  splay 
enough  to  admit  the  natural  width  of  tlie  stream  at  high  water, 
and  does  not  give  sufficient  protection  to  tlietoe  of  the  embankment. 
640.   Junction   of  Wings    and    Body.       With  a   culvert   of  the 

general  form  outlined  in  Fig.  104, 
there  are  two  methods  of  joining 
the  wings  to  the  body  of  the  cul- 
vert. The  more  common  method 
is  shown  in  Figs.  lOG  and  108;  and 
the  better,  but  less  common,  one  is 
shown  in  Figs.  107  and  109. 

The  form  shown  in  Figs.  106 
aiid  108  is  very  objectionable  because  (1)  the  corners  reduce  the 
c^'pacity  of  the  culvert,  and  (2)  add  to  its  cost.. 


Fig,  lOG. 


Fig.  lor. 


Fig.  :08. 


1.  The  sharp  angles  of  Fig.  106  materially  decrease  the  amount 
of  <yater  which  can  enter  under  a  given  head  and  also  the  amount 


ART.  3.]  ARCH    CULVERTS.  421 

Avhicli  can  be  discliarged.  It  is  a  well-established  fact  in  hydraulics 
that  the  discharging  capacity  of  a  pipe  can  be  increased  200,  or 
even  300,  per  cent,  simply  by  giving  the  inlet  and  outlet  forms  some- 
what similar  to  Fig.  107.  Although  nothing  h.<e  this  increase  can 
be  obtained  with  a  culvert,  one  finished  at  both  the  upper  and  the 
lower  end  like  Fig.  107  will  discharge  considerably  more  water  than 
one  like  Fig.  106.  The  capacity  of  Fig.  107  decreases  as  the  angle 
between  the  wing  and  the  axis  increases  ;  hence,  the  less  splay  the 
better,  provided  the  outer  ends  of  the  wings  are  far  enough  apart 
to  accommodate  the  natural  width  of  the  stream  at  high  water. 
Also  the  less  the  splay,  the  less  the  probability  of  the  culvert's  being 
choked  with  drift.  Fig.  106  is  very  bad  for  both  the  admission  and 
the  discharge  of  water,  and  also  on  account  of  the  great  liability 
that  drift  and  rolling  stones  will  catch  in  the  angles  between  the 
wings  and  the  end  walls.  In  this  latter  respect  Fig.  108  is  slightly 
better  than  Fig.  106. 

2.  Every  angle  adds  materially  to  the  cost  of  the  masonry.  In 
a  culvert  like  Fig.  106,  there  are  four  unnecessary  corners.  This 
form  probably  owes  its  prevalence  to  the  desire  to  have  a  uniform 
batter  on  the  face  of  the  wing,  and  to  have  the  face  of  the  wing 
wall  intersect  the  end  wall  back  of  the  arch  stones.  Satisfying  both 
of  these  conditions  gives  a  culvert  in  ground  plan  like  Fig.  106; 
and  satisfying  the  second  one  only,  gives  Fig.  108.  Practically 
there  is  but  little  difference  between  these  two  forms — both  are 
objectionable,  as  already  explained.  If  the  wing  of  Fig.  108  is 
moved  inward,  and  the  corner  of  the  wing,  which  would  other- 
wise project  into  the  water  way,  is  rounded  off  to  a  gentle  curve. 
Fig,  109  is  obtained.  This  form  is  simple,  efficient,  and.  on  the 
whole,  the  best. 

Plate  III  shows  another  method  of  joining  the  wing  to  the 
end  wall  without  having  an  unnecessary  angle.  In  this  case,  the 
face  of  the  wing  up  to  the  springing  line  of  the  arch  is  a  warped 
surface,  which  is  in  some  respects  undesirable,  although  it  saves 
a  little  masonry.  However,  the  face  of  the  wing  wall  could  be 
built  vertical  up  to  the  springing  line  and  then  battered;  or 
the  wing  could  be  moved  forward  and  the  corner  be  rounded  off 
as  in  Fig.  109. 

641.  Semi-circular  t'.y.  Segmental  Arches.  There  are  two 
classes  of  arches  employed  for  culverts,  viz.,  the  semi-circular  and 


422  C  UL VEETS.  [chap.  XVII. 

the  segmental.     The  first  is  by  far  the  more  common;  but  neverthe- 
less the  latter  is,  on  the  whole,  much  the  better. 

1.  For  the  same  span,  the  segmental  arch  requires  a  shorter  in- 
trados  (the  inside  curve  of  a  section  of  the  arch  perpendicular  to 
its  axis).  For  example,  the  culverts  shown  in  Plates  IV  and  V 
have  the  same  span,  but  the  intrados  of  the  semi-circular  arch  is 
15.71  ft.,  Avhile  that  of  the  segmental  arch  is  10.72  ft.;  that  is,  the 
intrados  of  the  segmental  is  only  68  per  cent,  of  the  intrados  of  the 
semi-circular  arch.  This  difference  depends  upon  the  degree  of 
flatness  of  the  segmental  arch.  The  above  example  is  an  extreme 
case,  since  the  segmental  arch  is  unusually  flat,  the  central  angle 
being  only  73°  44'.  (The  rise  is  one  sixth  of  the  span.)  With  a 
central  angle  of  120°,  the  intrados  of  the  segment  is  77  per  cent,  of 
the  semi-circle. 

Or,  to  state  the  above  comparison  in  another  and  better  form,  for 
the  same  length  of  intrados  the  segmental  arch  gives  the  greater 
span.  For  example,  a  segmental  arch  on  the  same  general  plan  as 
that  of  Plate  V,  but  having  an  intrados  equal  to  that  of  Plate  IV, 
would  have  a  span  of  14.64  ft.,  which  is  46  per  cent,  greater  than 
the  span  of  the  semi-circular  arch  shown  in  Plate  IV.  A  segmental 
arch  with  a  central  angle  of  120°  has  a  span  33  per  cent,  greater 
than  a  semi-circular  arch  having  the  same  length  of  intrados.  This 
difference  constitutes  an  important  advantage  in  favor  of  the  seg- 
mental arch  culvert,  since  the  wider  the  span  the  less  the  danger  of 
the  culvert's  being  choked  by  obstructions,  and  because  it  will  pass 
considerably  more  water  for  the  same  depth. 

2.  For  the  same  length  of  intrados,  the  segmental  arch  gives  the 
greater  water  way.  The  water  way  of  the  culvert  shown  in  Plate 
IV  is  87.6  square  feet ;  but  the  same  length  of  intrados  in  a  seg- 
mental arch  culvert  having  73°  44'  central  angle  (the  same  as  Plate 
V)  would  have  a  water  way  of  98.3  square  feet;  and  with  a  central 
Angle  of  120°  would  have  a  water  way  of  99.5  square  feet.  In  both 
examples  the  increase  is  one  eighth. 

3.  On  the  other  hand,  the  segmental  culvert  will  require  a 
thicker  arch.  It  will  be  shown  in  the  next  chapter  that  arches 
can  not  be  proportioned  strictly  in  accordance  with  mathematical 
formulas  ;  and  hence  the  exact  difference  in  thickness  of  arch  which 
should  exist  between  a  semi-circular  and  a  segmental  arch  can  not 
be  computed.     According  to  established  rules   of  practice,  small 


ART.  3.]  ARCH    CULVERTS.  423 

segmental  arches  are  from  10  to  25  per  cent,  thicker  than  semi- 
circular ones.  This  difference  is  not  very  great,  and  its  effect  upon 
the  cosu  of  the  culvert  is,  proportionally,  still  less,  since  the  cost 
per  yard  of  arch  masonry  is  less  for  the  thicker  arches.  Then,  we 
may  conclude  that,  since  for  the  same  span  the  intrados  of  seg- 
mental arches  is  from  20  to  40  per  cent,  shorter  than  the  semi- 
circle, the  segmental  arch  requires  a  less  volume  of  arch  masonry 
than  the  semi-circular,  and  also  costs  less  per  cubic  yard.  The  arch 
masonry  per  foot  of  length  of  the  segmental  arch  culvert  shown  in 
Plate  V  is  only  71  per  cent,  of  that  in  the  semi-circular  one  shown 
in  Plate  IV.  The  dimensions  and  contents  of  arch  culverts  of  the 
general  forms  shown  in  Plates  IV  and  V  are  given  in  Tables  51 
and  52  (pp.  430  and  431  respectively),  from  which  it  appears  that 
the  segmental  arch  contains  only  from  60  to  76  per  cent,  as  much 
masonry  as  the  semi-circular,  the  average  for  the  six  spans  being 
almost  exactly  70  per  cent.  The  cost  of  these  two  classes  is  shown 
in  Tables  56  and  57  (pages  437  and  438),  from  which  it  ajjpears  that 
the  average  cost  of  segmental  culverts  20  feet  long  and  of  different 
spans  is  only  59  per  cent,  of  the  cost  of  semi-circular  ones  of  the 
same  length  and  span;  and  the  average  cost  of  an  additional  foot 
in  length  for  the  segmental  is  only  86  per  cent,  of  that  for  a  circular 
one.  The  water  ways  of  the  semi-circular  culverts  are  a  little  the 
greater,  and  hence  the  difference  in  cost  per  square  foot  of  water 
way  is  not  as  great  as  above;  but,  on  the  other  hand,  the  form  of 
water  way  of  the  segmental  culvert  is  the  more  eflficient,  and  hence 
the  above  comparison  is  about  correct. 

4.  Will  the  segmental,  /.  e.,  the  flatter,  arch  require  heavier  abut- 
ments (side  walls)?  Unquestionably  the  flatter  the  arch  the 
greater  the  thrust, upon  the  abutment;  but  the  abutment  not  only 
resists  the  thrust  of  the  arch  which  tends  to  turn  it  over  outwards, 
but  also  the  thrust  of  the  embankment,  which  tends  to  push  it  in- 
wards. It  is  impossible  to  compute,  with  any  degree  of  accuracy, 
either  the  thrust  of  the  arch  or  of  the  embankment;  and  hence  it  is 
impossible  to  determine  either  the  relative  value  of  these  forces  or  the 
thickness  which  the  two  abutments  should  have.  Experience  seems 
to  indicate  that  the  thrust  of  the  earth  is  greater  than  that  of  the 
arch,  as  is  shown  by  the  fact  that  nearly  all  semi-circular  culverts 
have  abutments  of  much  greater  thickness  than  are  required  to  re- 
sist the  thrust  of  the  arch;  and  hence  we  may  conclude  that  expe- 


424  CULVERTS.  [chap.  XVII. 

rience  has  shown  that  the  thrust  of  the  earth  necessitates  a  heavier 
HDutment  than  does  the  thrust  of  the  arch.  If  this  be  true,  then 
the  abutment  for  segmental  arches  may  be  thinner  than  those  for 
semi-circular  ones;  for,  since  the  thrust  of  the  former  is  greater 
than  the  latter,  it  exerts  a  greater  force  outward,  which  counter- 
balances a  larger  part  of  the  inward  thrust  of  the  embankment,  and 
thus  leaves  a  less  proportion  of  the  latter  to  be  resisted  by  the  mass 
of  the  abutment.  Segmental  arch  culverts  are  not  often  built;  and 
designers  appear  to  have  overlooked  the  thrust  of  the  earth,  since 
the  side  walls  of  segmental  arches  are  generally  thicker  than  for 
semi-circular  ones  (compare  Plates  IV  and  V). 

The  conclusions  may,  therefore,  be  drawn  that  segmental  arch 
culverts  are  both  cheaper  and  more  eflficient  than  semi-circular  ones. 

642.  As  built,  many  semi-circular  arches  are  23i'actically  seg- 
mental; that  is,  the  side  walls  are  built  so  high,  or  the  backing  is 
made  so  heavy,  that  practically  the  abutments  are  less  than  120° 
apart,  and  hence  the  two  lower  ends  of  the  arch  are  really  only  a 
part  of  the  side  wall,  and  should  be  built  square. 

Further,  it  is  shown  in  §§  681-82  that  a  true  avch  of  more  than 
about  90  to  120  degrees  is  impossible. 

643.  Examples.  Under  this  head  will  be  given  a  brief  descrip- 
tion of  four  series  of  arch  culverts  which  are  believed  to  be  repre- 
sentative of  the  best  practice. 

644.  Illinois  Central  Arch  Culverts.  Plate  II  shows  the  gen- 
eral plan  of  the  standard  arch  culvert  employed  in  the  construction 
(185"2-53)  of  the  Chicago  branch  of  the  Illinois  Central  Railroad.* 
While  the  timber  in  the  foundation  is  apparently  still  in  good  con- 
dition, the  use  of  timber  for  such  shallow  foundations  can  not  be 
considered  as  the  best  construction.  However,  many  of  the  con- 
ditions, particularly  drainage,  have  greatly  changed  since  this  road 
was  built,  and  it  is  by  no  means  certain  that  this  use  of  timber 
was  not  good  practice  at  that  time  (see  §  636). 

Table  49  (page  425)  gives  the  dimensions  and  contents  for  the 
several  spans  of  this  form  of  culvert.  The  contents  of  the  end 
walls  were  computed  on  the  assumption  that  the  off-set  at  tha  back 
was  6  inches  for  each  foot,  counting  from  the  top,  until  the  full 
thickness  at  the  bottom  was  obtained  (see  Section  E-F,  Plate  II). 

*  Published  by  permission  of  J.  M.  Healey,  Division  Engineer. 


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426  CULVERTS.  [chap.  XVII. 

645.  Example  of  the  Use  of  Table  49.  To  illustrate  the  method 
of  using  the  above  table,  assume  that  an  estimate  of  the  amount  of 
material  in  an  8-ft.  arch  culvert  of  the  preceding  form  is  required. 
Assume  that  the  top  of  the  coj^ing  is  3  feet  below  sub-grade,  i.  e., 
that  there  is  4.25  feet  of  earth  above  the  crown  of  the  arch.  Assume 
also  that  the  road  bed  is  IG  feet,  and  that  the  slojie  of  the  embank- 
ment is  li  to  1.  Then  the  length  of  the  culvert  from  inside  to  in- 
side of  the  end  walls  will  be  16  -f  2  (|  X  3)  =  10  +  9  =  25  feet;  and 
from  out  to  out  of  end  walls,  the  length  will  be  25  +  2  X  2.5  =  30 
feet. 

Assuming  that  the  timbers  under  the  planking  are  8  X  10  inches, 
the  205  sq.  ft.,  as  per  the  table,  will  require  1,422  ft.  B.  M.  of  tim- 
ber, or  9  pieces  24  ft.  long.  Notice,  however,  that  in  practice  10 
pieces  would  be  used — 5  at  each  end  of  the  culvert.  The  length  of 
the  trunk  of  the  foundation  is  30  -  2  (4+ 1  +  1)  =  19  ft.  Hence 
the  area  under  the  trunk  of  the  foundation  to  be  covered  with  tim- 
ber is  19  X  8  (see  table)  =  152  sq.  ft. ;  and  if  8  X  10-inch  timbers 
are  used,  this  will  require  1,216  ft.  B.  M.,  or  12  pieces  14  feet  long. 
The  plank  under  the  wings  and  in  the  sheet  piling  is  1,493  feet  (see 
table),  and  that  in  the  trunk  is  32  (see  table)  X 19  =  608  ft.  B.  M. ; 
hence  the  total  plank  is  1,493  +  008  =  2,101  ft.  B.  M. 

The  masonry  in  the  end  wall  is  32.97  cu.  yds.,  as  in  table.  The 
masonry  in  1  foot  of  arch  is  (see  table)  0.6T3  +  0.284  =  0.957;  and 
in  30  ft.  it  is  0.957x30  =  28.71  cu.  yds.  The  masonry  in  the  side 
walls  (abutments  of  the  arch)  is  0.444  (see  table)  X30  =  13.32  cu. 
yds.     The  coping  is  117.0  cu   ft.  (see  table)  =  4.33  cu.  yds. 

Collecting  and  tabulating  the  preceding  results,  we  have  the 
following : 

Timber:— 10  pieces,  8  X  10  inches,  24  ft.  lone 1,600  ft.  B.  M. 

13      '•  "  '•        14  '•      "^ I.ISO  "      •• 

2-inchplank 2,101"      " 

Total  timber  in  culvert  25  ft.  long... .     4,821  "      " 

Masonry: — 2  end  walls 33 . 0  cu.  yds. 

coping 4.3  "       " 

side  walls  (abutments) 13.3  "       " 

arch  masonry 28 . 7  "       " 

Total  masonryin  culvert  2-5  ft.  long..     79.3  "       *' 


AKT,  3. J  ARCH    CULVERTS.  4:27 

646.  Chicago,  Kansas  and  Nebraska  Arch  Culverts. — The  culvert 
shown  in  Plate  III  is  the  standard  form  employed  on  the  Chicago, 
Kansas  and  Xebraska  Railroad.*  Xotice  that  the  slope  line  inter- 
sects the  inside  face  of  the  end  wall  at  a  considerable  distance  above 
the  back  of  the  crown  of  the  arch  (see  Side  View,  Plate  III).  This 
is  sometimes  urged  as  an  objection  to  this  form  of  construction,  on 
account  of  the  stipposed  liability  of  the. top  of  the  end  wall  being 
pushed  outward;  but  there  is  no  danger  of  this  method  of  failure, 
since  the  height  of  the  end  Avall  above  the  crown  of  the  arch  is,  ex- 
clusive of  the  coping,  only  equal  to  its  thickness,  and  in  iulu>:on  it 
is  buttressed  on  the  outside  by  the  wings.  The  advantage  of  this 
construction  is  that  it  requires  less  masonr}'  and  also  less  labor. 

Concerning  the  manner  of  joining  the  wings  to  the  body,  see  the 
last  paragraph  of  §  640  (page  431). 

Table  50  (page  428)  gives  the  dimensions  and  contents  for 
various  spans.  The  contents  of  the  wings  above  the  springing  line 
of  the  arch  were  computed  for  cotirses  1  foot  tlnck  and  for  an  earth 
slope  of  1^  to  1  (see  §557). 

647.  Example  of  the  Use  of  Table  50.  Assume  the  same  depth 
of  earth  over  the  crown  of  the  arch  as  in  the  example  in  §  G45, 
X.  e.,  4.25  ft.;  and  assume  also  that  the  slope  line  strikes  the  upper 
corner  of  the  coping  instead  of  the  lower  as  shown  in  Plate  III. 
The  to^o  of  the  coping  will  be  0.75  ft.  below  sub-grade;  and,  for  a 
16-ft.  road-bed,  the  length  of  the  arch — inside  to  inside  of  end 
walls— is  16  +  2(1  X  0.75)  =  18.25  ft.  With  the  above  data  and 
Table  50,  we  have  the  following  for  an  8  foot  culvert : 

Four  wiug  walls,  including  one  footing  course,      .     .  40.5  cu,  yds. 

Two  head      "  "  "        "  "  .     .  36.8  "      " 

Coping, 3.7  "      " 

Two  side  walls,  18i  ft.  @  1.382  cu.  yds.  per  foot,  .     .  25.2  "      " 

Arch  masonry,    "     "    "  1.184  "      "       "      "     .     ,  21.6  "      " 
Paving,  23.58  ft.  @  0.272  cu.  yd.  per  ft.,      ....        6.4  "       ' 

Total  masonry  in  culvert  l&J  ft.  long,    .     .    134.1   "      " 

In  attempting  to  make  comparisons  between  the  above  total  and 
that  of  §  645,  notice  that  the  culverts  are  of  very  difterent  style  (see 
§§  638  and  639)  and  that  the  water  ways  are  of  different  areas. 

*  Published  by  permission  of  H.  A.  Parker,  Chief  Engineer. 


428 


CULVERTS. 


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ART.  3.]  ARCH    CULVERTS.  429 

648.  Atchison,  Topeka  and  Santa  F6  Arch  Culverts.  Plates 
IV  iiiid  V  show  the  standard  semi-cireular  aud  segmental  arch  cul- 
verts used  by  the  Atchison,  Topeka  and  Santa  Fe  Eailroad.* 

Tables  51  and  52  give  the  dimensions  and  contents  for  the 
several  spans.  Notice  that  the  heights  of  the  end  walls  do  not  vary 
uniformly,  that  for  the  12-foot  span  being  proportionately  too  great; 
and  consequently  the  contents  of  the  end  walls  and  of  the  wings  do 
not  vary  uniformly.  The  contents  of  the  facing  of  the  wings  were 
computed  for  courses  18  inches  thick  (see  §  557),  and  the  backing 
was  computed  on  the  assumption  that  the  back  surface  was  a  plane 
such  that  the  dimension  at  the  outer  end  'and  also  where  a  plane 
parallel  to  the  section  E-F  passes  through  the  corner  of  the  end 
wall  is  as  in  the  diagram. 

In  computing  the  masonry  in  a  given  culvert,  these  tables  are  to 
be  employed  as  already  explained  for  Tables  49  and  50— see  §§  645 
and  G47. 

649.  Standard  Arch  Culvert.  The  culvert  shown  in  Plate  VI 
has  been  designed  iu  accordance  with  the  principles  laid  down 
in  the  preceding  discussion  (§§  638-41).  The  wings  are  joined  to 
the  body  in  such  a  manner  as  to  offer  the  least  possible  resistance 
to  the  passage  of  water  and  drift.  If  the  current  is  slow  and  not 
liable  to  scovir,  the  paving  may  be  omitted,  since  the  end  walls,  being 
continuous  under  the  ends  of  the  water  way,  will  prevent  under- 
mining of  the  side  walls;  or,  iu  long  culverts,  one  or  more  inter- 
mediate cross  walls  may  be  constructed.  But  ordinarily  the  money 
paid  for  paving  is  a  good  investment.  If  the  current  is  very  rapid, 
it  is  wise  to  grout  the  paving, — and  also  to  inspect  the  structure 
frequently. 

The  arch  ring  is  amply  strong  to  support  any  bank  of  earth  (see 
Table  63,  page  502,  particularly  Nos.  9,  12,  18,  53,  54,  and  61). 
The  strains  in  a  masonry  arch  can  not  be  computed  exactly;  but  the 
best  method  of  analysis  (§  688)  shows  that  if  the  earth  is  10  feet 
thick  over  the  crown,  the  maximum  pressure  is  not  more  than  55 
pounds  per  square  inch  (comimre  with  §222  and  also  ,§§  246-48). 
A  greater  thickness  of  earth  at  the  crown  would  doubtless  increase 
the  maximum  pressure  in  the  arch;  but  proportionally  the  pressure 
would  increase  much  less  rapidly  than  the  height  of  the  bank  (see 

*  Published  by  permission  of  A.  A.  Robinson,  Chief  Engineer. 


430 


CULVERTS. 


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432  CULVERTS.  [chap.  XVII. 

§  619).  The  arch  is  also  stable  uitder  any  position  of  the  moving 
load,  with  either  a  heavy  or  a  light  embankment.  The  joints  of 
the  abutment  are  radial,  to  prevent  any  possibility  of  failure  by 
tlie  sliding  of  one  course  on  another  (see  §  674). 

Table  53  (page  433)  gives  the  dimensions  and  contents  of  various 
•sizes.  In  each  case  the  rise  is  one  fifth  of  the  span,  the  central 
angle  is  87°  12\  and  the  height  of  the  opening  is  equal  to  half  the 
span.  The  paving  and  coping  were  each  assumed  to  be  1  foot  thick; 
but  for  any  other  thickness  it  is  only  necessary  to  increase  or  de- 
crease the  tabular  numbers  proportionally.  The  contents  of  the 
wings  were  computed  on  the  assumption  that  all  the  courses  were  1 
foot  thick  (see  §  557). 

650.  Quality  of  Masonry.  The  masonry  of  arch  culverts  is 
usually  divided  into  two  classes;  the  first  consists  of  the  masonry  in 
the  wings  and  end  walls  (parapet),  and  the  second  of  the  arch 
stones.  The  former  is  classified  as  first-class  or  second-class  ma- 
sonry (see  §  225).  Only  the  masonry  in  the  arch  stones  is  called 
arch  masonry.  The  arch  stones  which  show  at  the  end  of  the  arch 
are  called  rmg  stones,  and  the  remainder  of  the  arch  stones  the 
arch  sheeting.  The  arch  masonry  proper  is  usually  classified  as 
first-class  or  second-class  arch  masonry.  The  distinction  between 
these  two  classes  is  usually  about  as  in  the  specifications  below. 

651.  Specifications.*  Foundations.  "When  the  bottom  of  the  pit  is 
-common  earth,  gravel,  etc.,  the  foundations  of  arch  culverts  will  generally 
<!onsist  of  a  pavement  formed  of  stone,  not  less  than  twelve  inches  (13")  in 
.depth,  set  edgewise,  and  secured  at  the  ends  by  deep  curbstones  which  must 
be  protected  from  undermining  by  broken  stone  placed  in  such  quantity  and 
position  as  the  engineer  may  direct.  When  the  bottom  upon  which  a  culvert 
is  to  be  built  is  soft  and  compressible,  and  where  it  will  at  all  times  be 
covered  with  water,  timber  well  hewn,  and  from  eight  (8)  to  twelve  inches 
<12")  in  thickness,  according  to  the  span  of  the  culvert,  shall  be  laid  side  by 
side  crosswise  upon  longitudinal  sills;  and  when  the  position  of  the  culvert  is 
such  that  a  strong  ciu-rent  will  be  forced  through  during  floods,  three  courses 
of  sheet  piling  shall  be  placed  across  the  foundation — one  course  at  each  end, 
and  one  in  the  middle, — which  shall  be  sunk  from  three  (3)  to  six  feet  (6') 
l)elow  the  top  of  the  timber,  according  as  the  earth  is  more  or  less  compact,  "f 

652.  First- Class  Arcli  Masonry.  "  First-class  arch  masonry  shall  be  built 
in  accordance  with  the  speciticatioiis  for  first-class  masonry  [§  225],  with  the 
exception  of  the  arch  sheeting  and  the  ring  stones.     The  ring  stones  shall  be 

*  See  also  Specifications  for  Railroad  Masonry,  Appendix  1. 
■f-  Pennsylvania  Railroad. 


AKT.  3.] 


AKCH    CULVERTS. 


433 


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434  CULVERTS.  [chap,  xvil 

dressed  to  such  shape  as  the  engineer  shall  direct.  The  ring  stones  and  the 
arch  sheeting  shall  be  of  stone  not  less  than  ten  inches  (10")  thick  on  the 
inirados,  shall  be  dressed  with  three  eighths  of  an  inch  (f")  joints,  and  shall 
be  of  the  full  depth  specified  for  the  thickness  of  the  arch;  and  the  jointa 
shall  be  at  right  angles  to  the  surface  of  the  iutrados.  The  face  of  sheeting 
stones  shall  be  dressed  to  make  a  close  centering  joint.  The  ring  stones  and 
the  sheeting  shall  break  joints  not  less  than  one  foot  (1). 

"  The  wings  shall  be  neatly  stepped  with  selected  stones  of  the  full  width 
of  the  wing  and  of  not  less  than  ten  inches  (10")  in  thickness,  which  shall 
overlap  by  not  less  than  eighteen  inches  (18");  or  shall  be  finished  with  a 
neatly-capped  newel  at  the  free  end,  and  a  coping  course  on  the  wing.  The 
parapets  shall  be  finished  with  a  coping  course  not  less  than  ten  inches  (10") 
thick  and  of  the  full  width  of  the  parapet,  which  shall  project  six  inches  (6"). 
653.  Second-Class  Arch  Masonry.  "  Second-class  arch  masonry  is  the 
same  as  second-class  masonry  [§  225],  with  the  exception  of  the  arch  sheeting. 
The  stones  of  the  arch  sheeting  shall  have  a  good  bearing  throughout,  and 
shall  be  well  bonded  and  of  the  full  depth  of  the  thickness  of  the  arch.  No 
stone -shall  be  less  than  four  inches  (4")  in  thickness  on  the  intrados.  Ring^ 
stones  of  all  arches  over  eight  feet  (8)  span  shall  be  dressed  according  to 
specifications  for  first-class  arch  masonry  [§  651]."  * 

654.  Paving.  For  specifications  for  Paving,  see  §  219  (page  148),  and 
also  Specifications  for  Railroad  Masonr3^  Appendix  I. 

655.  Cost.  §§  326-38  contain  data  on  the  cost  of  masonry,  of 
which  the  last  is  a  snmmary.  Table  17  (page  159)  contains  a  de- 
tailed statement  of  the  actual  cost  of  the  masonry  in  an  arch 
culvert;  and  below  are  the  items  of  the  total  cost  of  that  culvert. 

613  cu.  yds.  of  masonry  @  $6.59,       $4,036.85 

Excavations — foundations  and  drainage,     ......  263.36 

Sheet  piling 19.69 

Concrete, 43.75 

Extra  allowance  on  sheeting  stones, 20.00 

Total  cost  of  culvert, $4,383.65 

The  total  cost  of  the  culvert  per  yard  of  masonry  is  $7.16, — which 
is  unusually  low. 

Below  is  the  total  actual  cost  of  the  8-ft.  culvert  (length  out  to 
out  of  end  walls  =  30  ft.)  for  which  the  quantities  were  estimated 
in  §  645  (page  426). 

*  Atchison,  Topeka  and  Santa  F6  R.  R. 


ART.  3.] 


ARCH   CULVERTS. 


435 


Wall  masonry— 48.7  cu.  yds.  @  $7.00, $340.90 

Arch  masonry— 28.7  "      "      "     8.50, 243.95 

Timber— 5,247  ft.,  B  M.,  @  $40.00, 209.88 

Excavating  foundations  and  straightening  stream  158  cu. 

yds.  @  50c., 79.00 

Total  cost  of  culvert $873.73 

The  total  cost  of  this  culvert  per  cubic  yard  of  masonry  is  $11.29. 
The  average  total  cost  of  a  number  of  representative  culverts  of 
this  style  was  111.46  per  cubic  yard  of  masonry,  being  practically 
constant  for  all  spans. 

656.  Illinois  Central  Culverts.  Table  54  gives  the  cost  of  cul- 
verts 25  feet  long — out  to  out  of  end  walls — of  various  spans  of  the 
general  plan  shown  in  Plate  II,  and  will  be  very  useful  in  estimat- 
ing the  cost  of  such  culverts.  The  quantities  of  masonry  necessary 
to  compute  Table  54  were  taken  from  Table  49  (page  425).  The 
prices  are  believed  to  be  fair  averages  (see  page  160)  for  the  first- 
class  masonry  described  in  §  651.  The  prices  are  the  same  as 
actually  paid  by  the  Illinois  Central  Railroad,  except  for  arch 
masonry  and  excavation,  for  which  68.50  and  50c.  respectively  were 
paid.  The  prices  used  in  deducing  the  table  are  given  therein,  and 
hence  the  results  can  be  modified  for  prices  differing  from  those 
there  employed  by  simply  taking  proportional  parts  of  the  tabulated 

TABLE  54. 

Cost  of  Illinois  Central  Arch  Culverts  25  Ft.  Long  from  Out  to 
Out  op  End  Walls,  and  also  op  each  Additional  Foot. 

FOR   description   SEE  PAGE  424. 


Span. 

6  ft. 

8  ft. 

10  ft. 

12  ft. 

Plain  masonry  @  SI'.OO  per  cu.  yd 

Arch  mason rv  @    8.IK1    •■     '•     "   

$237.35 

151.29 

150.88 

12.48 

8325.29 
191.29 

188.12 
15.85 

8424.97 

255.29 

232.84 

19.22 

8455.98 
362  82 

Timber  and  plank  at  $40  per  M.  ft,,  B.  M ... 

Excavating  foundation  @  25c.  per  cu.  yd 

268.92 
21.38 

Total  cost  of  culvert  25  ft.  long 

Cost  of  an  Additional  Foot  of  Length: 

Plain  masonry  @  $7.00  per  cu.  yd 

Arch  masonry  @    8.00    "      "     " 

Timber  and  plank  @  840  per  M  ft.,  B.  M 

Excavation  @  25c.  per  cu.  yd 

»552.00 

S;mi 

6.05 

3.36 

.32 

8720.55 

83.11 

7.65 

3.84 

.40 

8932.32 

83.11 

10.21 

5.04 

.48 

$1,109.10 

83.63 

14.51 

5.88 

.56 

Total  cost  of  1  additional  foot 

812.84 

815.00 

$18.84 

$24.48 

436 


CULVERTS. 


[chap.  XVII. 


quantities.  The  amount  of  excavation  used  in  computing  the  table 
is  the  mean  of  the  actual  quantities  for  a  number  of  representative 
culverts  as  constructed  on  the  above  road. 

657.  Chicago,  Kansas  and  Nebraska  Culverts.  Table  55  is  given 
to  facilitate  estimating  the  cost  of  culverts  of  the  general  form 
shown  in  Plate  III.  The  prices  are  about  the  average  for  the 
respective  kinds  of  work;  but  in  case  it  is  desired  to  determine  the 
cost  for  other  prices,  it  is  only  necessary  to  increase  or  decrease  the 
tabular  numbers  proportionally.  The  quantities  of  excavation  are, 
approximately,  averages  of  the  actual  amounts  for  a  number  of 
similar  culverts,  and  are  equivalent  to  a  pit  2  feet  2  inches  deep  and 
of  an  area  equal  to  the  area  of  the  foundation.  The  table  includes 
only  one  footing  course,  but  in  so  doing  it  is  not  intended  to  imply 
that  one  is  always,  or  even  generally,  enough.  Notice  that  the  cul- 
vert in  Table  55  is  25|  feet  long  from  outside  to  outside  of  end 
walls,  and  hence  is  one  third  of  a  foot  longer  than  that  presented  in 
Table  54. 

658.  A.,  T.  and  S.  F.  Semi-circular  Culverts.  Table  56  is  similar 
to  the  two  preceding  ones,  and  shows  the  cost  of  the  Atchison, 

TABLE  55. 

Cost  op  C    K.  and  N.  Arch  Culverts  20  Ft.  Long  from  Inside  to 

Inside  of  Coping,  and  also  of  each  Additional  Foot  of  Length. 

for  description  see  page  437. 

This  table  includes  one  footing  course. 


Span. 

3  ft. 

4  ft. 

6  ft. 

8  ft. 

10  ft. 

Plain  masonry  @  $7.00  per  cu.  yd 

Arch  masonry  @    8.00    "      "     " 

Paving               @    2.00    "      "     " 

Excavation       @      .25    "      "     " 

$217.98 

44.64 

4.42 

7.44 

$397.04 
79.03 
6.32 
9.33 

$657.23 

130.78 

10.16 

12.42 

$703.43 

2.39.92 

13.96 

13.95 

$853.16 

346.96 

17.76 

14.99 

Total  cost  of  culvert  20  ft.  long  . . . 

Cost  of  an  Additional,  Foot  of 
Length: 

Plain  masonry  @,  $7.00  per  cu.  yd 

Arch  masonry  ©    8.00    "      "     " 

Paving               @    2.00    "      "     " 

Excavation       @      .25    "     "     ". 

$274.48 

$4.14 

2.30 

.17 

.23 

$6.84 

$491.72 

$6.17 

4.06 

.25 

.27 

$810.59 

$9.33 

6.72 

.40 

.35 

$971.26 

$9.67 

10.06 

.54 

.39 

$1,232.87 

$10.9'> 

14.55 

.69 

.46 

Total  cost  of  1  additional  foot 

$10.75 

$16.80 

$20.66 

$26.66 

ART.  3.] 


AKCH    CULVERTS. 


437 


Topeka  and  Santa  Fe's  standard  semi-circular  arch  culvert  as  given 
in  Plate  IV  and  Table  51  (page  430).  The  excavation  is  only  ap- 
proximate, and  is  computed  on  the  assumption  of  a  pit  2  feet  2 
inches  deep  for  the  entire  foundation  including  the  paved  area;  /.  e., 
the  excavation  is  computed  on  the  same  basis  as  the  two  preceding. 
Notice  that  this  culvert  is  23  feet  between  the  outer  faces  of  the 
end  walls,  and  hence  is  1  foot  shorter  than  that  of  Table  54  and  2^ 
feet  shorter  than  that  of  Table  55. 

TABLE   56. 

Cost  of  A.  T.  and  S.  F.  Semi-circulak  Arch  Culverts  20  Ft.  Long 
FROM  Inside  to  Inside  of  the  Coping,  and  also  of  each  Addi- 
tional Foot  of  Length. 

FOR  description   SEE   PAGE   429. 

Thig  table  does  not  include  the  masonry  in  the  footings. 


Items. 

Span. 

6  ft. 

8  ft. 

10  ft. 

12  ft. 

14  ft. 

16  ft. 

Plain  masonry  @  $7.00  per  cu.  yd. 
Arch  masonry  "     8.00    "     "     " 
Paving                 "     2.00    "     "     " 
Excavation        "       .25    "     "     " 

$325.15 

140.72 

9.88 

6.93 

$766.42 

197.28 

13.15 

13.51 

$997.14 

270.32 

16.42 

16.01 

$1,071.42 

356.04 

19. 6S 

18.21 

$1,328.12 

418.40 

22.99 

21.10 

$1,785.91 

516. 4& 

26.26 

24  44 

Total  cost  of  culvert  20  ft.  long. 

Cost  of  an  Additional  Foot 
OF  Length: 
Plain  masonr\'  @  $7.00  per  cu.  yd. 
Arch  masonry  "     8.00    "     "     •• 
Paving                 "     200    "     "     " 
Excavation        "       .25    "'     "     " 

$482.68 

$5.44 

6.12 

.52 

.28 

$990.36 

$8.49 

8.58 

.69 

.32 

$1,299.89 

$11.66 

11.75 

.86 

.40 

$1,465.35 

$12.44 

15.48 

1.04 

.44 

$1,790.61 

$14.98 

18.19 

1.21 

.48 

$2,353.09 

$19.48 

22.46 

1..38 

.54 

Total  cost  of  1  additional  foot 

$12.36 

$18.08 

$24.68 

$29.. 50 

$34.86 

$43.86: 

659.  A.,  T.  and  S.  F.  Segmental  Culverts.  Table  57  is  similar  to- 
the  three  preceding,  and  is  given  to  facilitate  estimating  the  cost  of 
segmental  arch  culverts  of  the  standard  form  employed  by  the 
Atchison,  Topeka  and  Santa  Fe  Eailroad,  as  shown  in  Plate  V  and 
Table  52  (page  431).  The  excavation  is  only  approximate,  and  is 
computed  on  the  assumption  of  a  pit  2  feet  2  inches  deep  over  the 
entire  foundation,  including  the  paved  area.  Notice  that  this 
culvert  is  23  feet  between  the  outer  faces  of  the  end  walls,  and  is 
therefore  the  same  length  as  that  of  Table  56. 


438 


CULVERTS. 


[chap.    XVII. 


TABLE  57. 
Cost  of   A.   T.   and    S.  F.    Segmental   Auch  Culverts   20   Ft.    Long 
FROM  Inside  to  Inside  of  the   Coping,  and  also  of  each  Addi- 
tional Foot  of  Length. 

for  description  see  page  429. 
This  table  does  not  include  the  masonry  in  the  footings. 


Items. 

Span. 

6  ft.         8  ft. 

10  ft. 

12  ft. 

14  ft. 

16  ft. 

Plain  masonry  @,  $7.00  per  cu.  yd 

Arch  masonry   "     8.00    "     '"     " 

Paving                 "     2.00    "     "     " 

Excavation        "       .25    "     "     " 

$183.45 

99.18 

9.88 

7.33 

$470.34 

150.33 

13.15 

11.75 

$607.99 
190.99 
16.42 
14.11 

$657.83'  $641.41 

229. 12|     298.64 

19.68       22.99 

15.17,       16.45 

$669.13 

353  84 

26.26 

17.82 

Total  cost  of  culvert  20  feet  long.   . . . 

Cost  of  an  Additional  Foot  of 
Length : 

Plain  masonry  ©  $700  per  cu.  yd 

Arch  masonry   "     8.00    "     "     " 

Paving                "     2.00    ' 

Excavation        "       .25    "     "     " 

$299.84 

$5.44 

4.31 

.52 

.31 

$645.57 

$9.73 

6.54 

.69 

.39 

$829.51 

$12.96 

8.30 

.86 

.46 

$921.80 

$13.61 

9.96 

1.04 

.50 

$979.49 

$14.47 

12.98 

1.21 

.56 

$1,067.05 

$14.56 

15.38 

1.38 

.62 

Total  cost  of  1  additional  foot 

$10.58 

$17.35 

$22.58 

$25.11 

$29.22 

$31.94 

660.  Standard  Arch  Culvert.  Table  58  is  given  to  facilitate  the 
estimation  of  the  cost  of  culverts  of  the  general  form  shown  in  Plate 
VI.  The  prices  are  about  the  average  for  the  respective  kinds  of 
work ;  but  in  case  it  is  desired  to  determine  the  cost  for  other  prices, 

TABLE  58. 

Cost  of  Standard  Arch  Culverts  20  Ft.  Long  from  Inside  to  Inside 
OF  THE  Coping,  and  of  each  Additional  Foot  of  Length. 

FOR  description   SEE   PAGE   429. 

The  masonry  in  the  footings  is  not  included  in  this  table. 


Items. 

Span. 

6  ft. 

8  ft. 

10  ft. 

12  ft. 

14  ft.           16  ft. 

Plain  masonry  @  $7.00  per  cu.  yd  . . . 
Arch  masonry  "     8  00    "     "      "  — 
Paving                "     2.00    "     "      "  .... 
Excavation        '"       .25    "     "      " 

$233.11 

65.87 
6.83 
8.24 

$3.30.88 

92.72 

9.84 

9.53 

$496.79 

127.33 

12.65 

12.42 

$683.55 
184.00 
15.47 
15.42 

$912.45    $1,193.35 

238.64         305.62 

18.281          20.09 

18.33^          20.61 

Total  cost  of  culvert  20  feet  long  . . . 

Cost  of  an  Additional  Foot  of 

Length : 

Plain  masonry  @  $7.00  per  cu.  yd  — 

Arch  masonry  "     8.00    '•     "      " 

Paving                 "     2.00     "     "      "  .... 
Excavation        "       .25    "     "      "  — 

$314.05 

$3.88 

2.86 

.37 

.21 

$442.97 

$6.56 

4.03 

.52 

.26 

$649.19 

$9.85 

5.54 

.67 

.31 

$898.44 

$14.00 

8.00 

.81 

.36 

$1,187.70 

$18.75 

10.38 

.96 

.41 

$1,539.67 

$24.28 

13.29 

1.11 

.46 

Total  cost  of  1  additional  foot 

$7. 32 

$11.37 

$16.37 

$23.17 

$30.50 

$39.14 

AET.  3.]  ARCH   CULVERTS.  439 

it  is  only  necessary  to  increase  or  decrease  the  tabular  numbers 
proportionally.  The  quantities  of  excavation  are,  approximately, 
averages  of  the  actual  amounts  for  a  number  of  similar  culverts, 
and  are  equivalent  to  a  pit  2  feet  2  inches  deep  and  of  an  area  equal 
to  the  area  of  the  foundation.  Notice  that  the  culvert  in  Table  58 
]s  23  feet  between  the  outer  faces  of  the  end  walls;  and  is  therefore 
the  same  length  as  that  in  Tables  56  and  57,  and  is  1  foot  shorter 
than  that  of  Table  5-4  and  2^  feet  shorter  than  that  of  Table  55. 
Notice  also  that  in  Table  58  the  height  of  the  opening  is  in  each 
case  half  of  the  span  (see  Table  53,  page  433),  while  in  Tables  56 
and  57  the  height  of  the  opening  is  nearly  the  same  for  all  spans 
(see  Tables  51  and  52,  pages  430,  431). 


CHAPTEE  XVIII. 


ARCHES. 


661.  Definitions.  Parts  of  an  Arch.  Voussoirs.  The  wedge- 
shaped  stones  of  which  the  arch  is  composed  ;  also  called  the  arch- 
stone.s. 

Keystone.     The  center  or  highest  voussoir  or  arch-stone. 

Soffii.     The  inner  or  concave  surface  of  the  arch. 

Intrados,  The  concave  line  of  intersection  of  the  soffit,  with  a 
vertical  plane  perpendicular  to  the  axis  or  length  of  the  arch.  See 
Fig.  110. 

Extrados.  The  convex  curve,  in  the  same  plane  as  the  intrados, 
which  bounds  the  outer  extremities  of  the  joints  between  the 
voussoirs. 

Croivn.  The  highest  part 
of  the  arch. 

Skeioback.  The  inclined 
surface  or  joint  upon  which 
the  end  of  the  arch  rests. 

Abutment.  A  skewback 
and  the  masonry  which  sup- 
ports it. 

Springing  Line.  The  in- 
ner edge  of  the  skewback. 

Springer.     The  lowest  voussoir  or  arch-stone 

Hauncli.  The  part  of  the  arch  between  the  crown  and  th© 
skewback. 

Spandrel.  The  space  between  the  extrados  and  the  roadway. 
The  material  deposited  in  this  space  is  called  the  spandrel  filling y 
and  may  be  either  masonry  or  earth,  or  a  combination  of  them.  In 
large  arches  it  often  consists  of  several  walls  running  parallel  with 
the  roadway,  connected  at  the  top  by  small  arches  or  covered  with 
fiat  stones,  which  support  the  material  of  the  roadway. 

440 


Fig.  110. 


KINDS    OF    ARCHES.  441 


Span.  The  perpendicular  iistaaoe  '3etweeii  the  springing 
lines. 

Rise.  The  vertical  distance  between  the  highest  part  of  the 
intrados  and  the  plane  of  the  springing  lines. 

Bi7ig  Stones.  The  voussoirs  or  arch-stones  which  show  at  the 
ends  of  the  arch. 

Arch  Sheeting.  The  voussoirs  which  do  not  show  at  the  end 
of  the  arch. 

Backing.  Masonry,  usually  with  joints  horizontal  or  nearly 
so,  carried  above  the  skewbacks  and  outside  of  the  extrados. 

String  Course.  A  course  of  voussoirs  extending  from  one  end 
of  the  arch  to  the  other. 

Coursing  Joint.  The  joint  between  two  adjoining  string 
courses.  It  is  continuous  from  one  end  of  the  arch  to  the 
other. 

Heading  Joint.  A  joint  in  a  plane  at  right  angles  to  the  axis 
of  the  arch.     It  is  not  continuous. 

Ring  Course.  The  stones  between  two  consecutive  series  of 
heading  joints. 

662.  Kinds  of  Arches.  Circular  Arch.  One  in  which  the 
intrados  is  a  part  of  a  circle. 

Semi-circular  Arch.  One  whose  intrados  is  a  semi-circle;  also 
called  a  full-centered  arch. 

Segmental  Arch.  One  whose  intrados  is  less  than  a  semi- 
circle. 

Elliptical  Arch.  One  in  which  the  intrados  is  a  part  of  an 
ellipse. 

Basket-Handle  Arch.  One  in  which  the  intrados  resembles  a 
semi-ellipse,  but  is  composed  of  arcs  of  circles  tangent  to  each 
other. 

Pointed  Arch.  One  in  which  the  intrados  consists  of  two  arcs 
of  equal  circles  intersecting  over  the  middle  of  the  span.  For  ex- 
ample, see  Figs.  115  and  117,  page  447. 

Hydrostatic  Arch.  An  arch  in  equilibrium  under  the  vertical 
pressure  of  water. 

Geostatic  Arch.  An  arch  in  equilibrium  under  the  vertical 
pressure  of  an  earth  embankment. 

Catenarian  Arch.     One  whose  intrados  is  a  catenary. 

663.  Right  Arch.     A  cylindrical  arch,  either  circular  or  el- 


442 


AECHES. 


[chap.   XVIII. 


liptical,  terminated  by  two  planes,  termed  heads  of  the  arch,  at 
right  angles  to  the  axis  of  the  arch.     See  Fig.  111. 


FiQ.  111.— Right  Arch. 


Fig.  112.— Skew  Arch. 


Skeio  Arch.  One  whose  heads  are  oblique  to  the  axis.  See 
Fig.  112.  Skew  arches  are  quite  common  in  Europe,  but  are 
rarely  employed  in  the  United  States  ;  and  in  the  latter  when 
an  oblique  arch  is  required,  it  is  usually  made,  not  after  the 
European  method  with  spiral  joints  as  shown  in  Fig.  112,  but 
by  building  a  number  of  short  right  arches  or  ribs  in  contact 
with  each  other,  each  successive  rib  being  placed  a  little  to  one 
side  of  its  neighbor. 

Groined  and  Cloistered  Arches.  Those  formed  by  the  in- 
tersection of  two  or  more  cylindrical  arches.  The  spans  of 
the  intersecting  arches  may  be  different,  but  the  rise  must  be 
the  same  in  each;  and  their  axes  must  lie  in  the  same  plane, 
but  may  intersect  at  any  angle.  The  groined  arch  is  formed 
by  removing  those  portions  of  each  cylinder  which  lie  under 
the  other  and  between  their  common  curves  of  intersection, 
thus  forming  a  projecting  or  salient  angle  on  the  soffit  along 
these  curves.  The  cloistered  arch  is  formed  by  removing  those 
portions  of  each  cylinder  which  are  above  the  other  and  exterior 
to  their  common  intersection,  thus  forming  re-entrant  angles 
along  the  same  lines. 

Dome  and  Vault.  If  an  arch  revolves  around  a  vertical 
through  the  keystone,  a  dome  is  produced ;  and  if  it  moves  in 
a  straight  line  on  the  springer,  a  vault  is  produced.  Hence 
there  are  essentially  the  same  kinds  of  domes  and  vaults  as 
arches. 

Only  right  arches  will  he  considered  in  this  chapter. 


LIXE    OF   RESISTANCE. 


443 


664.  Line  of  Resistance.  If  the  action  and  reaction  between 
each  pair  of  adjacent  arch-stones  be  replaced  by  single  forces  so 
situated  as  to  be  in  every  way  the  equivalent  of  the  distributed 
pressures,  the  line  connecting  the  points  of  application  of  tliese 
several  forces  is  the  line  of  resistance  of  the  arch.  For  example, 
assume  that  the  half  arch  shown  in  Fig.  113  is  held  in  equilibrium 
by  the  horizontal  thrust  T — the  reaction  of  the  right-hand  half  of  the 
arch — applied  at  some  point  a  in  the  joint  OF.    Assume  also  that  the 


Fig.  113. 


several  arch-stones  fit  mathematically,  and  that  there  is  no  adhesion 
of  the  mortar.  The  forces  F^,  F^,  F^,  and  F^  represent  the  result- 
ants of  all  the  forces  (including  the  weight  of  the  stone  itself)  acting 
upon  the  several  voussoirs.  The  arch-stone  CIHF  is  in  equilib- 
rium under  the  action  of  the  three  forces,  T,  F^ ,  and  the  reaction 
of  the  voussoir  IHEG.  Hence  these  three  forces  must  intersect 
in  a  point,  and  the  direction  of  i?, — the  resultant  pressure  be- 
tween the  voussoirs  CIHF  and  IHEG — can  be  found  graphically 
as  shown  in  Fig.  113.  The  point  of  application  of  R^  is  at  b — 
the  point  where  R^  intersects  the  joint  HI.     The  voussoir  GEHl 


444  ARCHES.  [chap.  XVIII. 

is  in  equilibrium  under  the  action  of  R^,  F,,  and  R^ — the  resultant 
reaction  between  GEHI  and  GEDH, — and  hence  the  direction, 
the  amount,  and  the  point  of  application  (c)  of  R^  can  be  deter- 
mined as  shown  in  the  figure.  R^  and  R^  are  determined  in  the  same 
manner  as  R^  and  R^ . 

The  points  a,  b,  c,  d,  and  e,  called  centers  of  pressure,  are  the 
points  of  application  of  the  resultants  of  the  pressure  on  the  several 
joints  ;  or  they  may  be  regarded  as  the  centers  of  resistance  for  the 
several  joints.  In  the  latter  case  the  line  ahcde  would  be  called 
the  line  of  resistatice,  and  in  the  former  the  line  of  pressure. 
Strictly  speaking,  the  line  of  resistance  is  a  continuous  curve  cir- 
cumscribing the  polygon  ahcde.  The  greater  the  number  of 
joints  the  nearer  the  polygon  ahcde  approaches  this  curve.  Occa- 
sionally the  polygon  imiop  is  called  the  line  of  resistance.  The 
greater  the  number  of  joints  the  nearer  this  line  approaches  the 
line  of  resistance  as  defined  above.  For  an  infinite  number  of  joints 
the  polygons  ahcde  and  tmiop  coincide  with  the  curved  line  of  re- 
sistance, a,  h,  c,  d,  and  e  being  common  to  all  three. 

Notice  that  if  the  four  geometrical  lines  ah,  he,  cd,  and  de  were 
placed  in  the  relative  position  shown  in  Fig.  113,  and  were  acted 
upon  by  the  forces  T,  F^,  F^,  F^,  F^,  and  R,  as  shown,  they  would 
be  in  equilibrium  ;  and  hence  the  line  ahcde,  or  rather  a  curve 
passing  through  the  points  a,  b,  c,  d,  and  e,  is  sometimes  called  a 
linear  arch. 

Akt.  1.     Theory  of  the  Arch. 

665.  The  theory  of  the  masonry  arch  is  one  of  great  com- 
plexity. Numerous  volumes  have  been  written  on  this  subject,  and 
it  still  occupies  the  attention  of  mathematicians.  No  attempt  will 
be  made  here  to  give  an  exhaustive  treatise  on  the  arch  ;  but  the 
fundamental  principles  will  be  stated  as  clearly  as  possible,  and  tlie 
principal  solutions  of  the  problem  which  have  been  proposed  from 
time  to  time  will  be  explained  and  their  underlying  assumptions 
pointed  out. 

666.  The  External  Forces.  It  is  clear  that  before  we  can 
find  the  strains  in  a  proposed  arch  and  determine  its  dimensions, 
we  must  know  the  load  to  be  supported  by  it.  In  other  words, 
the   strength  and   stability  of  a  masonry  arch  depend   upon  the 


ART.   1.]  THEORY    OF   THE    ARCH.  445 

position  of  the  line  of  resistance  ;  and  before  this  can  be  deter^ 
mined,  it  is  necessary  that  the  external  forces  acting  upon  the  arch 
shall  be  fully  known,  i.  e.,  that  (1)  the  point  of  application,  (2)  the 
direction,  and  (3)  the  intensity  of  the  forces  acting  upon  each 
voussoir  shall  be  known.  Unfortunately,  the  accurate  determina- 
tion of  the  outer  forces  is,  in  general,  an  impossibility. 

1.  If  the  arch  supports  a  fluid,  the  pressure  upon  the  several 
voussoirs  is  perpendicular  to  the  extrados,  and  can  easily  be  found; 
and  combining  this  with  the  weight  of  each  voussoir  gives  the 
several  external  forces.     This  case  seldom  occurs  in  practice. 

2.  If  the  arch  is  surmounted  by  a  masonry  wall,  as  is  frequently 
the  case,  it  is  impossible  to  determine,  with  any  degree  of  accuracy, 
the  effect  of  the  spandrel  walls  upon  the  stability  of  the  arch.  It 
is  usually  assumed  that  the  entire  weight  of  the  masonry  above  the 
soffit  presses  vertically  upon  the  arch;  but  it  is  known  certainly 
that  this  is  not  the  case,  for  with  even  dry  masonry  a  part  of  the 
wall  will  be  self-supporting.  The  load  supported  by  the  arch  can 
be  computed  roughly  by  the  principle  of  §  250  (p.  168);  but,  as  this 
gives  no  idea  of  the  manner  in  which  this  pressure  is  distributed,  it 
is  of  but  little  help.  The  error  in  the  assumption  that  the  entire 
weight  of  the  masonry  above  the  arch  presses  upon  it  is  certainly  on 
the  safe  side;  but  if  the  data  are  so  rudely  approximate,  it  is  use- 
less to  attempt  to  compute  the  strains  by  mathematical  processes. 
The  inability  to  determine  this  pressure  constitutes  one  of  the  limi- 
tations of  the  theory  of  the  arch. 

Usually  it  is  virtually  assumed  that  the  extradosal  end  of  each 
voussoir  terminates  in  a  horizontal  and  vertical  surface  (the  latter 
may  be  zero) ;  and  therefore,  since  the  masonry  is  assumed  to  press 
only  vertically,  there  are  no  horizontal  forces  to  be  considered.  But 
as  the  extrados  is  sometimes  a  regular  curve,  there  would  be  active 
horizontal  components  of  the  vertical  pressure  on  this  surface;  and 
this  would  be  true  even  though  the  spandrel  masonry  were  divided 
by  vertical  Joints  extending  from  the  extrados  to  the  upper  limit  of 
the  masonry.  Further,  even  though  no  active  horizontal  forces  are 
developed,  the  passive  resistance  of  the  spandrel  masonry — either 
spandrel  walls  or  spandrel  backing — materially  affects  the  stability; 
of  an  arch.  Experience  shows  that  most  arches  sink  at  the  crown 
and  rise  at  the  haunches  when  the  centers  are  removed  (see  Fig. 
116,  p.  44?),  and  hence  the  resistance  of  the  spandrel  masonry  will 


446  AKCHES,  [chap.   XVITL 

materially  assist  in  preventing  the  most  common  form  of  failure. 
The  eflBciency  of  this  resistance  will  depend  upon  the  execution  of 
the  spandrel  masonry,  and  will  increase  as  the  deformation  of  the 
arch  ring  increases.  It  is  impossible  to  compute,  even  roughly,  the 
horizontal  forces  due  to  the  spaiidrel  masonry. 

Further,  in  computing  the  strains  in  the  arch,  it  is  usually 
assumed  that  the  arch  ring  alone  supports  the  masonry  above  it ; 
while,  as  a  matter  of  fact,  the  entire  masonry  from  the  intrados  to 
the  top  of  the  wall  acts  somewhat  as  an  arch  in  supporting  its  own 
weight. 

3.  If  the  arch  supports  a  mass  of  earth,  we  can  know  neither  the 
amount  nor  the  direction  of  the  earth  pressure  with  any  degree  of 
accuracy  (see  Chap.  XIV — Eetaining  Walls, — particularly  §  527, 
page  339).  We  do  know,  however,  that  the  arch  does  not  support 
the  entire  mass  above  it  (see  §§  618-20).  No  one  ever  thinks  of 
trying  to  make  a  tunnel  arch  strong  enough  to  sustain  the  weight  of 
the  entire  mass  above  it. 

In  the  theory  of  the  masonry  arch,  the  pressure  of  the  earth  is 
usually  assumed  to  be  wholly  vertical.  That  the  pressure  of  earth 
gives,  in  general,  active  horizontal  forces  appears  to  be  unquestion- 
able. An  examination  of  Fig.  113  (page  443)  will  show  how  the 
horizontal  forces  add  stability  to  an  arch  ring  whose  rise  is  equal  to 
or  less  than  half  the  span.  It  is  clear  that  for  a  certain  position 
and  intensity  of  thrust  T,  the  line  of  resistance  will  approach  the 
extrados  nearer  when  the  external  forces  are  vertical  than  when 
they  are  inclined.  We  know  certainly  that  the  passive  resistance  of 
the  earth  adds  materially  to  the  stability  of  masonry  arches  ;  for  the 
arch  rings  of  many  sewers  which  stand  without  any  evidence  of 
weakness  are  in  a  state  of  unstable  equilibrium,  if  the  vertical  press- 
ure of  the  earth  immediately  above  it  be  considered  as  the  only 
external  force  acting  upon  it. 

667.  Method  of  Failtjee  of  Arches.  A  masonry  arch  may 
yield  in  any  one  of  three  ways,  viz.:  (1)  by  the  crushing  of  the 
stone,  or  (2)  by  the  sliding  of  one  voussoir  on  another,  or  (3)  by 
rotation  about  an  edge  of  some  joint.  1.  An  arch  will  fail  if  the 
pressure  on  any  part  is  greater  than  the  crushing  strength  of  the 
material  composing  it.  2.  Figs.  114  and  115  represent  the  second 
method  of  failure ;  in  the  former  the  haunches  of  the  arch  slide 


ART.  1.] 


THEORY   OF   THE   ARCH. 


447 


out  and  the  crown  slips  down,  and  in  the  latter  the  reverse  is 
shown.  If  the  rise  is  less  than  the  span  and  the  arch  fails  by  the 
sliding  of  one  voussoir  on  the  other,  the  crown  will  usually  sink; 
but  if  the  rise  is  more  than  the  span,  the  haunches  will  generally 


Fig.  114. 


Fig.  115. 


be  pressed  inward  and  the  crown  will  rise.     3.  Figs.  116  and  117 
show  the  two  methods  by  which  an  arch  may  give  way  by  rotation 


Fig.  116. 


Fig.  117. 


about  the  joints.  As  a  rule  the  first  case  is  most  frequent  for  flat 
arches  and  the  second  for  pointed  ones. 

However,  more  arches  fail  on  account  of  unequal  settlement  of 
the  foundation  than  because  of  a  faulty  design  of  the  arch  proper. 

668.  Ceiteria  of  Safety.  There  are  three  criteria,  corre- 
sponding to  the  three  modes  of  failure,  by  which  the  stability  of  an 
arch  may  be  Judged.  (1)  To  prevent  overturning,  it  is  necessary 
that  the  line  of  resistance  shall  everywhere  lie  between  the  intrados 
and  the  extrados.  (2)  To  prevent  crushing,  the  line  of  resistance 
should  intersect  each  joint  far  enough  from  the  edge  so  that  the 
maximum  pressure  will  be  less  than  the  crushing  strength  of  the 
masonry.  (3)  To  prevent  sliding,  the  angle  between  the  line  of 
resistance  and  the  normal  to  any  joint  should  be  less  than  the  angle 
of  repose  (''angle  of  friction")  for  those  surfaces;  that  is  to  say, 
the  tangent  of  the  angle  between  the  line  of  resistance  and  the 
normal  to  any  joint  should  be  less  than  the  co-efficient  of  friction 
(§  489). 


448  ARCHES.  [chap.   XVIII. 

669.  Stability  against  Rotation.  An  arch  composed  of  incom- 
pressible voussoirs  can  not  fail  by  rotation  as  shown  in  Fig.  116, 
unless  the  line  of  resistance  touches  the  intrados  at  two  points  and 
the  extrados  at  one  higher  intermediate  point  (see  Fig.  120,  page 
454);  and  an  arch  can  not  fail  by  rotation  as  shown  in  Fig.  117, 
unless  the  line  of  resistance  touclies  the  extrados  at  two  points 
and  the  intrados  at  one  higher  intermediate  point  (see  Fig.  120). 
The  factor  of  safety  against  rotation  about  any  point  is  equal 
to  half  the  length  of  the  joint  divided  by  the  distance  between 
the  center  of   pressure  and   the  center  of  the  joint  ;    that  is  to 


1  I 
the  factor  of  safety  = —-, (1) 


in  which  I  is  tlie  length  of  the  joint  and  d  the  distance  between 
the  center  of  pressure  and  the  center  of  the  joint.  For  example,  if 
the  center  of  pressure  is  at  one  extremity  of  the  middle  third  of  the 
joint,  d  =^  \  I ;  and,  by  equation  (1),  the  factor  of  safety  is  three. 
If  the  center  of  j^ressure  is  5  ^  from  the  middle  of  the  joint,  the 
factor  of  safety  is  two. 

It  is  customary  to  require  that  the  line  of  resistance  shall  lie 
within  the  middle  third  of  tlie  arch  ring,  which  is  equivalent  to 
specifying  that  the  minimum  factor  of  safety  for  rotation  shall  not 
be  less  tlian  three. 

670.  Stability  against  Crushing.  The  method  of  determining 
the  pressure  on  any  part  of  a  joint  has  already  been  discussed  in  the 
chapter  on  masonry  dams  (see  pp.  320-26).  When  the  total  press- 
ure and  its  center  are  known,  the  maximum  pressure  at  any  part 
of  the  joint  is  given  by  formula  (23),  page  323.     It  is 

P=^+^, .  .  .  „) 

in  which  P  is  the  maximum  pressure  on  the  joint  per  unit  of  area  ; 
W  is  the  total  normal  pressure  on  the  joint  per  unit  of  length  of  the 
arch  ;  I  is  the  depth  of  the  joint,  i.  e.,  the  distance  from  intrados  to 
extrados  ;  and  d  is  the  distance  from  the  center  of  pressure  to 
the  middle  of  the  joint.     This  formula  is  general,  provided  the 


ART.   l.J  THEORY   OF   THE   ARCH.  449 

masonry  is  capable  of  resisting  tension  ;  and  if  the  masonry  is 
assumed  to  be  incapable  of  resisting  tension,  it  is  still  general,  pro- 
vided d  does  not  exceed  ^  /. 

For  the  case  in  which  the  masonry  is  incapable  of  resisting  ten- 
sion and  d  exceeds  \  I,  the  maximum  pressure  is  given  by  formula 
(24),  page  324.     It  is 

3(i/-f/) ^"•' 

If  the  line  of  resistance  for  any  arch  can  be  drawn,  the  maximum 
pressure  can  be  found  by  (1)  resolving  the  resultant  reaction  per- 
pendicular to  the  given  joint,  and  (2)  measuring  the  distance  d  from 
a  diagram  of  the  arch  similar  to  Fig.  113  (page  443),  and  (3)  sub- 
stituting these  data  in  the  proper  one  of  the  above  formulas  (the 
one  to  be  employed  depends  upon  the  value  of  d),  and  computing 
P.*  This  pressure  should  not  exceed  the  compressive  strength  of 
the  masonry. 

It  is  customary  to  presci'ibe  that  the  line  of  resistance  shall  lie 
within  the  middle  third  of  each  joint,  and  also  that  the  result 
obtained  by  dividing  the  total  pressure  by  the  area  of  the  joint  shall 
not  be  more  than  one  twentieth  of  the  ultimate  crushing  strength 
of  the  stone.  Under  these  conditions  the  maximum  pressure  is 
twice  the  mean,  and  hence  using  the  above  limits  is  equivalent  to 
saying  that  the  maximum  pressure  shall  not  be  more  than  one  tenth 
of  the  ultimate  crushing  strengtli  of  the  stone.  The  mean  pressure 
in  arches  is  usually  not  more  than  one  fortieth  or  one  fiftieth,  and 
sometimes  only  one  hundredth,  of  the  ultimate  compressive  strength 
of  the  stone  or  brick  of  which  it  is  constructed. 

671.  Unit  Pressure.  In  the  present  state  of  our  knowledge  it 
is  not  possible  to  determine  the  value  of  a  safe  and  not  extravagant 
unit  working-pressure.  The  customary  unit  appears  less  extrava- 
gant, when  it  is  remembered  (1)  that  the  crushing  strength  of 
masonry  is  considerably  less  than  that  of  the  stone  or  brick  of  which 
it  is  composed  (see  §§  221-22  and  §§  246-47  respectively),  and  that 
we  have  no  definite  knowledge  concerning  either  the  ultimate  or 
the  safe  crushing  strength  of  stone  masonry  (§  223)  and  but  little 

*  For  a  numerical  example  of  the  method  of  doiag  this,  see  2,  §  690. 


450  ARCHES.  [chap.   XVIII. 

concerning  that  of  brickwork  (§  248)  ;  and  (2)  that  all  the  data  we 
have  on  crushing  strength  are  for  a  load  perpendicular  to  the 
pressed  surface^  while  we  have  no  experimental  knowledge  of  the 
effect  of  the  component  of  the  pressure  parallel  to  the  surface  of  the 
joint,  although  it  is  probable  that  this  component  would  have  some- 
what the  same  effect  upon  the  strength  of  the  voussoirs  as  a  sheet 
of  lead  has  when  placed  next  to  a  block  of  stone  subjected  to  com- 
pression (§  12). 

On  the  other  hand,  there  are  some  considerations  which  still 
further  increase  the  degree  of  safety  of  the  usual  working-j^ressure. 
(1)  When  the  ultimate  crushing  strength  of  stone  is  referred  to,  the 
crushing  strength  of  cubes  is  intended,  although  the  blocks  of  stone 
employed  in  actual  masonry  have  less  thickness  than  width,  and 
hence  are  much  stronger  than  cubes  (see  §  15,  paragraph  2  §  60,  and 
§  273).  To  prevent  the  arch  stones  from  flaking  off  at  the  edges, 
the  mortar  is  sometimes  dug  out  of  the  outer  edge  of  the  joint. 
This  procedure  diminishes  the  area  under  pressure,  and  hence 
increases  the  unit  pressure  ;  but,  on  the  other  hand,  the  edge  of 
the  stone  which  is  not  under  pressure  gives  lateral  support  to  the 
interior  portions,  and  hence  increases  the  resistance  of  that  portion 
(see  §  273).  It  is  imiDossible  to  compute  the  relative  effect  of  these 
elements,  and  hence  we  can  not  theoretically  determine  the  efficiency 
of  thus  relieving  the  extreme  edges  of  the  joint.  (2)  The  preceding 
formulas  (2  and  3)  for  the  maximum  pressure  neglect  the  effect  of 
the  elasticity  of  the  stone  ;  and  hence  the  actual  pressure  must  be 
less,  by  some  unknown  amount,  than  that  given  by  either  of  the 
formulas. 

672.  Notice  that  the  distance  which  the  center  of  pressure  may 
vary  from  the  center  of  the  joint  without  the  masonry's  being 
crushed  depends  upon  the  ratio  between  the  ultimate  crushing 
strength  and  the  mean  pressure  on  the  joint.  In  other  words,  if 
the  mean  pressure  is  very  nearly  equal  to  the  ultimate  crushing 
strength,  then  a  slight  departure  of  the  center  of  pressure  from  the 
center  of  the  joint  may  crush  the  voussoir  ;  but,  on  the  other  hand, 
if  the  mean  pressure  is  small,  the  center  of  pressure  may  de- 
part considerably  from  the  center  of  the  joint  without  the  stone's 
being  crushed.     This   can  be   shown  by   equation    (2),  page  448. 

W 
If  both  P  and  -y  are  large,  d  must  be  small ;  but  if  F  is  large  and 


mm/imm/m/mm/mi/i/m////ii/\ 


ART.    ].]  THEORY    OF   THE    ARCH.  451 

^ small,  then  d  may  be  large.     Essentially  the  same  result  can  be 

deduced  from  equation  (3),  page  449. 

Even  though  the  line  of  resistance  approaches  so  near  the  edge 
of  the  Joint  that  the  stone  is  crushed,  the  stability  of  the  arch  is  not 
necessarily  endangered.  For  example,  conceive  a  block  of  stone 
resting  upon  an  incompressible  plane, 
AB,  Fig.  118,  and'  assume  that  the 
center  of  pressure  is  at  X  Then  the  Jl^mmd^ 
pressure  is  applied  over  an  area  pro- 
i ?ctod  in  A  V,  such  that  AX=^  A  V. 
The  pressure  at  A  is  represented  by  « 
AK,    and    the   area   of    the   triangle  Fi«-  ^'^^■ 

AKV  represents  the  total  pressure  on  the  joint.  Assume  that 
AK  is  the  ultimate  crushing  strength  of  the  stone,  and  that  the 
center  of  pressure  is  moved  to  JV'.  The  pressure  is  borne  on  an 
area  projected  in  A  V\  The  pressure  in  the  vicinity  of  A  is 
uniform  and  equal  to  the  crushing  strength  ^^j  and  the  total 
pressure  on  the  joint  is  represented  by  the  area  of  the  figure 
A  K  G  V,  which  has  its  center  of  gravity  in  the  vertical 
through  X'.  Eventually,  when  the  center  of  pressure  approaches 
so  near  A  that  the  area  in  which  the  stone  is  crushed  becomes 
too  great,  the  whole  block  will  give  way  and  the  arch  will 
fall.* 

673.  Open  Joints.  It  is  frequently  prescribed  that  the  line  of - 
resistance  shall  pass  through  the  middle  third  of  each  joint,  "  so 
that  the  joint  may  not  open  on  the  side  most  remote  from  Ibe  line 
of  resistance."  If  the  line  of  resistance  departs  from  the  middle 
third,  the  remote  edge  of  the  joint  will  be  in  tension  ;  but  since 
cement  mortar  is  now  quite  generally  emj^loyed,  if  the  masonry  is 
laid  with  ordinary  care  the  joint  will  be  able  to  bear  considerable 
tension  (see  Table  13,  page  94);  and  hence  it  does  not  necessa- 
rily follow  that  the  joint  will  open. 

*  Rankine  saj-s :  "  It  is  true  that  arches  have  stood,  and  still  stand,  in  which  the 
centers  of  resistance  of  joints  fall  be.vond  the  middle  third  of  the  depth  of  the  arch 
ring ;  but  the  stability  of  such  arches  is  either  now  precarious,  or  must  have  been 
precarious  while  the  mortar  was  fresh."  The  above  is  one  reason  whj-  the  stability 
of  the  arch  is  not  necessarily  precarious,  and  other  reasons  are  found  in  §  666  and 
also  in  the  subsequent  discussion.  A  reasonable  theory  of  the  arch  will  not  make  a 
structure  appear  instable  which  shows  every  evidence  of  security. 


452  ARCHES.  [chap.    XVIII. 

If  the  line  of  pressure  departs  from  the  middle  third  and  the 
mortar  is  incapable  of  resisting  tension,  the  joint  will  open  on  the 
side  farthest  from  the  line  of  resistance.  For  example,  if  the 
center  of  pressure  is  at  W,  Fig.  118,  then  a  jDortion  of  the  joint 
AV  {=  3  AN)  is  in  compression,  while  the  portion  VB  has  no  force 
acting  upon  it  ;  and  hence  the  yielding  of  the  portion  A  Fwill  cause 
the  joint  to  open  a  little  at  B.  This  opening  will  increase  as  the 
center  of  pressure  approaches  J,  and  when  the  material  at  that 
point  begins  to  crush  the  increase  will  become  comparatively  rapid. 

Notice  that  if  there  are  open  joints  in  an  arch,  it  is  certain 
tliat  the  actual  line  of  resistance  does  not  lie  within  the  middle 
third  of  such  joints.  Notice,  however,  that  the  opening  of  a  joint 
does  not  indicate  that  the  stability  of  the  arch  is  in  danger.  In 
most  cases,  an  open  joint  is  no  serious  matter,  particularly  if  it  is  in 
the  soffit.  If  in  the  extrados,  it  is  a  little  more  serious,  since  water 
might  get  into  it  and  freeze.  To  guard  against  this  danger,  it  is 
customary  to  cover  the  extrados  witli  a  layer  of  puddle  or  some 
coating  impervious  to  water  (§  204). 

674.  Stability  against  Sliding.  If  the  effect  of  the  mortar  is 
neglected,  an  arch  is  stable  against  sliding  when  the  line  of  resist- 
ance makes  with  the  norma  an  angle  less  than  the  angle  of  friction. 
According  to  Table  36  (page  315)  the  co-efficient  of  friction  of 
masonry  under  conditions  the  most  unfavorable  for  stability — /.  e., 
while  the  mortar  is  wet — is  about  0.50,  which  corresponds  to  an  angle 
of  friction  of  about  25°.  Hence  if  the  hne  of  pressure  makes  an 
angle  with  the  normal  of  more  than  25°,  there  is  a  possibility  of 
one  voussoir's  sliding  on  the  other.  This  possibility  can  be  elimi- 
nated by  changing  the  joints  to  a  direction  more  nearly  at  right 
angles  to  the  line  of  pressure. 

However,  there  is  no  probability  that  an  arch  will  receive  its  full 
load  before  the  mortar  has  begun  to  set ;  and  hence  the  angli^  of 
friction  is  virtually  much  greater  than  25°.  It  is  customar;  to 
arrange  the  joints  of  the  arch  at  least  nearly  perpendicular  to  the 
line  of  resistance,  in  which  case  little  or  no  reliance  is  placed  on  the 
resistance  of  friction  or  the  adhesion  of  the  mortar. 

675.  Conclusion.  From  the  preceding  discussion,  it  will  be 
noticed  that  the  factors  of  stability  for  rotation  and  for  crushing 
are  dependent  upon  each  other  ;  while  the  factor  for  sliding  is 
independent  of  tlie  other  conditions  of  failure,  and  is  dependent 


AKT.  1.]  THEORY    OF   THE    ARCH.  453 

only  upon  the  direction  given  to  the  joints.  A  theoretically  perfect 
design  for  an  arch  would  be  one  m  which  the  three  factors  of  safety 
were  equal  to  each  other  and  uniform  throughout  the  arch.  As 
arches  are  ordinarily  built,  the  factor  for  rotation  is  about  three,  or 
a  little  more  ;  the  nominal  factor  for  crushing  is  ten  to  forty  ;  and 
the  nominal  factor  for  sliding  is  one  and  a  half  to  two. 

It  is  evident  that  before  any  conclusions  can  be  drawn  concern- 
ing the  strength  or  stability  of  a  masonry  arch,  the  position  of  the 
line  of  resistance  must  be  known  ;  or,  at  least,  limits  must  be  found 
within  which  the  true  line  of  resistance  must  be  proved  to  lie. 

676.  Location  of  the  True  Line  of  Resistance.  Tlie  de- 
termination of  the  line  of  resistance  of  a  semi-arch  requires  that  the 
external  forces  shall  be  fully  knoAvn,  and  also  that  (1)  the  amount, 
(3)  the  point  of  application,  and  (3)  the  direction  of  the  thrust  at 
the  crown  shall  be  known.  The  determination  of  the  external 
forces  is  a  problem  independent  of  the  theory  of  the  arch  ;  and  for 
the  present  it  will  be  assumed  that  they  are  fully  known,  although 
as  a  matter  of  fact  they  can  not  be  known  with  any  considerable 
degree  of  accuracy  (see  §  666). 

Each  value  for  the  intensity  of  the  thrust  at  the  crown  gives  a 
different  line  of  resistance.  For  example,  in  Fig.  113  (page  443), 
if  the  thrust  T  be  increased,  the  point  I — where  R^  intersects  the 
plane  of  the  joint  HI — will  approach  /;  and  consequently  c,  d, 
and  e  will  approach  G,  H,  and  A  respectively.  If  T  be  increased 
sufficiently,  the  line  of  pressure  will  pass  through  A  or  H  (usually 
the  former,  this  depending,  however,  upon  the  dimensions  of  the 
arch  and  the  values  and  directions  of  F^,  F^,  and  F^),  and  the  arch 
will  be  on  the  point  of  rotating  about  the  outer  edge  of  one  of  these 
joints.  This  value  of  T  is  then  the  maximum  thrust  at  a  consistent 
with  stability  of  rotation  about  the  outer  edge  of  a  joint,  and  the 
corresponding  line  of  resistance  is  the  line  of  resistance  for  maxi- 
mum thrust  at  a.  Similarly,  if  the  thrust  T'be  gradually  decreased, 
the  line  of  resistance  will  approach  and  finally  intersect  the  intrados, 
in  which  case  the  thrust  is  the  least  possible  consistent  with  stabil- 
ity of  rotation  about  some  point  in  the  intrados.  The  lines  of 
resistance  for  maximum  and  minimum  thrust  at  a  are  shown  in 
Fig.  119  (page  454). 

If  the  point  of  application  of  the  force  T'be  gradually  lowered 
and  at  the  same  time  its  intensity  be  increased,  a  line  of  resistance 


454 


ARCHES. 


[chap.  XVIIT. 


may  be  obtained  ■which  will  have  one  point  in  common  with  the 
intrados.  This  is  the  line  of  resistance  for 
maximum  thrust  at  the  crown  joint.  Simi- 
larly, if  the  point  of  application  of  T  be 
gradually  raised,  and  at  the  same  time  its 
intensity  be  decreased,  a  line  of  resistance 
may  be  obtained  which  will  have  one  point 
in  common  with  the  extrados.  This  is  the 
line  of  resistance  for  minimum  thrust  at 
The  lines  of  resistance 
the   crown   are   shown    in 


Fig.  119. 


Fig.  120. 


the  crown  joint. 
for  maximum  and  minimum  thrust  at 
Fig.  120. 

Similarly  each  direction  of  the 
thrust  T  will  give  a  new  line  of  re- 
sistance. In  short,  every  different 
value  of  each  of  the  several  factors, 
und  also  every  combination  of  these 
values,  will  give  a  different  position 
for  the  line  of  resistance.  Hence,  the 
problem  is  to  determine  which  of  the 
infinite  number  of  possible  lines  of 
resistance  is  the  actual  one.  This 
problem  is  indeterminate,  since  there  are  more  unknown  quantities 
than  conditions  (equations)  by  which  to  determine  them.  To 
meet  these  difficulties  and  make  a  solution  of  the  problen;i  possible, 
various  hypotheses  have  been  made  ;  but  there  is  no  unanimity  of 
opinion  among  authorities  regarding  the  position  of  the  true  line 
of  resistance.  Some  of  these  hypotheses  will  now  be  considered 
briefly. 

677.  Hypothesis  of  Least  Pressure.  Some  writers  have  assumed 
the  true  line  of  resistance  to  be  that  which  gives  the  smallest  abso- 
lute pressure  on  any  joint.  This  principle  is  a  meta-physical  one, 
and  leads  to  results  unquestionably  incorrect.  Of  the  four  hypo- 
theses here  discussed  this  is  the  least  satisfactory,  and  the  least 
frequently  employed.     It  will  not  be  considered  further. 

For  an  explanation  of  Claye's  method  of  drawing  the  line  of 
pres'^ure  according  to  this  theory,  see  Van  Nostrand's  Engineering 
Magazine,  vol.  xv,  pp.  33-36.  For  a  general  discussion  of  the 
theory  of  the  arch  founded  on  this  hypothesis,  see  an  article  by  Pro- 


ART.   1.]  THEORY    OF   THE    ARCH.  455 

lessor  Du  Bois  in  A"an  Xostrand's  Engineering  Magazine,  vol.  xiii, 
pp.  341-46,  and  also  Du  Bois's  '•Graphical  Statics,"  Chapter  XV. 

678.  Hypothesis  of  Least  Thrust  at  the  Crown.  According  to 
this  iiypothesis  the  true  line  of  resistance  is  that  for  which  the 
thrust  at  the  crown  is  the  least  possible  consistent  with  equilibrium. 
This  assumes  that  the  thrust  at  the  crown  is  a  passive  force  called 
into  action  by  the  external  forces  ;  and  that,  since  there  is  no  need 
for  a  further  increase  after  it  has  caused  stability,  it  will  be  the  least 
possible  consistent  with  equilibrium. 

This  principle  alone  does  not  limit  the  position  of  the  line  of 
resistance;  but,  if  the  external  forces  are  known  and  the  direction 
of  the  thrust  is  assumed,  this  hypothesis  furnishes  a  condition  by 
which  the  line  of  resistance  corresponding  to  a  minimum  thrust  can 
be  found  by  a  tentative  process.  The  principle  of  least  crown 
thrust  was  first  proposed  by  Moseley,*'  was  amplified  by  Scheffler,f 
and  has  been  adopted  more  generally 

by   writers    and   engineers   than   any         jwj       Wj      [w/,       I*"! 
other.  .  ^..^._x;^^=— r — |C 

679.  The  portion  of  the  arch  shown  \i^""'j^  '     \    !     1  ! 
m  Fig.  121  is  held  in  equilibrium  by  (1)          y<C  j;    VH""*^  :       ii 
the  vertical  forces,  w,,  w^,  etc.,  (2)  by     r/t   \i^     •  i        \ 

the  horizontal  forces  7i J, /<„,  etc.,  (3)  by  /^-^X/C.-.m j-, -: ^*- 

the  reaction  R  at  the  abutment,  and  (4)      A)C— \—i,  \ 

bv  the  thrust   r  at  the  crown.     The  !* -x,-—--->! 

r  .  Fig.  121. 

direction  of  R  is  immaterial  in  this 

discussion.  Let  a  and  l  represent  the  points  of  application  of  T 
and  R,  respectively,  although  the  location  of  these  points  is  yet  un- 
determined.    Let 

7^=  the  thrust  at  the  crown; 

a'j  =  the  horizontal  distance  from  h  to  the  line  of  action  of  ii\ ; 
x^  =  the  same  for  u\,  etc. ; 

*  Philosophical  Magazine,  Oct.,  1833 — see  Moseley's  Mechanical  Principles  of  En- 
gineering, 2d  American  ed.,  p.  430. 

t  "  Theorie  der  Gewolbe,  Futtermauern,  und  eisernen  Briicken.''  Braunschweig, 
1857.  A  French  translation  of  this  work  is  entitled  "  Traits  de  la  Stability  des  con- 
structions  ;  Ire  partie,  Theorie  des  Voutes  et  des  Murs  de  Soutenement,"  Paris,  1864. 
Cain's  "  A  Practical  Theory  of  Voussoir  Arches  " — No.  12  of  Van  Nostrand's  Science 
Series — New  York,  1874,  is  an  exposition  of  a  theory  of  the  arch  based  upon  thia 
hypothesis. 


4:^6  ARCHES.  [chap.   XVIII. 

y  =  the  perpendicular  distance  from  b  to  the  line  of  action  of  T; 
k^  =  the  perpendicular  distance  from  b  to  the  line  of  action  of 

h^-  k^  =  the  same  for  h^;  etc. 
Then,  by  taking  moments  about  b,  we  have 

Ti/  =  w^  rr,  +  ii\  2\  +  etc.  +  h^  h^  +  A,  ^%  +  etc. ;     .      (4) 
hence 

r^^^WX  2hh 

y  y 

1.  The  value  of  T  depends  upon  ^  h  h — the  sum  of  the 
moments  of  the  horizontal  component  of  the  external  forces; — but 
we  know  neither  the  nature  of  the  material  over  the  arch  nor  the 
value  of  ^hkiov  any  particular  material  (see  §§  527-31).  In 
discussing  and  applying  this  principle,  the  term  ^  h  Tc  is  usually 
neglected.  Ordinarily  this  gives  an  increased  degree  of  stability; 
but  this  is  not  necessarily  the  case.  The  omission  of  the  effect  of 
the  horizontal  component  makes  the  computed  value  of  Tless  than 
it  really  is,  and  causes  the  line  of  resistance  found  on  this  assump- 
tion to  approach  the  infrados  at  the  haunches  nearer  than  it  does  in. 
fact ;  and  hence  the  conditions  may  be  such  that  the  actual  line  of 
resistance  will  be  unduly  near  the  cxtrados  at  the  haunches,  and 
consequently  endanger  the  arch  in  a  new  direction. 

2.  For  simplicity  of  discussion,  and  because  the  error  involved  in 
the  discussion  immediately  to  follow  is  immaterial,  we  will  tempo- 
rarily omit  the  effect  of  the  horizontal  components  of  the  external 
forces.  If  the  horizontal  forces  are  disregarded,  equation  (5) 
becomes 

T=?^'^ (G) 

y 

From  equation  (6)  we  see  that,  other  things  remaining  the  same, 
the  larger  //  the  smaller  T ;  and  hence,  for  a  minimum  value  of  7', 
a  should  be  as  near  c  as  is  possible  without  crushing  the  stone  (see 
§§  670-72).  Usually  it  is  assumed  that  ac  is  equal  to  one  third  of 
the  thickness  of  the  arch  at  the  crown  ;  and  hence  the  average 
pressure  per  unit  of  area  is  to  be  equal  to  one  half  of  the  assumed 
unit  working  pressure  ;  or,  in  other  words,  twice  the  thrust  T 
divided  by  tlie  thickness  of  the  crown  is  to  be  equal  to  the  unit 
working  pressure. 


ART.   1.]  THEORY   OF   THE    ARCH.  457 

3.  To  determine  y.  it  is  necessary  that  the  direction  of  T  should 
be  known.  It  is  usually  assumed  that  T  is  horizontal.  If  the  arch 
is  symmetrical  and  is  loaded  uniformly  over  the  entire  span,  this 
assumption  is  reasonable  ;  but  if  the  arch  is  subject  to  heavy  moving 
loads,  as  most  are,  the  thrust  at  the  crown  is  certainly  not  hori- 
zontal, and  can  not  be  determined. 

4.  If  the  joint  A  B  is  horizontal,  then  h  is  to  be  taken  as  near 
A  as  is  consistent  with  the  crushing  strength  of  the  stone,  or  at, 
say,  one  third  of  the  length  of  the  joint  ^4  B  from  ^4.  Xotice  that 
if  the  springing  line  is  inclined,  as  in  general  it  will  be  (see  last 
two  paragraphs  of  §  682,  p.  463),  moving  h  toward  A  decreases  .r, 
and  will  at  the  same  time  increase  y.  Hence  the  position  of  b  cor- 
responding to  a  minimum  value  of  T  can  be  found  only  by  trial. 
It  is  usual,  however,  to  assume  that  Ah  is  one  third  of  AB.  what- 
ever the  inclination  of  the  joint. 

680.  Joint  of  Rupture.  The  joint  of  rupture  is  that  joint  for 
which  the  tendency  to  oj^en  at  the  extrados  is  the  greatest.  The 
joint  of  rupture  of  an  arch  is  analogous  to  the  dangerous  section  of 
a  beam.  Practically,  the  joint  of  rupture  is  the  springing  line  of 
the  arch,  the  arch  masonry  below  that  joint  being  virtually  only  a 
part  of  the  abutment. 

That  no  joint  may  open  at  the  extrados,  the  thrust  at  the  crown 
must  be  at  least  equal  to  the  maximum  value  of  T  as  determined 
by  equation  (5),  page  45G.  If  the  thrust  is  less  than  this,  the  joint 
of  rupture  will  open  at  the  extrados  ;  and  a  greater  value  is  incon- 
sistent Avith  the  hypothesis  of  minimum  crown  thrust.  Since  the 
moment  of  the  horizontal  components  of  the  external  forces  is 
indeterminable,  the  position  of  the  true  joint  of  rupture  can  be 
found  only  by  trial  for  assumed  values  and  positions  of  tlie  hori- 
zontal forces. 

681.  As  an  example,  assume  that  it  is  required  to  determine  the 
joint  of  rupture  of  the  16-foot  arch  shown  in  Fig.  122,  which  is 
the  standard  form  employed  on  the  Chicago,  Kansas  &  Xebraska 
I\.  R.  (see  page  427  and  Plate  III).  Assume  that  the  arch  supports 
an  embankment  of  earth  extending  10  feet  above  the  crown,  and 
that  the  earth  weighs  TOO  pounds  per  cubic  foot  and  the  masonry 
100.  For  simplicity,  consider  a  section  of  the  arch  only  a  foot 
thick  periDcndicular  to  the  plane  of  the  paper.  The  half-arch  ring 
and  the  earth  embankment  above  it  are  divided  into  eight  sections, 


4'58 


ARCHES. 


[chap.  XVIII. 


■which  for  a  more  accurate  determination  of  the  joint  of  ruj^ture 
are  made  smaller  near  the  supposed  position  of  that  joint.  The 
weight  of  the  first  section  rests  upon  the  first  joint,  that  of  the  first 
two  upon  the  second  joint,  etc.     The  values  and  the  positions  of 


Fig.  123. 


the  lines'  of  action  of  the  weights  of  the  several  sections  are  given  in 
the  second  and  third  columns  of  Table  59.* 


*  The  center  of  gravity  of  the  arch  stone  is  found  by  the  method  explained  in 
§  494  (page  318);  and  the  center  of  gravity  of  the  prism  of  earth  resting  upon  each  arch 
6tone  may,  without  sensible  error,  be  taken  as  acting  through  its  medial  vertical  line. 
The  center  of  gravity  of  the  combined  weight  of  the  arch  stone  and  the  earth  resting 
upon  it  may  be  found  by  either  of  the  two  following  methods,  of  which  the  first  is 
the  shorter  and  more  accurate  : 

1.  The  center  of  gravity  of  the  two  masses  may  be  found  by  the  following  well- 
known  principle  of  analytical  mechanics  : 


Wj  Xi  -\-  u'2  a"j 


(7) 


in  which  x  is  the  horizontal  distance  from  any  point,  say  the  crown,  to  the  vertical 
through  the  center  of  gravity  of  the  combined  masses,  k',  and  u\  are  the  weights  of 
the  two  masses,  and  z^  and  x^  the  horizontal  distances  from  any  point,  say  the  crown, 
to  the  verticals  through  the  centers  of  gravity  of  the  separate  masses  respectively. 
The  same  method  can  be  employed  for  finding  the  center  of  gravity  of  any  number 
of  masses,  by  simply  adding  the  corresponding  term  or  terms  in  the  numerator  and 
the  denominator  of  equation  (7). 

2.  Since  the  principles  employed  in  the  second  method  of  finding  the  center  of 
gravity  of  each  arch  stone  and  its  load  are  frequently  employed,  in  one  form  or 


ART.   1.] 


THEORY   OF   THE    ARCH. 


469 


TABLE  59. 
To  FIND  THE  Joint  of  Ruptuke  of  the  Arch  Ring  shown  in  Fig.  122. 


FOSITIO.N  OF  THE 

Data  for  Ver- 

Data for  Hori- 

Center of 

tical 

Forces. 

zontal  Forces. 

Pressure  for 

Thrust 

AT  the  Crown. 

each 

Joint.     , 

55 

<D 

tk«    r^ 

(D 

-,  •-  *  c 

^-c 

J3 

■°ot 

JS 

gft5s 

si. 

0 

3    D 

0  U 

=  0  as  0 

0  s  5'-s 

■s 

a 

3  (6 

0  0 

as 

ertical  distai 
f  point  of  ap 
aiion  from 
op  of  the  crc 
oint. 

3  PI 

1.  H  0 
0  e8  - 

%W  X 

y 

Total 
thrust. 

55*- -^ 

<!'" 

m-oi- 

>'^« 

Lbs. 

Feet. 

Lbs. 

Feet. 

Feet. 

Feet. 

Lbs. 

Lbs. 

Lbs 

1 

2.9.38 

1.20 

66 

0.10 

2.20 

1.18 

3.866 

94 

3.960 

2 

3.04.5 

3.. 57 

243 

0.155 

4  27 

1.86     ' 

7.744 

308 

8.052 

3 

1.644 

5.33 

192 

1.17 

5.27 

2.42 

8,518 

424 

8,942 

4 

1.716 

6.45 

259 

1,78 

6.17 

3.11 

S.74.f 

662 

Q.410 

5 

1.825 

7.50 

315 

2.. 53 

6.98 

3.90 

8,577 

700 

!i.277 

6 

1,888 

8.47 

415 

3.40 

7.71 

4.81 

8.407 

941 

9..S48 

7 

3,939 

9.77 

1,030 

5.02 

8.85 

6.84 

7.506 

1.407 

8,911 

8 

4,098 

11.05 

1,624 

7.70 

9.50 

9.25 

1 

S,<?^ 

1,983 

7,973 

another,  in  discussions  of  the  stabilitj-  of  the  masonry  arch,  this  method  will  be  ex- 
plained a  little  more  fully  than  is  required  for  the  problem  in  hand. 

The  first  step  is  to  reduce  the  actual  load  upon  an  arch  (including  the  weight  of 
the  arch  ring  itself)  to  an  equivalent  homogeneous  load  of  the  same  density  as  the 
arch  ring.  The  upper  limit  of  this  imaginary  loading  is  called  the  red  need -load  contour. 
For  example,  suppose  it  is  required  to  find  the  reduced-load  contour  for  the  arch 
loaded  as  in  Fig.  123.    Assume  that  the  weight  of  the  arch  ring   is  160  pounds  per 


Fig.  123. 


Fig.  124. 


cubic  foot ;  that  of  the  rubble  backing,  140  ;   and  that  of  the  earth,  100.     Then  the 
ordinate  at  a  to  the  load  contour  of  an  equivalent  load  of  the  density  of  the  arch  ring 


is  equal  to  ab  +  bc 


140 


+  CI 


100 


=  >  say,gf.    The  value  of  ^/is  laid  off  in  Fig.  124. 


160    '   '  "  160 

Computing  the  ordinates  for  other  points  in  the  load  contour  gives  the  line  E  F,  Fig. 
124,  which  is  the  reduced-load  contour  for  the  load  shown  in  Fig.  123.  The  area 
between  the  intrados  and  the  reduced-load  contour  is  proportional  to  the  load  on  the 
arch.  In  a  similar  manner,  a  live  load  (as,  for  example,  a  train)  can  be  reduced  to 
an  equivalent  load  of  masonry,— in  which  case  the  reduced-load  contour  would  con- 
sist of  a  line  O  H  above  and  parallel  to  E I  for  that  part  of  tbe  spau  covered  by  the 


460  AKCHES.  [chap.   XVIII. 

The  value  and  position  of  the  horizontal  components  of  the 
external  forces  are  somewhat  indeterminate  (see  §§  528-31).  Ac- 
cording to   Eankine's   theory   of    earth   pressure,*  the   horizontal 

pressure  of  earth  at  any  point  can  not  be  greater  than  — - — . — — 

times   the    vertical   pressure   at   the    same    point,    nor   less    than 

times   the   vertical    pressure, —  0   being    the    angle    of 

1  -f-  sin  0 

repose.  +  If  0  =  30°,  the  above  expression  is  equivalent  to  saying 
that  the  horizontal  pressure  can  not  be  greater  than  three  times 
the  vertical  pressure  nor  less  than  one  third  of  it.  Evidently 
the  horizontal  component  will  be  greater  the  harder  the  earth 
spandrel-filling  is  rammed  into  place.  The  condition  in  which  the 
earth  will  be  deposited  behind  the  arch  can  not  be  foretold,  but  it 
is  probable  that  at  least  the  minimum  value,  as  above,  will  always 
be  realized.  Hence  we  will  assume  that  the  horizontal  intensity 
is  at  least  owe  tliird  of  the  vertical  intensity ;  that  is  to  say, 
/i  =z  1  e  c?  ?,  in  which  e  is  the  weight  of  a  cubic  unit  of  earth — which 
was  assumed  above  at  100  pounds, — d  the  depth  of  the  center  of 
pressed  surface  below  the  top  of  the  earth  filling,  and  /  the  vertical 
dimension  of  the  surface.  The  values  and  the  positions  of  the 
horizontal  forces  acting  on  the  respective  sections  of  the  arch  ring 
are  given  in  the  second  double  column  of  Table  59. 

To  find  the  least  thrust  at  the  crown  consistent  with  stability  of 
rotation,  assume  that  the  center  of  pressure  on  any  joint  is  at  a 
distance  from  the  intrados  equal  to  one  third  of  the  IcDgth  of  the 
joint  (see  paragraph  4,  page  45T).  The  co-ordinates  to  the  several 
centers  of  pressures  are  given  in  the  third  double  column  of  Table 
59.  Notice  that  the  several  values  of  x  and  h  are  simply  the  differ- 
ences between  two  quantities  given  in  the  table.  The  thrust  at  the 
crown  is  supposed  to  be  applied  at  the  upper  limit  of  the  middle 
third  of  the  crown  joint.  The  length  of  the  crown  joint  is  1.25  feet; 
and  hence  the  several  values  of  y  are  the  respective  quantities  in  the 

train  :  while  for  the  remainder  of  the  span,  the  line  I F  is  the  reduced-load  contour. 
The  second  step  is  to  draw  the  arch  ring  and  its  reduced-load  contour  on  thick 
paper,  to  a  large  scale,  and  then,  with  a  sharp  knife,  carefully  cut  out  the  area  repre- 
senting the  load  on  each  arch  stone.  The  center  of  gravity  of  each  piece,  as  ij  k  I  m  a. 
Fig.  124,  can  be  found  by  balancing  it  on  a  knife-edge  ;  and  then  the  position  of  the 
center  of  gravity  is  to  be  transferred  to  the  drawing  of  the  arch. 

*  See  ^  .544,  page  .348. 

+  Rankine's  Civil  Engineering,  p.  320. 


»RT.  1.]  THEORY    OF    THE    ARCH.  461 

seventh  column  of  Table  59  minus  ^  of  1.20  feet.  The  last  three 
columns  of  the  table  contain  the  values  of  the  crown  thrust  as 
computed  by  equation  (5),  page  456. 

Au  inspection  of  the  results  in  the  last  column  of  Table  59 
shows  that  the  thrust  is  a  maximum  for  joint  4.  A  repetition  of 
the  computations,  using  smaller  divisions  of  the  arch  ring,  might 
show  that  the  absolute  maximum  occurs  a  little  to  one  side  or  the 
other  of  this  joint;  but  the  uncertainty  in  the  data  for  both  the 
vertical  and  the  horizontal  forces  is  too  great  (see  §  G19  and  §§  527-31 
respectively)  to  justify  an  attempt  at  absolute  accuracy,  and  hence 
we  will  assume  that  joint  4  is  the  true  joint  of  rupture.  The 
angular  distance  of  this  joint  from  the  crown  is  45°,  which  quantity 
is  termed  the  angle  of  rupture. 

Any  increase  in  the  assumed  intensity  of  the  horizontal  com- 
ponents  increases  the  computed  value  of  the  angle  of  rupture. 
For  example,  if  the  quantities  in  the  next  to  the  last  column  of 
Table  59  be  doubled,  the  thrust  for  joint  7  will  be  the  maximum. 
Probably  this  condition  could  be  realized  by  tightly  tamping  the 
earth  spandrel-filling. 

Notice  that  the  preceding  discussion  of  the  position  of  the 
joint  of  rupture  is  for  a  uniform  stationary  load.  The  angle  of 
rui)ture  for  a  concentrated  moving  load  will  differ  from  the  results 
found  above;  but  the  mathematical  investigation  of  the  latter  case 
is  too  complicated  and  too  uncertain  to  justify  attempting  it. 

682.  In  discussions  of  the  position  of  the  joint  of  rupture,  the 
horizontal  components  are  usually  neglected.*  This  phase  of  the 
subject  will  be  considered  only  briefly.  The  following  is  the 
method  usually  employed  f  in  investigating  the  position  of  the  joint 
of  rupture,  and  is  based  on  the  assumption  that  the  crown  thrust  is 
correctly  given  by  equation  (6),  page  456. 

Let  ir=the  total  weight  resting  on  any  joint;  .t  =  the  hori- 
zontal distance  of  the  center  of  gravity  of  this  weight  from  the 
origin  of  moments;  and  //  =  the  arm  of  the  crown  thrust.  Then 
equation  (6)  becomes 

^=T" (') 

*So  far  as  observed,  Rankine's  investigation  is  the  only  exception;  and  it  is,  in 
fact,  only  an  apparent  exception  (see  paragraph  2,  page  490). 

tFor  example,  see  Sonnet's  Dictionnaire  des  Mathdmatique  Appliqudes,  pp. 
1084-85. 


^ -     (9) 


462  ARCHES.  ^[CHAP.    XVIIL 

To  determine  the  condition  for  a  maximum,  it  is  assumed  that  W, 
X,  and  i/  are  independent  variables.     Differentiating  equation  (8), 

dy  ~  y      dij  y^  ' 

but  d{  Wx)  =  Wdx  -\-  dW .  ^ dx  =  Wdx,  and  then 

dT_^dx_  Wx 

dy  ~   y  dy        y 

Hence  the  condition  for  a  maximum  crown  thrust  is 

'^=^- •  .  (10) 

dy      y  ^     ' 

The  usual  interpretation  of  equation  (10)  is:  "  The  joint  of  rup- 
ture is  that  joint  at  Avhich  the  tangent  to  the  intrados  passes 
through  the  intersection  of  T  and  the  resultant  of  all  the  vertical 
forces  above  the  joint  in  question." 

The  position  of  the  joint  of  rupture  can  be  found  by  the  above 
principle  only  by  trial.  This  method  possesses  no  advantage  over 
the  one  explained  in  the  preceding  section,  and  is  less  convenient  to 
apply.  The  preceding  investigation  is  approximate  for  the  following 
reasons:  1.  The  effect  of  the  horizontal  forces  is  omitted.  2.  ]]\ 
X,  and  ?/are  dependent  variables,  and  not  independent  as  assumed. 
3.  In  the  interpretation  of  equation  (10),  instead  of  "the  tangent 
to  the  intrados,"  should  be  employed  the  tangent  to  the  line  of 
resistance. 

In  applying  this  method,  a  table,  computed  by  M.  Petit,  which 
gives  the  angle  of  rupture  in  terms  of  the  ratio  of  the  radii  of  the 
intrados  and  the  extrados,  is  generally  employed.  The  table  in- 
volves the  assumption  that  a,  Fig.  121  (p.  455),  is  in  the  extrados 
and  b  in  t]ie  intrados;  and  also  that  the  intrados  and  extrados  are 
parallel  According  to  this  table,  '' a  semi-circular  arch  of  which 
the  thickness  is  uniform  throughout  and  equal  to  the  span  divided 
by  seventeen  and  a  half  is  the  thinnest  or  lightest  arch  that  can 
stand.  A  thinner  arch  would  be  impossible."  If  the  line  of  re- 
sistance is  restricted  to  the  middle  third,  then,  according  to  this 
theory,  the  thinnest  semi-circular  arch  which  can  stand  is  one 
whose  span  is  five  and  a  half  times  the  uniform  thickness.     Many 


AET.   1.]  THEORY   OF   THE    ARCH.  463 

arches  in  which  the  thickness  is  much  less  than  one  seventeenth 
of  the  span  stand  and  carry  heavy  loads  without  showing  any  evi- 
dence of  weakness.  For  example,  in  arch  No.  26  of  Table  63  (pp. 
502-3),  which  is  frequently  cited  as  being  a  model,  the  average  thick- 
ness is  3.25  ft.,  or  about  one  twenty-JiftJi  of  the  sj^an;  and  since  no 
joints  open,  the  line  of  resistance  must  lie  in  the  middle  third, 
even  though  the  thickness  is  only  one  fifth  of  that  required  by  the 
table.  Owing  to  the  approximations  involved,  and  also  to  the  limi- 
tations to  arches  having  intrados  and  extrados  parallel,  the  ordi- 
nary tables  for  the  position  of  the  joint  of  rupture  have  little,  if 
any,  practical  value.  The  only  satisfactory  way  to  find  the  angle 
of  rupture  is  by  trial  by  equation  (5),  as  explained  in  §  681. 

According  to  M.  Petit's  table,  if  the  thickness  is  one  fortieth  of 
the  diameter,  the  angle  of  rupture  is  46°  12';  if  the  thickness  is  one 
twentieth,  the  angle  is  53°  15';  and  if  one  tenth,  59°  41'. 

In  conclusion,  notice  that  the  investigations  of  both  this  and  the 
preceding  section  show  that  an  arch  of  more  than  about  90°  to  120° 
central  angle  is  impossible. 

683.  Winkler's  Hypothesis.  Prof.  Winkler,  of  Berlin,— a  well- 
known  authority — published  in  1879  in  the  "  Zeitschrift  des  Archi- 
telcten  unci  Ingenienr  Vereins  zu  Hannover,"  page  199,  the  follow- 
ing theorem  concerning  the  position  of  the  line  of  resistance:  "For 
an  arch  ring  of  constant  cross  section  that  line  of  resistance  is 
approximately  the  true  one  Avhich  lies  nearest  to  the  axis  of  the 
arch  ring,  as  determined  by  the  method  of  least  squares."  * 

The  only  proof  of  this  theorem  is  that  by  it  certain  conclusions 
can  be  drawn  from  the  voussoir  arch  which  harmonize  with  the 
accepted  theory  of  solid  elastic  arches.  The  demonstration  de- 
pends upon  certain  assumptions  and  approximations,  as  follows: 
1.  It  is  assumed  that  the  external  forces  acting  on  the  arch  are 
vertical;  whereas  in  many  cases,  and  perhaps  in  most,  they  are 
inclined.  2.  The  loads  are  assumed  to  be  uniform  over  the  entire 
span  ;  whereas  in  many  cases  the  arch  is  subject  to  moving  con- 
centrated loads,  and  sometimes  the  permanent  load  on  one  side  of 
the  arch  is  heavier  than  that  on  the  other.  3.  It  is  assumed  that 
the  load  included  between  the  lines  PGD  and  NHC,  Fig.  122 
(page  458),  is  equal  in  all  respects  to  that  included  between  PG2 

*  This  theorem  was  first  brought  to  the  attention  of  American  readers  in  1880,  by 
Professor  Swain  in  an  article  in  Van  Nostrand's  Engln'g  Mag.,  vol.  xxiii,  pp.  26.5-74 


464  ARCHES.  [chap.   XVIII. 

and  NIIl.  The  error  thus  involved  is  inappreciable  at  the  crown, 
but  at  the  springing  of  semicircular  arches  is  considerable.  4.  The 
conclusions  drawn  from  the  voussoir  (masonry)  arch  only  approxi- 
mately agree  with  the  theory  of  elastic  (solid  iron  or  wood)  arches. 
5.  Masonry  arches  do  not  ordinarily  have  a  constant  cross  section 
as  required  by  the  above  theorem;  but  it  usually,  and  properly, 
increases  toward  the  springing.  6.  The  phrase  "  as  determined  by 
the  method  of  least  squares "  means  that  the  true  line  of  resist- 
aiice  is  that  for  which  the  sum  of  the  squares  of  the  vertical 
deviations  is  a  minimum.  Since  the  joints  must  be  nearly  perpen- 
dicular to  the  line  of  resistance,  the  deviations  should  be  measured 
normal  to  that  line.  For  a  uniform  load  over  the  entire  arch,  the 
lines  of  resistance  are  comparatively  smooth  curves;  and  hence,  if 
the  sum  of  the  squares  of  the  vertical  deviations  is  a  minimum, 
that  of  the  normal  also  Avould  probably  be  a  minimum.  But  for 
eccentric  or  concentrated  loads  it  is  by  no  means  certain  that  such  a 
relation  would  exist.  7.  The  degree  of  approximation  in  this  theorem 
is  le.-s  the  flatter  the  arch. 

684.  To  apply  Winkler's  theorem,  it  is  necessary  to  (1)  con- 
struct a  line  of  resistance,  (2)  measure  its  deviations  from  the  axis, 
and  (3)  compute  the  sum  of  the  squares  of  the  deviations;  and  it  is 
then  necessary  to  do  the  same  for  all  possible  lines  of  resistances, 
the  one  for  which  the  sum  of  the  squares  of  the  deviations  is  least 
being  the  "  true"  one. 

Instead  of  applying  Winkler's  theorem  as  above,  many  writers 
employ  the  following  principle,  which  it  is  asserted  follows  directly 
from  that  theorem:  "If  any  line  of  resistance  can  be  constructed 
within  the  middle  third  of  the  arch  ring,  the  true  line  of  resistance 
lies  within  the  same  limits,  and  hence  the  arch  is  stable."  This 
assertion  is  disputed  by  Winkler  himself,  who  says  it  is  not,  in  geu- 
.eral,  correct.*  It  does  not  necessarily  follow  that  because  one  line 
of  resistance  lies  within  the  middle  third  of  the  arch  ring,  the 
■"  true"  line  of  resistance  also  does;  for  the  "  true"  line  may  coin- 
cide very  closely  with  the  axis  in  one  part  of  the  arch  ring  and 
•  depart  considerably  from  it  in  another  part,  and  still  the  sum  of  the 
squares  of  the  deviations  be  a  minimum.  This  method  of  applying 
"Winkler's  theorem  is  practically  nothing  more  or  less  than  an  appli- 

*  Prof.  Swain's  review  of  Winkler's  Theorem — Van  Nostrand's  Engineering  Magar 
zine,  vol.  xxiii.  p.  275. 


ART.    1.]  THEORY   OF   THE   ARCH.  465 

cation  of   the   conclusions  derived  from   the   hypothesis  of   least 
resistance  (§  677). 

685.  Navier's  Principle.  It  is  well  known,  from  the  principles 
of  fluid  pressure,  that  the  tangential  thrust  at  any  point  of  a  circle 
pressed  by  normal  forces  is  equal  to  the  pressure  per  unit  of  area 
multiplied  by  the  radius.  "  The  condition  of  an  arch  of  any  form 
at  any  point  where  the  pressure  is  normal  is  similar  to  that  of  a  cir- 
cular rib  of  the  same  curvature  under  a  normal  pressure  of  the  same 
intensity;  and  hence  the  following  principle:  tlie  thrust  at  any 
nor laalhj  pressed  point  of  a  linear  arch  is  the  product  of  the  radius 
of  curvature  by  the  intensity  of  the  pressure  at  that  point.  Or, 
denoting  the  radius  of  curvature  by  p,  the  normal  pressure  per 
unit  of  length  of  intrados  by  ^;,  and  the  thrust  by  T,  we  have 

T=pp:' (11) 

The  above  relation,  due  originally  to  Navier,  has  in  itself  nothing 
to  do  with  the  position  of  the  line  of  resistance;  but  is  employed  by 
writers  who  assume  that  an  arch  is  stable  if  a  line  of  resistance  can 
be  drawn  anywhere  within  the  middle  third  of  the  arch  ring,  to 
determine  the  crown  thrust.  Xotice,  however,  that  under  these 
conditions  the  radius  of  curvature  is  known  only  within  limits.  An 
example  of  its  application  will  be  referred  to  later  (§  704;  and  8, 
§  705; — pp.  482  and  486  respectively). 

686.  Theories  of  the  Arch.  Various  theories  have  been 
proposed  from  time  to  time,  which  differ  greatly  in  the  fundamental 
principles  involved.  Unfortunately,  the  underlying  assumptions 
are  not  usually  stated ;  and,  as  a  rule,  the  theory  is  presented  in  such 
a  wav  as  to  lead  the  reader  to  believe  that  each  particular  method 
''is  free  from  any  indeterminateness,  and  gives  results  easily  and 
accurately."  Every  theory  of  the  masonry  arch  is  approximate, 
owmg  to  the  uncertainty  concerning  the  amount  and  distribution 
of  the  external  forces  (§  666).  to  the  indeterminateness  of  the  posi- 
tion of  the  true  line  of  resistance  (§§  676-85),  to  the  neglect  of  the 
influence  of  the  adhesion  of  the  mortar  and  of  the  elasticity  of  the 
material,  and  to  the  lack  of  knowledge  concerning  the  strength  of 
masonry;  and,  further,  the  strains  in  a  masonry  arch  are  indeter- 
minate owincr  to  the  effect  of  variations  in  the  material  of  which  the 


466  ARCHES.  [chap.  XVIII. 

arch  is  composed,  to  the  effect  of  imperfect  workmanship  in  dress- 
ing and  bedding  the  stones,  to  the  action  of  the  center — its  rigidity, 
the  method  and  rapidity  of  striking  it, — to  the  spreading  of  the 
abutments,  and  to  the  settling  of  the  foundations.  These  elements 
are  indeterminate,  and  can  never  be  stated  accurately  or  adequately 
in  a  mathematical  formula  ;  and  hence  any  theory  can  be  at  best 
only  an  approximation.  The  influence  of  a  variation  in  any  one  of 
these  factors  can  be  approximated  only  by  a  clear  comprehension  of 
the  relation  which  they  severally  bear  to  each  other  ;  and  hence  a 
thorough  knowledge  of  theoretical  methods  is  necessary  for  the. 
intelligent  design  and  construction  of  arches. 

A  few  of  the  most  important  theories  will  now  be  stated,  and 
the  fundamental  principles  involved  in  each  explained. 

687.  To  save  repetition,  it  may  be  mentioned  here,  once  for  all, 
that  every  theory  of  the  arch  is  but  a  method  of  veritication.  The 
first  step  is  to  assume  the  dimensions  of  the  arch  outright,  or  to 
make  them  agree  with  some  existing  arch  or  conform  to  some  em- 
pirical formula.  The  second  step  is  to  test  the  assumed  arch  by  the 
theory,  and  then  if  the  line  of  resistance,  as  determined  by  the 
theory,  does  not  lie  within  the  prescribed  limits — usually  the  middle 
third, — the  depths  of  the  voussoirs  must  be  altered,  and  the  design 
must  be  tested  again. 

688.  Rational  Theory.  The  following  method  of  determining 
the  line  of  resistance  is  based  ujoon  the  hypothesis  of  least  crown 
thrust  (§  678),  and  recognizes  the  existence  of  the  horizontal  com- 
ponents of  the  external  forces.  Unfortunately,  the  results  found 
by  this  method,  as  well  as  those  by  all  others,  are  rendered  some- 
what uncertain  by  the  indeterminateness  of  the  external  forces 
(§  666). 

689.  Symmetrical  Load.  General  Solution.  As  an  example 
of  the  application  of  this  theory,  let  us  investigate  the  stability  of 
the  serai-arch  shown  in  Fig.  125  (page  467).  The  first  step  is  to 
determine  the  line  of  resistance.  The  maximum  crown  thrust  was 
computed  in  Table  59  (page  459),  as  already  explained  (§  681). 
To  construct  the  force  diagram,  a  line  7?0  is  drawn  to  scale  to 
represent  the  maximum  thrust  as  found  in  the  fourth  line  of  the 
last  column  of  Table  59.  From  0,  w,  is  laid  off  vertically  upwards  ; 
and  from  its  extremity,  li^  is  laid  off  horizontally  to  the  left.  Then 
the  line  from  0  to  the  left-hand  extremity  of  7i,  (not  shown  in  this 


ART.   l.J 


EATIOXAL   THEORY    OF   THE   ARCH. 


467 


particular  case)  represents  the  direction  and  amount  of  the  external 
force  F^  acting  upon  the  first  division  of  the  arch  stone ;  and  the 
line  7?j  from  B  to  the  upper  extremity  of  F^  represents  the  resultant 
pressure  of  the  first  arch  stone  upon  the  one  next  below  it.  Simi- 
larly, lay  off  u\  vertically  upwards  from  the  left-hand  extremity  of 
//,,  and  lay  off  h^  horizontally  to  the  left;  then  a  line  F^  from  the 
upper  end  of  u\  to  the  left-hand  end  of  li^  represents  the  resultant 
of  the  external  forces  acting  on  the  second  divisions  of  the  arch, 
and  a  line  E^  from  the  upper  extremity  of  F„  represents  the  resultant 
pressure  of  the  second  arch  stone  on  the  third.  The  force  diagram 
is  completed  by  drawing  lines  to  represent  the  other  values  of 
ii\  //,  F^  and  the  corresponding  reactions. 


Fig.  125. 

In  f  he  diagram  of  the  arch,  the  points  in  which  the  horizontal 
«nd  vertical  forces  acting  upon  the  several  arch  stones  intersect,  are 
marked  7, ,  g„ ,  etc.,  respectively  ;  and  the  oblique  line  through  each 
of  these  points  shows  the  direction  of  the  resultant  external  force 
acting  on  each  arch  stone. 

To  construct  tlie  line  of  resistance,  draw  through  U — the  upper 


468  ARCHES.  [chap.  XVIII. 

limit  of  the  middle  third  of  the  crown  joint — a  horizontal  line  to  an 
intersection  with  the  oblique  force  through  g^  ;  and  from  this  point 
draw  a  line  parallel  to  R^ ,  and  prolong  it  to  an  intersection  with  the 
oblique  force  through  g^.  In  a  similar  manner  continue  to  the 
springing  line.  Then  the  intersection  of  the  line  parallel  to  i?, 
with  the  first  joint  gives  the  center  of  pressure  on  that  joint ;  and 
the  intersection  of  R^  with  the  second  joint  gives  the  center  of 
pressure  for  that  joint, — and  so  on  for  the  other  joints.  Each 
center  of  pressure  is  marked  by  a  circular  dot.  A  line  connecting 
these  centers  of  pressure  would  be  the  line  of  resistance;  but  the 
line  is  not  shown  in  Fig.  125. 

690.  The  next  step  is  to  determine  the  degree  of  stability. 

1.  Since  the  line  of  resistance  lies  within  the  middle  third  of  the 
arch  ring,  and  touches  the  inner  limit  of  that  third  at  two  jjoiuts 
and  its  outer  limit  at  an  intermediate  and  higher  point,  the  factor 
against  rotation  is  3  (see  §  6G9). 

2.  The  unit  working  pressure  is  found  by  applying  equation  (2), 

2  W 
page  448.     At  the  crown,  d  ■=^  \l,  and  hence  P  =  -^—  ;   or,  since 

W  =  9,400  pounds  and  I  =  1.25  feet,  P  =  15,040  pounds  per  square 
foot  =  104  pounds  per  square  inch.  At  the  springing,  W  =  21,700 
pounds,  /  =  4,5  feet,  and  d  —  0.10  feet ;  and  therefore 

p       21.700    ,    6  X  21,700X0.10       .  q^.    ,   . .„       .  ..^ 
P  =  -~^-^-  H ^y =  4,820  +  643  =  5,463. 

That  is,  F  =  5,463  pounds  per  square  foot,  or  38  pounds  per  squar^ 
inch.  Except  for  a  particular  kind  of  stone  and  a  definite  quality 
of  masonry,  it  is  impossible  even  to  discuss  the  probable  factor  of 
safety  ;  but  it  is  certain  that  in  this  case  the  nominal  factor  is 
excessive  (see  §  223),  while  the  real  factor  is  still  more  so  (see 
§§  G71-72). 

If  the  maximum  pressure  at  the  most  compressed  joint  had  been 
more  than  the  safe  bearing  power  of  the  masonry,  it  would  have 
been  necessary  to  increase  the  depth  of  the  arch  stones  and  repeat 
the  entire  process.  Notice  that  the  total  pressure  on  the  joints 
increases  from  the  crown  toward  springing,  and  that  hence  the 
depth  of  the  arch  stones  also  should  increase  in  the  same  direc- 
tion. 

3.  To  determine  the  degree  of  stability  against  sliding,  notice 


ART.   1.]  RATIONAL   THEORY.  46& 

that  the  angle  between  the  resultant  pressure  on  any  joint  and 
the  joint  is  least  at  the  springing  joint ;  and  hence  the  stability 
of  this  joint  against  sliding  is  less  than  that  for  any  other.  The 
nominal  factor  of  safety  is  equal  to  the  co-efficient  of  friction 
divided  by  tan  (90°  —  72°)  =  tan  18°  =  0.33.  An  examination  of 
Table  36  (page  315)  shows  that  when  the  mortar  is  still  wet  the 
co-efficient  is  at  least  0.50  ;  and  hence  the  nominal  factor  for  the 
joint  in  question  is  at  least  1^,  and  probably  more,  wliile  the  real 
factor  is  still  greater.  The  nominal  factor  for  joint  7  is  at  least  34, 
and  that  for  joint  3  is  about  5.  There  is  little  or  no  probability  tiiat 
an  arch  will  be  found  to  be  stable  for  rotation  and  crushing,  and 
unstable  for  sliding.  If  such  a  condition  should  occur,  the  direc- 
tion of  the  assumed  joint  could  be  changed  to  give  stability.*  The 
actual  joints  should  be  as  nearly  perpendicular  to  the  line  of  resist- 
ance as  is  consistent  with  simplicity  of  workmanship  and  with 
stability.  For  circular  arches,  it  is  ordinarily  sufficient  to  make  all 
the  joints  radial.  In  Fig.  125,  the  joints  are  radial  to  the  intrados  ; 
but  if  they  had  been  made  radial  to  the  extrados  or  to  an  intermedi- 
ate curve,  the  stability  against  sliding,  particularly  at  the  springing 
joint,  would  have  been  a  little  greater. 

691.  Special  Solution.  The  folloAving  entirely  graphical  solution 
is  useful  when  it  is  desired  to  find  a  line  of  resistance  which  will 
pass  through  two  predetermined  points. 

For  example,  assume  that  it  is  desired  to  pass  a  line  of  resistance 
through  ?7and  «,  Fig.  126  (page  470),  the  former  being  the  upper 
extremity  of  the  middle  third  of  the  crown  joint  and  the  latter  the 
inner  extremity  of  the  middle  third  of  joint  4. 

The  value  and  positions  of  the  external  forces,  which  are  the 
same  as  those  employed  in  Fig.  125,  are  given  in  Table  59  (page 
459).  Construct  a  load  line,  as  shown  in  the  force  diagram,  hy 
laying  off  iv^  and  h^ ,  and  w^  ^^^  ^'2  ?  ^tc,  in  succession,  and  drawing 
F^,  F^,  etc.  Since  the  load  is  symmetrical,  we  may  assume  that  the 
thrust  at  the  crown  is  horizontal ;  and  hence  we  may  choose  a  pole 
at  any  point,  say  P',  horizontally  opposite  0.  Draw  lines  from  F' 
to  the  extremities  of  F^,  F^,  etc.  Construct  a  trial  equilibrium 
polygon  by  drawing  through  CT^aline  parallel  to  the  line  F'O,  of 
the  force  diagram,  and  prolong  it  to  b  where  it  intersects  F^ .     From 

*  Strictly  any  change  in  the  direction  of  the  joints  will  necessitate  a  recomputatioa 
of  the  entire  problem  ;  but,  except  in  extreme  cases,  such  revision  is  unnecessary. 


470 


ARCHES. 


[chap.   XVIII. 


b  draw  a  line  he  parallel  to  R\  of  the  force  diagram  ;  from  r,  the 
point  where  he  intersects  the  line  of  F^,  draw  a  line  c d  parallel  to 
R\  ;  from  d,  the  point  where  c  d  intersects  F^ ,  draw  a  line  d  e 
parallel  to  R\ ;  and  from  e,  the  point  where  d  e  intersects  F^ ,  draw 
a  line  ef  parallel  to  R\ .     Prolong  the  line  fe  to  r/,  the  point  in 


Fig.  1-X. 

which  it  intersects  the  prolongation  of  Ub  ;  and  then,  by  the  prin- 
ciples of  graphical  statics,  ^  is  a  point  on  the  resultant  of  the  forces 
F^,F,,  F^,  and  F^. 

The  section  of  the  arch  from  the  crown  joint  to  joint  4  is  at 
rest  under  the  action  of  the  crown  thrust  7\  the  resultant  of  the 
external  forces,  and  the  reaction  of  joint  4.  Since  the  first  two 
intersect  at  g,  and  since  it  has  been  assumed  that  the  center  of 
pressure  for  joint  4  is  at  a — the  inner  extremity  of  the  middle  third, 
— a  line  ag  must  represent  the  direction  of  the  resultant  reaction  of 
joint  4;  and  hence  the  line  R^,  in  the  force  diagram  drawn  from 
the  upper  extremity  of  F^ ,  parallel  to  a  g,  to  an  intersection  with 
P'  0,  reprasents,  to  the  scale  of  the  load  line,  the  amount  of  the 
reaction  of  joint  4.  Then  PO,  to  the  same  scale,  represents  the 
crown  thrust  corresponding  to  the  line  of  resistance  passing  through 
U  and  a  ;    and  a  line — not  shown  in  Fig.  12G^from   the  upper 


ART.  1.]  RATIONAL   THEORY.  471 


eAtremity  of  F^  to  the  lower  extremity  of  F^ ,  would  represent,  in 
both  direction  and  amount,  the  resultant  of  F^,  F^,  F^,  and  F^ . 

Having  found  the  thrust  at  the  crown,  complete  the  force  dia- 
gram by  drawing  the  lines  E^,  R^,  R^,  etc.  ;  and  then  construct  a 
new  equilibrium  polygon  exactly  as  was  described  above  for  tlie 
trial  equilibrium  polygon.  The  construction  may  be  continued  to 
the  springing  line.  Tlie  equilibrium  polygon  shown  in  Fig.  126  by 
a  solid  lino  was  obtained  in  this  way. 

The  amount  of  the  pressure  on  any  Joint  is  given  by  the  length 
of  the  corresponding  ray  in  the  force  diagram.  The  points  in  which 
the  sides  of  tne  equilibrium  polygon  cut  the  joints  are  the  centers 
of  pressure  on  the  respective  joints.  The  stability  of  the  arch  may 
be  discussed  as  in  §  690. 

692.  One  of  the  most  useful  applications  of  the  method  described 
in  the  preceding  section  is  in  determining  the  line  of  resistance  for 
a  segmental  arch  having  a  central  angle  so  small  as  to  make  it 
obvious  that  the  joint  of  rupture  (§§  680-81)  is  at  the  springing. 

For  example,  assume  that  it  is  required  to  draw  the  line  of 
resistance  for  the  circular  arch  shown  in  Fig.  127  (p.  472).  The  span 
is  50  feet,  the  rise  10  feet,  che  depth  of  voussoirs  2.5  feet,  and  the 
height  of  the  earth  above  the  summit  of  the  arch  ring'is  10  feet. 
The  angular  distance  of  the  springing  from  the  crown  is  43°  45' ; 
and  since  the  angle  of  rupture  is  nearly  always  more  than  45°,  it  is 
safe  to  assume  that  the  joint  of  rupture  is  at  the  springing. 

The  method  of  determining  tlie  line  of  resistance  is  the  same 
as  that  explained  in  §  691,  and  is  sufficiently  apparent  from  an 
inspection  of  Fig.  127. 

693.  Unsymmetrical  Load.  The  design  for  an  arch  ring 
should  not  be  considered  perfect  until  it  is  found  that  the  criteria 
of  safety  (§§  668-75)  are  satisfied  for  the  dead  load  and  also  for 
■every  possible  position  of  the  live  load.  A  direct  determination  of 
the  line  of  resistance  for  an  arch  under  an  unsymmetrical  load  is 
impossible.  To  find  the  line  of  resistance  for  an  arch  under  a 
symmetrical  load,  it  was  necessary  to  make  some  assumption  con- 
cerning (1)  the  amount  of  the  thrust,  (2)  its  point  of  application, 
and  (3)  its  direction  ;  but  when  the  load  is  unsymmetrical,  Ave 
neither  know  any  of  these  items  nor  can  make  any  reason;ible 
iiypothesis  by  which  they  can  be  determined.  For  an  unsymmetri- 
cal load  we  know  nothing  concerning  the  position  of  the  joint  of 


472 


AECHES. 


[chap.  XVIII. 


rupture,  and  know  that  the  thrust  at  the  crown  is  neither  horizontal 
nor   applied   at   one   third  of    the    deptli  of    that  joint    from  the 


Fig.  127 


crown  :  and  hence  the  preceding  methods  can  not  be  employed. 
When  the  load  is  not  symmetrical,  the  following  method  may  be 
employed  to  find  a  line  of  resistance  ;  but  it  gives  no  indication  as 
to  which  of  the  many  possible  lines  of  resistance  is  the  true  one. 

Let  it  be  required  to  test  the  stability  of  a  symmetrical  arch  hav- 
ing a  uniform  live  load  covering  half  the  span.  Divide  the  arch  and 
its  load  into  sections,  as  shown  in  Fig.  128.  The  live  load  is  a  ver- 
tical force,  and  the  earth  pressure  Avould  give  a  horizontal  compo- 
nent. The  approximate  reduced-load  contour  for  the  vertical  forces 
is  shown  in  Fig.  128,  and  the  horizontal  and  vertical  components 
are  laid  off  in  the  force  diagram.  An  equilibrium  polygon  can  be 
made  to  pass  through  any  three  points  ;  and  therefore  we  may  as- 
sume three  points  for  a  trial  equilibrium  polygon, — as,  for  example, 
(1)  the  lower  limit  of  the  middle  third  of  the  joint  at  the  abutment 
A,  (2)  the  middle,  C,  of  the  crown  joint,  and  (8)  the  upper  limit 
of  the  middle  third  of  the  joint  at  B. 


ART.  1.] 


RATIONAL  THEORY. 


473 


Construct  a  force  diagram  by  laying  off  the  external  forces  suc- 
cessively from  0  in  the  usual  way  (§  689),  selecting  a  pole,  P',  at  any 
point,  and  drawing  lines  connecting  F'  with  the  points  of  division 
of  the  load  line.  Then,  commencing  at  A,  construct  an  equilib- 
rium polygon  through  A,  C ,  and  B' ,  by  the  method  explained  in 
§§  091-92. 

It  is  then  necessary  to  move  the  pole  of  the  force  diagram  in 
such  a  way  that  the  equilibrium  polygon  will  pass  through  b  instead 
of  B'.  To  do  this,  draw  a  line  through  the  pole  P' ,  parallel  to  A  B' 
— the  closing  line  of  the  trial  equilibrium  poh'gon, — and  then 
tlirough  H — the  intersection  of  the  preceding  line  with  the  load 
line— draw  HP  parallel  to  AB.     The  new  pole,  P.  is  at  a  point 


Fig.  128. 

on  this  line  such  that  HP  i.s  to  the  horizontal  distance  from  P'  to 
the  load  line  as  CD'  is  to  CD.  From  P  draw  lines  to  the  points 
of  division  of  the  load  line,  and  then  construct  an  equilibrium 
polygon  through  A,  C,  and  B.  If  the  resulting  line  of  resistance 
does  not  lie  within  the  middle  third,  try  some  other  position  of  the 
three  points  A,  G,  and  B  instead  of  as  above.  If  a  line  of  resistance 
can  not  be  drawn  (see  §  694)  within  the  prescribed  limits,  then  the 
section  of  the  arch  ring  must  be  changed  so  as  to  include  tlie  line 
of  resistance  within  the  limits. 

694.  Criterion.  If  the  line  of  resistance,  when  constructed  by 
any  of  the  preceding  methods,  does  not  lie  within  the  middle  thn-d 
of  the  arch  ring,  the  following  process  may  be  employed  to  deter- 
mine whether  it  is  possible,  or  not,  to  draw  a  line  of  resistance  in 
ihe  middle  third. 

Assume,  for  example,  that  the  line  of  resistance  of  Fig.  129  lies 


474  ARCHES.  [chap,  xviii. 

outside  of  the  middle  third  at  a  and  I,  Next  draw  a  line  of  resist- 
ance through  c  and  d,  the  points  where 
normals  from  a  and  h  intersect  the  outer 
and  inner  boundary  of  the  middle  third 
respectively.  To  pass  a  line  of  resistance 
through  c  and  d,  it  is  necessary  to  deter- 
mine the  value  and  point  of  application  of 
the  corresponding  crown  thrust.  The 
condition  which  makes  the  line  of  resist- 
ance pass  through  c  is:  the  thrust  multi- 
FiQ.  129.  PLIED  BY  the  vertical  distance  of  its  point 

of  application  above  c  is  equal  to  the  load  on  the  joint  at  c  multi- 
plied BY  its  horizontal  distance  from  c.  The  condition  that  makes 
the  line  of  resistance  pass  through  d  is:  the  thrust  multiplied 
BY  the  sum  of  the  distance  its  point  of  application  is  above  c  and 
of  the  vertical  distance  between  c  and  d  is  equal  to  the  load  on 
the  joint  at  d  multiplied  by  its  horizontal  distance  from  d.  These 
conditions  give  two  equations  which  contain  two  unknown  quanti- 
ties— the  thrust  and  the  distance  its  point  of  application  is  above  c. 
After  solving  these  equations,  the  line  of  resistance  can  be  drawn 
by  any  of  the  methods  already  explained. 

If  this  new  line  of  resistance  lies  entirely  within  the  prescribed 
limits,  it  is  plain  that  it  is  possible  to  draw  a  line  of  resistance 
therein ;  but  if  the  second  line  does  not  lie  within  the  prescribed 
limits,  it  is  not  at  all  probable  that  a  line  of  resistance  can  be  drawn 
therein.  The  possibility  of  finding,  by  a  third  or  subsequent  trial, 
a  line  of  resistance  within  the  limits  can  not,  in  general,  be  answered 
definitely,  since  such  a  possibility  depends  upon  the  form  of  the 
section  of  the  arch  ring. 

If  the  line  of  resistance  drawn  through  TJ  and  V  goes  outside  of 
the  arch  ring  beyond  the  extrados  only,  as  at  a,  the  second  line  of 
resistance  should  be  drawn  through  c  and  F;  and  if,  on  the  other 
hand,  it  goes  outside  below  the  intrados  only,  as  at  Z*,  the  second 
line  should  be  drawn  through  TJ  and  d. 

695.  SCHEFFLER'S  THEORY.*  This  theory  is  the  one  most  fre- 
quently employed.  It  is  based  upon  the  hypothesis  of  least  crown 
thrust  (§§  678-82),  and  assumes  that  the  external  forces  are  vertical. 

*  See  the  second  foot-note  page  455, 


ART.  1.] 


SCHEFFLER  S   THEORY. 


475 


This  theory  is  frequently  referred  to  as  assuming  that  the  arch 
stones  are  incompressible;  but,  fairly  considered,  such  is  not  the 
case.  Dr.  Scheffler  develops  the  theory  of  the  position  of  the  line 
of  pressures  for  incompressible  voussoirs;  but  subsequently  states 
that  the  compressibiliiy  of  the  arch  stones  causes  the  line  of  resist- 
ance to  retreat  within  the  arch  ring  at  points  where  it  would  other- 
wise reach  the  edge.  He  also  says  that,  if  a  line  of  resistance  can 
be  drawn  within  the  arch  ring,  that  nowhere  approaches  nearer  the 
edges  of  the  joint  than  one  fourth  of  its  depth,  the  stability  of  the 
arch  is  assured. 

This  theory  will  be  illustrated  by  two  examples. 

696.  First  Example.  Assume  that  it  is  required  to  determine, 
in  accordance  with  this  theory,  the  line  of  resistance  for  the  circular 
segmental  arcli  shown  in  Fig.  130.     The  span  is  50  feet,  and  th** 


Fig.  130. 

rise  is  10  feet.  The  voussoirs  are  2  feet  6  inches  deep,  and  the 
spandrel  wall  rises  2  feet  10  inches  above  the  summit  of  the  arch 
ring.  In  this  example  we  will  follow  the  explanation  used  by 
Scheffler.* 

The  first  step  is  to  find  the  amount  and  the  point  of  application 
of  the  resultant  of  the  external  forces  acting  on  the  portion  of  the 
arch  above  the  successive  joints.  Divide  the  semi-arch  and  the 
spandrel  wall  into  any  convenient  number  of  parts  by  vertical  lines 

*  Cain's  "  Practical  Theory  of  the  Arch,"  pp.  38-44. 


476 


AECHES. 


[chap.   XVIII. 


through  F,  G,  H,  I,  J,  and  K,  as  shown.  The  positions  of  the  act- 
ual joints  are  assumed  to  be  not  yet  fixed;  but,  for  temporary  pur- 
poses, assume  radial  joints  to  be  drawn  through  F,  G,  H,  I,  J, 
and  A".  Then  the  load  on  any  part  of  the  arch  is  assumed  to  be 
proportional  to  the  area  above  it, — for  example,  the  load  on  CHGR 
is  assumed  to  be  proportional  to  the  area  CNPD.* 

Having  determined  the  area  representing  the  loads,  it  is  then 
necessary  to  determine  (1)  the  numerical  values  of  the  several  loads 
and  the  distances  of  their  centers  of  gravity  from  a  vertical  through 
the  crown,  and  (2)  the  amount  and  the  position  of  the  center  of 
gravity  of  the  loads  above  any  joint.  The  steps  necessary  for  this 
are  given  in  Table  60. 

The  quantities  in  column  2  of  Table  GO  are  the  lengths  of  the 
medial   lines  of   the  several   trapezoids.     Column  6    contains   the 

*  Notice  that  reallj'  the  load  on  the  joint  SH,  for  example,  is  SHXPGR,  and  not 
CJ^PD  as  above.  The  error  is  least  near  the  crown  of  flat  segmental  arches,  and 
greatest  near  the  springing  of  semi-circular  ones.  The  error  could  be  eliminated  (1) 
by  finding  the  weights  of  GPNH  and  RGHS  separately  and  combining  them  into 
a  single  resultant  for  the  weight  on  the  joint  SH.  as  was  done  in  §681;  or  ('2)  by 
drawing  the  arch  to  a  large  scale  on  thick  paper  and  cutting  out  the  several  six-sided 
figures  which  represent  the  loads,  when  the  amounts  of  the  several  loads  can  be 
determined  readily  from  the  weights  of  corresponding  sections  of  the  paper,  and  the 
center  of  gravity  of  each  section  can  be  found  by  balancing  it  on  a  knife  edge. 

Scheffler  gives  the  following  empii-ical  and  approximate  method  of  altering  the 
position  of  the  joints  to  correct  this  error.  Let  I)CG,  Fig.  131,  be  the  side  of  the 
trapezoid,  and   CH  the  uncorrected  joint.    From  b,  the  middle  point  of  GH,  draw 


Fig.  131. 


Fig.  132. 


bD ;  and  draw  Gc  parallel  to  bD,  and  ch  parallel  to  CH.  Then  will  ch  be  the  corrected 
joint.  Conversely,  having  given  the  joint  CH,  Fig.  133,  to  find  the  sid«  of  the  trape- 
zoid which  limits  the  portion  of  the  load  upon  it,  through  Cdraw  DG  vertical,  and 
draw  Cii  parallel  to  Db  {b  being  the  middle  point  of  GH)  ;  then,  from  g,  draw  ay  ver- 
tical, and  5ve  have  the  desired  side  of  the  trapezoid. 


,-,KT.  1."] 


SCHEFFLER^S   THEORY. 


477 


TABLE  60. 

Application  of  Scheffler's  Theory  to  the  Arch  Ring   shown  in 
Fig.  130,  page  475. 


1 

2 

3 

4 

5 

6 

7 

8 

9 

0 

n 

f.  o 

H  ns  S 
o  W  « 

The  Amount,  and  Position  op  the 

Center  of  Gravity,  of  the 

Several  Loads 

To  find  the  Amount,  and  the  Center 

OP  Gravity,  of  the  Loads  above 

the  Several  Joints. 

i 

Dimensions  of  the  sections. 

Horizontal  distance 
of  centei'  of  gravity 
of  eacli  sectiou 
from  U. 

1       6 
1       S 

I      1^ 

1.1 

tn 

Is 
©3 

cS  o  1* 

|S3 
C  3  CS 

ill 

o  (u     .a   . 

C    OttJ  -    !0 

ScDO®a 

-D  ■"  .-S  ^  -^i 

Height. 

Width. 

Area. 

©    —  >;    C    3> 

'c  s  1-  *  u 

1 

2 
3 
4 

5 
6 

5.4 
6.1 
7.6 
9.8 
13.2 
14.5 

5 
5 
5 
5 
5 
1.75 

27.0 
30.5 
38.0 
49.0 
66  0 
25.4 

2.5 
7  5 
12.5 
17.5 
22.5 
25.9 

67. 50 
228  75 
475  00 
857.. 50 
1,485.00 
657.86 

27.0 
.57.5 
95  5 
]4t.5 
210.5 
2.35.9 

67.50 

296  25 

771.25 

1,628.75 

3,113.75    . 

3,771.61 

2  5 
5  1 

8.1 
11.3 
14.7 
16.0 

products  of  the  numbers  ill  columns  4  and  5.  Column  7  contains 
the  continued  sums  of  the  quantities  in  column  4.  Column  8  con- 
tains the  continued  sums  of  the  quantities  in  column  6.  Column  9 
is  found  by  the  principle  of  analytical  mechanics  :  the  distance 
of  the  center  of  parallel  forces  from  any  point  is  equal  to  tJie  sum 
of  the  moments  of  the  several  forces  about  that  point  divided  by 
the  sum  of  the  several  forces  ;  and  hence  the  numbers  in  column 
9  are  found  by  dividing  the  quantities  in  column  8  by  the  corre- 
sponding quantity  in  column  7. 

697.  The  second  step  is  to  find  the  minimum  thrust  which 
applied  at  U  (  UF  ^  ^  FE)  is  sufficient  to  prevent  the  semi-arch 
from  rotating.  The  origin  of  moments  is  considered  as  being  i?j 
the  successive  joints  at  one  third  of  the  depth  of  each  from  the 
intrados. 

li  T  =  the  thrust  and  y  =  its  arms,  and  W  =  the  load  above 
any  joint  and  x  =  its  arm,  then  for  equilibrium  about  any  joint 


T. 


Wx 


(12) 


It  is  required  to  find  the  maximum  value  of  T. 


478 


ARCHES. 


[chap.   XVIII. 


The  W — in  terms  of  the  weight  of  a  cubic  foot  of  the  masonry — 
for  each  joint  is  the  corresponding  number  in  column  7  of  Table  60, 
and  is  for  convenience  repeated  in  cohimn  2  of  the  table  below.  The 
X  for  each  joint  is  the  horizontal  distance  between  the  resultant  of 
the  load  above  each  joint  and  the  center  of  moments  for  that  joint; 
and  is  equal  to  the  horizontal  distance  from  U  to  the  points  1,  2, 
etc.,  minus  the  respective  quantities  in  column  9  of  Table  60. 
The  first  of  these  quantities  is  given  in  column  3  of  Table  61,  the 
second  in  column  4,  and  their  difference  in  column  5.  The  /y  for 
each  joint  is  given  in  column  6  of  Table  61.  The  value  of  the 
thrust,  obtained  by  substituting  the  above  data  successively  in  equa- 
tion (12)  and  solving,  is  given  in  column  7  of  Table  61. 

TABLE  61. 

Application  of  Scheffler's  Theory  to  the  Arch  Ring  shown  in 
Fig.  130,  page  475. 


1 

2         1 

3 

4 

5 

6 

7 

No.  OF  THE  .TOINT, 
COUNTING  FROM 
THE   ONK   NEXT  TO 

THE  Crown. 

i|ii 

Horizontal  dis- 
tance from  U  to 
1,2,  3,  etc.,  re- 
spectively. 

Horizontal  dis- 
tance from  [7  to 
the  center  of 
fjravity  of  the 
loads  above  the 
successive  joints. 

Arm  of  the  load 
about  the  center 
of  resistance  of 
the  successive 
joints  (  —  x). 

Arm  of  the  thrust 
about  the  center 
of  resistance  of 
each  joint  (  =  y) 

Horizontal  thrust 
required  to  pi'e- 
vent  rotation 
about  tlie  suc- 
cessive joints 

1 
2 
3 
4 
5 
6 

27.0 
57.5 
95.5 
144.5 
210.5 
235.9 

4.8 
9.6 
14.4 
19.2 
24.0 
25.6 

2.5 
5.1 
8.1 
11.3 
14.7 
16.0 

2.3 

4.5 
6.3 
7.9 
9.3 
9.6 

1.15 
2.09 
3.72 
6.16 
9.60 
11.00 

54.0 
123.6 
116.9 
185.3 
204  0 
205.9 

The  horizontal  thrust  for  joint  6  is  the  greatest,  and  hence  that 
joint  is  the  joint  of  rupture.  This  result  might  have  been  antici- 
pated, since  the  angle  of  rupture  ordinarily  varies  between  45^ 
and  60°  (see  last  paragraph  of  §  682,  page  463),  while  the  angular 
distance  of  joint  6  from  the  crown  is  only  43°  35'. 

698.   The  second  step  is  to  construct  the  line  of  resistance. 

To  find  the  center  of  pressure  on  joint  1,  Fig.  130,  page  475,  draw 

a  horizontal  line  through  U,  and  lay  off,  to  any  convenient  scale,  a 

distance  Ua  to  the  left  equal  to  the  first  quantity  in  column  4  of 

Table   61.      a  is  a  point  through  which  the  weight  of  DEQP* 

*  Assumed  to  be  equal  to  REQPO  (see  foot-note,  page  476). 


ART.    i    ,  SCHEFFLER's   THEORY.  479 

acts.  Lay  off,  vertically,  a  distance  ab  equal  to  the  first  quantity 
in  column  2  of  Table  61;  this  line  represents  the  weight  of  the  first 
voussoir  and  the  load  resting  upon  it.  From  h  lay  oflf,  horizontally 
to  the  right,  a  distance  he  equal  to  the  last  quantity  in  column  T  of 
Table  61.  This  line  represents  the  horizontal  pressure  at  the  crown. 
Then,  by  the  principle  of  the  triangle  of  forces,  a  line  ca  repre- 
sents the  resultant  pressure  on  the  joint  IIG\  and  this  line  pro- 
longed intersects  the  joint  RG  2X  d,  which  is,  therefore,  the  center 
of  pressure  on  that  joint. 

To  find  the  center  of  pressure  on  the  second  joint,  lay  off  from 
JJ,  horizontally  to  the  left,  a  distance  equal  to  the  second  quantity 
in  column  4  of  Table  61;  erect  a  vertical  equal  to  the  second  quan- 
tity in  column  2;  and  from  the  point  thus  found  lay  off,  horizon- 
tally to  the  right,  a  quantity  equal  to  the  last  quantity  in  column  7. 
Then  draw  the  third  side  of  the  triangle  of  forces,  and  prolong  it 
until  it  intersects  the  joint  at  e. 

By  a  similar  construction,  the  centers  of  pressure  for  the  several 
joints  are  determined  to  be  U,  d,  e,f,  g,  h,  and  6,  as  shown  in  Fig. 
130.  A  line  joining  these  points  is  the  line  of  resistance  (not  shown 
in  the  figure). 

699.  The  preceding  method  of  drawing  the  line  of  resistance 
has  two  advantages :  (1)  The  center  of  pressure  on  any  joint  may 
be  found  at  once;  and  (2)  any  small  error  in  draughting  is  confined 
to  the  joint  where  it  first  occurs.  Notice,  however,  that  the  method 
is  applicable  only  when  the  horizontal  component  of  the  pressure  on 
the  several  joints  is  constant;  that  is,  this  method  is  applicable  only 
when  the  external  forces  are  assumed  to  be  vertical. 

Having  determined  the  line  of  resistance  by  the  above  method, 
the  stability  of  the  arch  can  be  discussed  as  described  in  §  690. 

700.  Second  Example.  Let  us  construct,  according  to  this 
theory,  the  line  of  resistance  for  the  semi-arch  shown  in  Fig.  133, 
page  480,  which  is  the  same  one  discussed  in  §  681,  where  it  was 
shown  that  joint  4  is  the  joint  of  rupture,  and  that,  if  the  horizon- 
tal forces  be  disregarded,  the  maximum  crown  thrust  is  8,748 
pounds  (see  Table  59,  page  459). 

The  crown  thrust  is  laid  off,  to  any  convenient  scale,  from  S 
to  0  ;  and  the  loads  as  given  in  Table  59  are  laid  off,  to  the  same 
scale,    successively  from  0  downwards.      The    remainder    of    the 


480 


ARCHES. 


[chap.   XVIII. 


construction — shown    by   dash    lines — is   exactly   similar    to   that 
described  in  §  689  in  connection  with  Fig.  125,  page  467. 


I   ! 


I        I 
I         i 


tr      V 


!      A. 


I     ui 


h:-. 


I    ! 


14^ 


i  \    ^' 

i  / 

y  / 

a 

•  /         ''  / 

i  ;           /    / 

1 // 

9 

/  /  / 

/     ^'       / 

/    /        / 

'    /              /    ToporrooriNd 

10 

0 

C                 S  i 

•^a^'^^'^y'^/Vn 

^*^     ^  y^y^y/ I] 

lX<^^%f/  I 

■■''■■<> 

'^''yX/f      I 

yf/nl 

\.>^  ///a 

y  ///    /    / 

^■■'  ^/^ / 

//^■W  J    1     1 

A7  / 

A//  /  •■/ 

11        I 

'  Y  /    /   ■  / 

1              / 

H/.-/:    /l 

/    / 

'T/l      1  1 

'      / 

''4^  // 

h? 

:\-     1   / 

1 

i/^ 

/ 

7     .'    \    1 

/ 

^      ■■'■  l\/ 

/ 

'  •■  ■/'■'Y 

1 

•■ '    \ 

1 

■  /    i  v 

1 

//      i  \ 

•/        •    \    / 

:l              ■       \ 

i               ■■        \ 

>                :  7..   V 

Fig.  183. 


701.  Erroneous  Application.  Frequently  the  principle  of  the 
joint  of  rupture  is  entirely  and  improperly  neglected  in  applying 
this  theory;  that  is  to  say,  the  crown  thrust  employed  in  determin- 


ART.   1.]  SCHEFFLER's   THEORY.  4S1 

ing  the  line  of  resistance  is  that  which  would  produce  equilibrium 
of  rotation  about  the  springing  line,  instead  of  that  which  would 
produce   equilibrium  about  the  joint  of  rupture.     For  example, 

instead  of  employing  the  maximnm  value  in  the column  of 

y 

Table  59,  page  459,  the  last  quantity  in  that  column  is  used. 

The  line  of  resistance  obtained  by  this  method  is  shown  in  Fig. 
133  (page  480)  by  the  dotted  line,  the  crown  thrust  (5,990,  as  com- 
puted in  Table  59,  page  459)  being  laid  off  from  C  to  0,  to  the  scale 
employed  in  laying  off  the  load  line. 

702.  The  error  of  this  method  is  shown,  incidentally,  in  §§  678- 
82  and  §§  688-701,  and  needs  no  further  explanation. 

The  amount  of  the  error  is  illustrated  in  Fig.  133.  According 
to  this  analysis,  the  line  of  resistance  is  tangent  to  the  intrados, 
which  seems  to  show  that  the  arch  can  not  stand  for  a  moment. 
However,  many  such  arches  do  stand,  and  carry  a  heavy  railroad 
traffic  without  any  signs  of  weakness  ;  and  further,  any  reasonable 
method  of  analysis  shows  that  the  arch  is  not  only  safe,  but  even 
extravagantly  so  (§  690). 

This  method  of  analysis  certainly  accounts  for  some,  and  per- 
haps many,  of  the  excessively  heavy  arches  built  in  the  past.  For 
examjDle,  compare  8  and  9,  17  and  18,  33  and  34,  52  and  54,  etc., 
of  Table  63  (page  502). 

703.  Reliability  of  Scheffler's  Theory.  For  the  sake  of  com- 
parisons, the  line  of  resistance  according  to  the  Eational  Theory 
(§§  688-94),  as  determined  in  Fig.  125  (page  467),  is  shown  in  Fig. 
133  by  the  solid  lines.  (K'otice  that  Fig.  133  gives  the  lines  of  re- 
sistance, and  not  the  equilibrium  polygons  as  in  Fig.  125.)  In  this 
particular  case,  the  difference  between  the  two  lines  above  the  joint 
of  rupture  is  not  material  ;  but  the  difference  below  that  joint  has 
a  very  important  effect  upon  the  thickness  of  the  arch  at  the  spring- 
ing, and  also  upon  the  thickness  of  the  abutment  (§  712). 

If  the  maximum  ratio  of  the  horizontal  to  the  vertical  compo- 
nent of  the  external  forces  (see  first  paragraph  on  page  460)  had 
been  employed  in  determining  the  crown  thrust  and  the  line  of 
resistance,  there  would  have  been  a  material  difference  in  the  posi- 
tion of  both  the  joint  of  rupture  and  the  line  of  resistance  above 
that  joint.  Although  the  horizontal  components  of  the  external 
forces  can  not  be  accurately  determined,  any  theory  that  disregards 


482  AECHES.  [chap.   XVIII. 

the  existence  of  these  forces  can  not  be  considered  more  than  a 
loose  approximation. 

704.  Rankine's  Theory.  Although  this  theory  has  long  been, 
before  the  public  and  is  in  some  respects  much  superior  to  the  one 
in  common  use,  it  is  comparatively  but  little  employed  in  i)ractice. 
This  is  probably  due,  in  part  at  least,  to  the  fact  that  Eankine's 
discussion  of  the  theory  of  the  masonry  arch  is  not  very  simjjle  nor 
very  clearly  stated,  besides  being  distributed  throughout  various 
parts  of  his  works.* 

Eankine  determines  the  thrust  at  the  crown  by  Navier's  princi- 
ple (§  685) ;  but  he  makes  no  special  assumption  as  to  the  point  of 
application  of  this  thrust,  further  than  to  assume  that  if  a  line  of 
resistance  can  be  drawn  anywhere  within  the  middle  third  of  the 
arch  ring,  the  arch  is  stable. 

In  that  part  of  his  books  which  precedes  the  discussion  of  arches, 
Rankine  investigates  the  various  curves  which  a  cord  will  assume 
under  different  distributions  of  the  load  ;  and  subsequently  adopts, 
these  curves  as  the  form  which  the  line  of  resistance  of  an  arch 
similarly  loaded  should  have.  The  discussion  of  these  curves  con- 
stitutes the  most  valuable  part  of  his  investigations  concerning  the 
stability  of  the  masonry  arch. 

705.  Curvature  of  the  Linear  Arch.  Tlio  curves  assumed  by 
a  cord  under  the  various  conditions  of  loading,  can  be  ajDplied  to 
linear  arches  (the  line  of  resistance  of  actual  arches)  by  imagining 
that  the  curve  of  the  cord  is  reversed,  and  that  the  cord  itself  is 
replaced  by  a  thin  metal  strip,  which,  like  the  cord,  shall  be  prac- 
tically without  transverse  strength,  but  wliich,  unlike  the  cord, 
shall  be  able  at  every  point  to  resist  a  compressive  force  in  the  di- 
rection of  its  length.  The  amount  and  distribution  of  the  external 
forces  are  the  same  in  both  cases  ;  but  with  the  cord  they  act  out- 
ward, while  with  the  linear  arch  they  act  inward.  The  formulas 
and  diagrams  are  essentially  the  same  in  both  cases.  The  curves 
assumed  by  a  suspended  cord  under  various  distributions  of  the 
load  will  now  be  briefly  considered.  In  each  case  it  will  be  assumed 
that  the  ends  of  the  suspended  cord  and  also  of  the  corresponding 
linear  arch  are  in  the  same  horizontal  line. 

1.  If  the  cord  is  acted  upon  by  vertical  loads  distributed  uni- 

*  "Civil  Engineering,"  and  "  Applied  Mechanics.'' 


ART.    1.]  KANKINE's   THEORY.  483 

formly  along  the  horizontal,  it  will  assume  the  form  of  a  parabola. 
This  case  does  not  occur  with  masonry  arches. 

2.  If  the  load  is  vertical  and  distributed  uniformly  along  the 
curve,  the  resulting  curve  is  the  common  catenary,  of  which  the 
equation  is 

y  =  y(^-+^    -], (13) 

in  which  i/  is  the  ordinate  to  any  point,  w  the  ordinate  to  the  apex, 
E  the  base  of  the  Naperian  logarithms,  and  x  the  abscissa  corre- 
sponding to  I/.  Approximately,  this  case  may  occur  with  masonry 
arches,  since  the  above  law  of  loading  is  nearly  that  of  an  arch 
whose  intrados  is  the  common  catenary  and  which  supports  a  span- 
drel wall  of  masonry  having  a  horizontal  upper  surface  (see  3,  page 
445). 

3.  Three  points  fix  the  common  catenary  ;  and  hence,  if  the  posi- 
tion of  the  springing  lines  and  the  crown  are  assumed,  the  depth  of 
the  load  at  the  crown  is  fixed  by  the  equation  of  the  curve.  This 
limitation  would  often  interfere  with  the  use  of  the  common  cate- 
nary in  building  arches.  To  meet  this  difficulty,  Eankine  trans- 
forms the  common  catenary  by  the  principle  of  what  he  calls  paral- 
lel projections,  i.  e.,  by  increasing  or  decreasing  one  set  of  the 
rectangular  co-ordinates  to  the  curve  without  changing  the  other, 
and  obtains  the  transformed  catenary.  The  equation  of  the 
curve  is 

y  =  lI^\E^  +  B-^\, (14) 


in  which  ?/„  is  the  ordinate  to  the  apex,  and  m  is  the  modulus  of  the 
curve  and  is  found  by  the  formula 

X 


m  = 


hyp-  log.  1-^-  + 


/^ 


(16) 


I/O  2/o 

The  determination  of  values  of  y  by  equation  (14)  is  not  easy  except 
with  either  a  table  of  Naperian  logarithms  or  a  table  of  results 
deduced  therefrom,  and  even  then  it  is  tedious. 

With  this  curve  we  may  assume  the  springing  lines,  the  crown, 
and  the  depth  of  load  at  the  crown,  and  then  compute  the  curve  of 
equilibrium.  The  transformed  catenary  differs  from  a  circular  arc 
between  the  same  points  only  in  being  slightly  (and  frequently  only 


484  ARCHES.  [chap.  XVIII. 

very  slightly)  sharper  in  the  haunches  ;  and  hence  it  is  not  neces- 
sary to  discuss  it  further.* 

4.  If  the  load  is  uniform  and  normal  at  every  point,  the  curve 
of  equilibrium  is  plainly  a  circle.  An  example  of  this  case  would  be 
an  empty  masonry  shaft  standing  in  water. 

5.  The  ellipse  is  the  form  assumed  by  a  cord  under  a  load  com- 
posed of  horizontal  and  vertical  components  which  are  constant 
along  the  horizontal  and  vertical  lines,  but  which  differ  from  each 
other  in  intensity.  There  is  no  case  in  ordinary  practice  where  the 
pressures  upon  an  arch  are  strictly  identical  with  those  which  give 
an  elliptical  curve  of  equilibrium.  The  curve  of  "equilibrmm  of  a 
tunnel  arch  through  earth,  wiien  the  depth  below  the  surface  is 
great  compared  with  the  rise  of  the  arch  itself,  approximates  to  an 
ellipse.  The  load  is  nearly  uniform  along  the  horizontal,  while  the 
horizontal  force  at  any  point  is  some  fractional  part  of  the  vertical 
one  at  the  same  point ;  and  therefore  the  horizontal  forces  are 
nearly  uniform.  It  is  readily  shown  that  the  intensity  (the  pressure 
per  unit  of  area  perpendicular  to  the  force)  of  the  vertical  com- 
ponent is  to  that  of  the  horizontal  component  as  the  square  of  the 
vertical  diameter  of  the  ellipse  is  to  the  square  of  its  horizontal 
diameter ;  f  that  is  to  say. 


the  horizontal  axis  _  . /intensity  of  horizontal  component 
the  vertical  axis  intensity  of  vertical  component  '         ^ 

6.  If  the  forces  acting  on  the  linear  arch  are  normal  and 
increase  in  intensity  in  proportion  to  the  distance  of  the  points  of 
application  from  a  horizontal  line,  the  curve  is  a  hydrostatic  arch. 
A  tunnel  under  water  is  an  example  of  this  method  of  loading. 
The  form  of  the  curve  is  shown  in  Fig.  134,  of  which  only  the  portion 

ff ^^  6^  is  available  in  the  construction  of 

arches.     The  equation  of  the  curve  is 

p  p  =  lojJo  Po  =  a  constant,    .   (17) 

I  ^  in  which  jt?  is  the  normal  pressure  on  a 
^°-  ^34.  unit  area  at  any  point,  p  the  radius  of 

*  For  two  numerical  examples  of  the  method  of  employing  the  transformed  cate- 
nary in  the  design  of  an  arch,  see  an  article  by  W.  H.  Booth  in  Van  Nostrand's 
Engin'g  Mag.,  vol.  xxxi,  pp.  1-10  ;  and  for  another,  see  an  editorial  in  Engineering 
News,  vol.  xviii,  p.  372. 

+  Rankine's  Civil  Engineering,  p.  205. 


ART.   1.]  RAXKINE's   THEORY.  485 

curvature  at  the  same  point,  y  the  distance  from  the  line  0  (the 
surface)  to  any  point,  po  and  y^  the  values  of  p  and  y  for  the  point 
A,  and  lo  the  weight  of  a  unit  of  volume  of  the  loading. 

"  The  true  semi-ellipse  of  a  given  span  and  rise  differs  from  the 
hydrostatic  arch  by  being  of  somewhat  sharper  curvature  at  the 
crown  and  springing  and  of  somewhat  flatter  curvature  at  the 
haunches,  and  by  enclosing  a  somewhat  less  area.  The  application 
of  the  hydrostatic  arch  to  practice  is  founded  on  the  fact  that  every 
arch,  after  having  been  built,  subsides  at  the  crown,  and  spreads, 
or  tends  to  spread,  at  the  haunches,  which  therefore  press  horizon- 
tally against  the  filling  of  the  spandrels  ;  from  which  it  is  inferred 
as  probable  that,  if  an  arch  be  built  of  a  figure  suited  to  equilibrium 
under  fluid  pressure — /'.  c,  pressure  of  equal  intensity  in  all  direc- 
tions,— it  will  spread  horizontally,  and  compress  the  masonry  of  the 
spandrels  until  the  horizontal  pressure  at  each  point  becomes  of 
equal  intensity  to  the  vertical  pressure,  and  is  therefore  sufficient  to 
keep  the  arch  in  equilibrio."  * 

7.  If  the  vertical  and  the  horizontal  comijoncnts  of  the  normal 
force  differ  from  each  other  but  both  vary  as  the  distance  of  the 
point  of  application  from  a  horizontal  line,  tlie  curve  of  equilbrium 
is  the  geostatic  arch.  An  arch  in  clean  dry  sand  is  the  best  example 
of  this  form  of  loading.  The  geostatic  arch  bears  the  same  relation 
to  the  hydrostatic  arch  that  the  ellipse  does  to  the  circle.  The 
geostatic  curve  can  be  produced  from  that  of  the  hydrostatic  curve 
by  increasing  or  decreasing  one  set  of  ordinates  without  altering  the 
other.  If  px  be  the  horizontal  intensity  of  the  forces  acting  on  the 
hydrostatic  arch  and  2^'x  be  that  for  the  geostatic  arch,  then 
p^  z=  cp'x  ',  and  if  X  is  the  horizontal  diameter  at  any  point  of 
the  hydrostatic  curve  and  x'  the  same  for  the  geostatic,  then 
x'  =  cx.\ 

8.  Rankine  next  discusses  the  following  more  general  problem  : 
"  Given  the  curve  of  a  linear  arch  and  the  vertical  components  of  a 
symmetrical  load,  to  find  the  intensity  and  distribution  of  the 
horizontal  components  necessary  to  produce  equilibrium. 

*  Rankine's  Civil  Engineering,  pp.  419-20. 

+  For  a  numerical  example  of  the  method  of  employing  the  geostatic  curve  for  the 
intrados  of  tunnel  arches,  see  an  article—"  The  Employment  of  Mathematical  Curves 
as  the  Intrados  of  Arches  "—by  W.  H.  Booth  in  Van  Nostrand's  Engin'g  Mag.,  vol. 
XXX,  pp.  335-60. 


486 


ARCHES. 


[chap.  XVIII. 


"Let  V  =  the  vertical   load  on  any  arc  DC, — represented   in 
Fig.  135  by  the  line  £0; 
Vi  —  the  vertical  load  on  the  serai-arch  A  C; 
H  =  the  horizontal  load  on  any  arc  DC, — represented  by 

the  line  GF,  Fig.  135  ; 
Hi  =  the  horizontal  load  on  the  semi-arch  A  C; 
Ho  =  the  compression  at  the  crown  C, — represented  by  the 

line  BC,  Fig.  135 ; 
C  =  the  compression  on  the  rib  at  any  point  D, — rej^re- 
sented  by  FD,  Fig.  135  ; 
=  the  intensity  of  the  horizontal  force,  i.  e.,  the  force 
per  unit  of  area  perpendicular  to  its  line  of  action; 
=  the  intensity  of  the  vertical  force; 
=  the  value  of  py  at  the  crown  C; 
=  the  radius  of  curvature  at  the  crown  C; 
=  the  angle  that  the  tangent  of  the  linear  arch  at  any 
point  makes  with  the  horizontal, — that  is,  i  =  the 
angle  BDG,  Fig.  135. 


Pv 
Po 
Po 


Fig.  135. 

"Then       V  =--  fj i\dx', (18) 

(7=  Fcosec  i\ (19) 

H-Vcoii', (20) 

-  ^  -  _  ^(^CQtQ  _  _  ^  v'd'y] 
^'~    dy  ~  dy        ~  dy      '    '     '     ^^  ' 

•*  The  integration  constant  for  (21)  is  Hq  ;  and  is  found  by  equa- 
tion (11),  page  465,  which,  in  the  above  nomenclature,  becomes 

Ho=PoPo-"  .     .     .     .   • (S2) 


ART.    1.]  RANKINE's   THEORY.  487 


However,  before  concluding  this  phase  of  tlie  discussion  of 
arches,  it  is  well  to  state  that  the  only  arches  in  common  use  are 
tlie  circular — cither  semi-circular  or  segmental — and  the  elliptic. 

706.  Stability  of  any  Proposed  Arch.  To  apply  the  preceding 
principles  in  designing  an  arch,  it  is  necessary  to  know  both  the 
vertical  and  the  horizontal  forces  acting  on  the  arch.  Rankine 
assumes*  (1)  that  the  vertical  force  acting  on  any  part  is  the  weight 
of  the  masonry,  earth,  or  other  load  vertically  above  the  same;  and 
(^))  that  the  horizontal  pressure  of  earth  is  given  by  the  formula 

7 1  —  sin  0 1 

Px=iod^    ,     .,\ (23) 

1  +  sm  0  ^     ' 

in  which  p^  is  the  horizontal  intensity  at  any  point,  w  the  weiglit  of 
a  unit  of  the  earth,  d  the  depth  of  earth  over  the  point,  and  0  the 
angle  of  repose.  In  the  above  nomenclature,  the  vertical  inten- 
sity is 

Pv=2ud (24) 

By  an  application  of  these  two  principles  are  to  be  determined  the 
amount  and  distribution  of  the  vertical  and  the  horizontal  forces 
acting  on  the  arch;  and  then  the  equilibrium  curve  corresponding 
to  this  form  of  loading  (see  §  705)  is  to  be  adopted  for  the  intrados 
of  the  proposed  arch. 

For  an  example,  take  the  case  of  an  arch  under  a  high  bank  of 
earth  whose  angle  of  repose  is  30°.  Strictly,  the  curve  of  equi- 
librium is  the  geostatic  arch  (see  paragraph  7,  §  705) ;  but  it  will 
be  more  simple  and  sufficiently  exact,  if  we  assume  it  to  be  an 
ellipse,  which  is  equivalent  to  assuming  that  the  rise  of  the  arch  is 
inconsiderable  in  comparison  with  the  depth  of  earth  over  it.  The 
intrados  is  then  to  be  an  ellipse  in  which 


the  vertical  axis    _  a/~P^  _  i/l  +  sin  0  _  ^- 
tfie  horizontal  axis  Px  1  —  sin  0  ""        •  *     v     / 

*'  If  the  earth  is  firm,  and  little  liable  to  be  disturbed,  the  propor- 
tion of  the  half-span — or  horizontal  semi-axis — to  the  rise — or  ver- 

*  Civil  Engineering,  p.  434. 

+  Rankine  states  (Civil  Engineering,  p.  320)  that  the  horizontal  pressure  can  not 

l+sin<^  ,  1  -  sin<i> 

be  greater  than  w  h— -. — -,  nor  less  than  w  h-— — : — -.     Notice  that  the  value  employed 

1-sin*  l+siQ"*  ' 

above  is  the  minimum. 


488  ARCHES.  [chap.   XVIII. 

tical  semi-axis — may  be  made  (/rm^er  than  is  given  by  the  preced- 
ing equation,  and  the  earth  will  still  resist  the  additional  horizontal 
thrust ;  but  that  proportion  should  never  be  made  less  than  the 
value  given  by  the  equation,  or  the  sides  of  the  archway  will  be  in 
danger  of  being  forced  inwards.^'  * 

"There  are  numerous  cases  in  which  the  form  of  the  linear  rib 
suited  to  sustain  a  given  load  may  at  once  be  adopted  for  the  in- 
trados  of  a  real  arch  for  sustaining  the  same  load,  with  sufficient 
_f  exactness  for  practical  purposes.     The  follow- 
-''       -],  ing  is  the  test  whether  this  method  is  appli- 
cable in  any  given  case.     Let  A  CB  in  Fig. 
136  be  one  half  of  the  ideal  rib  which  it  is 
proposed  to  adopt  as  the  intrados  of  a  real 
arch.      Draw  A  a  normal  to  the  rib  at  the 
/q  crown,  so  as  to  represent  a  length  not  ex- 

FiG.  136.  ceeding  two  thirds  of  the  intended  depth  of 

the  keystone.  Draw  a  normal  Bb  at  the  springing  of  a  length 
such  that 

Bb  _  thrust  along  rib  at  ^  „  , 

^«  ~  thrust  along  rib  at  iy*     '     '     *     •     •     v~  / 

The  thrust  at  A  is  found  by  equation  (11),  page  465  ;  and  the  thrust 
at  any  other  point  is  given  by  equation  (19),  page  486.  Construct, 
a  line  acb  such  that  its  perpendicular  distance  from  the  intrados  at 
any  point,  cC,  is  inversely  as  the  thrust  along  the  rib  at  that  point. 
Then  if  acb  lies  within  the  middle  third  of  the  proposed  arch  ring, 
the  ideal  rib  A  CB  is  of  a  suitable  form  for  the  intrados. 

707.  Eankine's  general  method  of  determining  the  stability  of 
a  proposed  arch  is  as  follows :  J 

"  The  first  step  towards  determining  whether  a  proposed  arch 
will  be  stable,  is  to  assume  a  linear  arch  parallel  to  the  intrados  or 
soffit  of  the  proposed  arch,  and  loaded  vertically  with  the  same 
weight,  distributed  in  the  same  manner.  Then  by  equation  (21), 
page  486,  determine  either  a  general  expression,  or  a  series  of  val- 
ues, of  the  intensity  j^x  of  the  conjugate  pressure,  horizontal  or 
oblique  as  the  case  may  be,  required  to  keep  the  arch  in  equilibrio 

*  Rankine's  Civil  Engineering,  p.  434. 
^  Ibid.,  p.  417. 
X  Ibid.,  pp.  421-22. 


ART.    l.J 


RANKINE  S   THEORY. 


489 


imder  the  given  vertical  load.  If  that  pressure  is  nowhere  nega- 
tive, a  curve,  similar  to  the  assumed  arch,  drawn  through  the  middle 
of  the  arch  ring  will  be,  either  exactly  or  very  nearly,  the  line  of 
pressure  of  the  proposed  arch;  p^  will  represent,  either  exactly  or 
very  nearly,  the  intensity  of  the  lateral  pressure  which  the  real 
arch,  tending  to  spread  outwards  under  its  load,  will  exert  at  each 
point  against  its  spandrel  and  abutments;  and  the  thrust  along  the 
linear  arch  at  each  j^oint  will  be  the  thrust  of  the  real  arch  at  the- 
corresponding  joint. 

"  On  the  other  hand,  if  ])x  has  some  negative  values  for  the 
assumed  linear  arch,  there  must  be  a  pair  of  points  in  that  arch 
where  that  quantity  changes  from  positive  to  negative,  and  is  ecpial 
to  nothing.  The  angle  of  inclination  i  at  that  point,  called  the 
(Dujlc  of  rupture,!?,  to  be  determined  by  placing  the  second  member 
of  equation  (21),  page  480,  equal  to  zero  and  solving  for  cot  i.  The 
corresponding  joints  in  the  real  arch  are  called  the  joints  of  i-up- 
ture  ;  and  it  is  below  those  joints  that  conjugate  pressure*  from 
without  is  required  to  sustain  the  arch  and  that  consequently  the- 
backing  must  be  built  with  squared  side-joints. 

"In  Fig.  137,  let  BC'A  represent  one  half  of  a  symmetrica.* 
arch,  KLDE  an  abutment,  and  C 
the  joint  of  rupture — found  by  the 
method  already  described.  The  point 
of  rupture,  which  is  the  center  of  re- 
sistance of  the  joint  of  rupture,  is 
somewhere  within  the  middle  third 
of  the  depth  of  that  joint;  and  from 
that  point  down  to  the  springing  joint 
B,  the  line  of  pressure  is  a  curve  sim- 
ilar to  the  assumed  linear  arch,  and  E^ 
parallel  to  the  intrados,  being  kept  in 
equilibrio  by  the  lateral  pressure  between  the  arch,  and  its  spandrel 
and  abutment. 

"  From  the  joint  of  rupture  C  to  the  crown  A,  the  figure  of  the 
true  line  of  pressure  is  determined  by  the  condition  that  it  shall  be 

*  A  minus  value  oipx  will  correspond  to  an  outward  pull,  and  consequently  tho- 
backing  below  the  joint  of  rupture  should  be  capable  of  resisting  tension. 


Fig. 137 


490  AECHES.  [CHAP.    XYIII. 

X  linear  arch  balanced  under  vertical  forces  only  ;  *  that  is  to  say, 
tlie  horizontal  component  of  the  thrust  along  it  at  each  point  is  a 
constant  quantity,  and  equal  to  the  horizontal  component  of  the 
thrust  along  the  arch  at  the  joint  of  rupture. 

"  The  only  point  in  the  line  of  pressure  above  the  joint  of 
rupture  which  it  is  important  to  determine  is  that  of  the  crown  of 
the  arch.  A}  and  it  is  found  in  the  following  manner  :  Find  the 
center  of  gravity  of  the  load  between  the  joint  of  rupture  6' and  the 
crown  A  ;  and  draw  through  that  center  of  gravity  a  vertical  line. 
Then  if  it  be  possible,  from  any  point,  such  as  M,  in  that  vertical 
line,  to  draAV  a  pair  of  lines,  one  parallel  to  a  tangent  to  the  soffit  at 
the  joint  of  rupture  and  the  other  parallel  to  a  tangent  to  the  soffit 
at  the  crown,  so  that  the  former  of  those  lines  shall  cut  the  joint  of 
rupture  and  the  latter  the  keystone,  in  a  pair  of  points  which  are 
both  within  the  middle  third  of  the  depth  of  the  arch  ring,  the 
stability  of  the  arch  will  be  secure  ;  and  if  the  first  point  be  the 
point  of  rupture,  the  second  will  be  the  center  of  resistance  at 
the  crown  of  the  arch  and  the  crown  of  the  true  line  of  pressures. 

"When  the  pair  of  points,  related  to  each  other  as  above,  do  not 
fall  at  opposite  limits  of  the  middle  third  of  the  arch  ring,  their 
exact  positions  are  to  a  small  extent  uncertain  ;  but  that  uncertainty 
is  of  no  consequence  in  practice.  Their  most  probable  positions  are 
equidistant  from  the  middle  line  of  the  arch  ring. 

*' Should  the  pair  of  points  fall  beyond  the  middle  third  of  the 
arch  ring,  the  de])th  of  the  arch  stones  must  be  increased." 

708.  Reliability  of  Rankine's  Theory.  1.  This  theory  is  ap- 
proximate since  it  makes  no  attempt  to  determine  the  true  line  of 
resistance,  but  finds  only  a  line  of  resistance  wliich  lies  within  the 
middle  third  of  the  arch  ring. 

2.  The  value  of  the  radius  of  curvature  to  be  used  in  finding 
the  crown  thrust  is  indeterminate.  It  is  frequently,  but  erroneously, 
taken  as  the  radius  of  the  intrados  at  the  crown. 

3.  The  method  of  finding  the  center  of  pressure  at  the  crown 
and  also  at  the  joint  of  rupture  assumes  that  the  portion  CM  A, 
Fig.  137,  is  acted  upon  by  only  three  forces  ;  viz.,  the  vertical  load, 
the  thrust  at  the  crown,  and  the  pressure  on  the  joint  of  rupture. 

*  From  this  it  appears  that  Rankine  himself  disregards,  for  that  part  of  the  areh 
above  the  joint  of  rupture,  the  principal  characteristic  of  his  theory,  viz. :  the  recog- 
nition of  the  horizontal  components  of  the  external  forces ;  and  hence  this  theory 
is,  in  fact,  the  same  as  Scheffler's  (§§  695-703). 


ART.    L]  RAXKIXE'S   THEORY.  491 


This  is  erroneous  (a)  because  it  neglects  the  horizontal  components; 
of  the  external  forces,  and  hence  the  actual  center  of  pressure  at 
the  joint  of  rupture  is  nearer  the  intrados  than  the  position  of  0 
as  found  in  Fig.  137  ;  and  (h)  because  it  finds  a  new  value  for  the 
thrust  at  the  crown  which,  in  general,  will  differ  from  that  employed 
in  finding  the  position  of  the  Joint  of  rupture. 

4.  Rankine  himself  says  that  the  method  of  §  TOT  is  inapplical)le 
to  a  circular  arch  greater  than  90°,  and  gives  a  complicated  formula 
for  that  case. 

Eankine's  theory  is  more  complicated  and  less  accurate  thau 
either  Scheffler's  (§  695)  or  the  rational  theory  (§  688). 

709.  Other  Theories  of  the  Arch.  There  are  several  methods^ 
in  more  or  less  common  use,  of  determining  the  stability  of  the  vous- 
soir  arch,  many  of  which  are  but  different  combinations  of  the  pre- 
ceding principles,  while  some  have  a  much  less  satisfactory  basis. 
It  is  not  necess..ry  to  discuss  any  of  these  at  length  ;  but  there  is 
one  which,  owing  to  the  frequency  with  which  it  is  employed, 
requires  a  few  words.  It  is  the  same  as  Scheffier's  (§§  695-T03),  ex- 
cept in  assuming  that  the  line  of  resistance  passes  through  the 
m  iddle  of  the  crown  Joint  and  also  through  the  m  iddle  of  the  spring- 
ing Joint.  The  line  of  resistance  is  then  determined  in  any  one  of 
a  number  of  ways  ;  and  the  arch  is  said  to  be  stable,  if  the  line  of 
resistance  lies  in  the  middle  third  of  the  section  of  the  arch  ring. 
This  theory  is  much  less  satisfactory  than  Scheffler's  and  possesses 
no  advantage  over  it. 

710.  Theory  of  the  Elastic  Arch.  It  has  long  been  recognized 
that  all  theories  for  the  voussoir  arch  are  very  unsatisfactory  ;  and 
hence  it  has  been  proposed  to  consider  the  masonry  arch  as  an 
elastic  curved  beam  fixed  at  its  ends,  and  examine  its  stability  by 
the  principles  employed  in  computing  the  strains  in  arches  of  iron 
or  wood.  There  is  no  essential  difference,  as  far  as  the  theory  is 
concerned,  between  the  iron  and  the  stone  arch  ;  but  there  is  great 
difficulty  in  applying  the  mathematical  theory  of  elasticity  to  the 
masonry  arch.  The  theory  of  elasticity  when  applied  to  the 
masonry  arch  has  the  following  sources  of  error,  in  addition  to  those 
of  the  ordinary  theory  of  the  elastic  arch  :  1.  There  is  great  un- 
certainty as  to  the  external  forces  (§  666).  2.  TVc  have  no  definite 
knowledge  concerning  either  the  modulus  of  elasticity  (§§  16  and 
146)  or  the  ultimate  strength  of   masonry  (§§  221-23,  and  §§  246- 


492 


ARCHES. 


[chap.   XVIII. 


49).  3.  The  stone  arch  is  not  homogeneous  ;  i.  e.,  the  modulus  of 
elasticity  is  not  constant,  but  varies  between  that  of  the  stone  and 
the  mortar.  4.  Slight  imperfections  in  the  workmanship — as,  for 
example,  a  projection  on  the  bearing  surface  of  an  arch  stone  or  a 
pebble  in  the  mortar — would  break  the  continuity  of  the  arch,  and 
render  the  theory  inapplicable.  5.  The  stability  of  the  arch  would 
be  greatly  influenced  by  the  action  of  the  center, — its  rigidity,  the 
method  of  loading  it  to  prevent  deformation,  and  the  method  and 
rapidity  of  striking  it. 

The  application  of  the  theory  of  elasticity  to  stone  arches  has 
been  considerably  discussed  in  late  years  ;  but  it  is  generally  con- 
ceded that  the  results  are,  for  the  most  part,  illusory,  since  the 
much  simpler  methods  give  results  equally  reliable.  The  explana- 
tion of  the  theory  of  the  elastic  masonry  arch  as  given  by  Professor 
Greene  in  Part  III — Arches — of  his  ''' Trusses  and  Arches"  is  all 
that  can  be  desired;  and  hence  this  theory  will  not  be  discussed 
here. 

711.  Stability  of  Abutments  and  Piers.  The  stability  of  the 
abutment  is  in  a  measure  indeterminate,  since  it  depends  upon  the 
position  of  the  line  of  resistance  of  the  arch.  The  stability  of 
the  abutment  may  be  determined  most  easily  by  treating  it  as  a 
part  of  the  arch,  i.  e.,  by  extending  the 
load  line  so  as  to  include  the  forces  acting 
upon  it  and  drawing  the  reactions  in  the 
usual  way  ;  or  its  stability  may  be  deter- 
mined as  follows  :  Assume  that  it  is  re- 
quired to  test  the  stability  of  the  abutment 
shown  in  Fig.  138.  Let  qc  represent  the 
direction  of  the  resultant  pressure  on  the 
]omt  AB.  g  is  the  center  of  gravity  of  the 
section  ABC  of  the  abutment,  and  g.y  that 
for  the  section  ABED.*  At  a^the  point 
Fig.  13S.  where  a  vertical  through  g  intersects  qCi 

prolonged — lay  off,  to  scale,  a  line  ad  equal  to  the  weight  of  ABC, 
and  also  a  line  ab  equal  to  the  pressure  qc^  ;  then  r.^  — the  point 
where  the  diagonal  ea  pierces  A  C — is  the  center  of  pressure  on  A  C. 


*  For  a  method  of  finding  the  center  of  gravity  when  the  section  is  a  trapezoid, 
see  the  third  paragraph  of  §  494  (page  318). 


A.RT.   1.]  STABILITY    OF   THE   ABUTMENT.  493 

In  a  similar  manner,  C3  is  found  to  be  the  center  of  pressure  on 
DE. 

The  amount  of  the  pressure  on  ^  C  is  given  by  the  length  of  the 
line  ae ;  and  the  stability  of  the  joint  against  crushing  can  be  de- 
termined as  described  in  §§  670-73  and  paragraph  2  of  §  690. 
The  stability  against  rotation  may  be  determined  as  described  in 
§  669  and  paragraph  1  of  §  690.  A  line — not  shown^connecting 
Ci,  C2,  C3,  is  the  line  of  resistance  of  the  abutment,  to  which  the 
joints  should  be  nearly  perpendicular  (see  §  674  and  division  3  of 
§  690). 

712.  In  Fig.  133  (page  480)  is  shown  the  line  of  resistance  for 
the  abutment  according  to  the  rational  theory  of  the  arch  (§§  688- 
94),  and  also  that  according  to  Scheffler's  theory  (§§  695-703), — 
the  former  by  the  solid  line  and  the  latter  by  the  broken  one. 
Since  to  overestimate  the  horizontal  components  of  the  external 
forces  would  be  to  err  on  the  side  of  danger,  in  apj^lying  the  former 
theory  in  Fig.  133,  the  liorizontal  component  acting  against  the 
abutment  was  disregarded  on  the  assumption  that  the  abutment 
might  be  set  in  a  pit  without  greatly  disturbing  the  surrounding 
earth.  If  the  horizontal  component  had  been  considered,  the  dif- 
ference between  the  lines  of  resistance  according  to  the  two  theories 
would  have  been  still  greater.  Xotice  that  the  analysis  which 
recognizes  the  existence  of  the  horizontal  forces,  i.  e.,  the  rational 
theory,  permits  a  lighter  abutment  than  the  theory  which  assumes 
the  external  forces  to  be  entirely  vertical. 

The  omission  of  the  horizontal  components  assumes  that  the 
only  object  of  the  abutment  is  to  resist  the  thrust  of  the  arch  ;  and 
that  consequently  the  flatter  the  arch  the  greater  the  thrust  and  the 
heavier  the  abutment.  Ordinarily  the  abutment  must  resist  the 
thrust  of  the  arch  tending  to  overthrow  it  and  to  slide  it  outward, 
and  must  act  also  as  a  retaining  wall  to  resist  the  lateral  pressure  of 
the  earth  tending  to  overthrow  it  and  to  slide  it  inward.  For 
large  arches  the  former  is  the  more  important ;  but  for  small 
arches,  particularly  under  high  embankments,  the  latter  is  the  more 
important.  Hence,  for  large  arches  or  for  an  arch  having  a  light 
surcharge,  the  abutment  should  be  proportioned  to  resist  the  thrust 
of  the  arch;  but  for  small  arches  under  a  heavy  surcharge  of 
earth,  the  abutment  should  be  proportioned  as  a  retaining  wall 
(Chap.  XIV). 


494  ARCHES.  (chap.   XVIII. 

Although  the  horizontal  pressure  of  the  earth  can  not  be  com- 
puted accurately,  there  are  many  conditions  under  which  the 
horizontal  components  should  not  be  omitted.  For  example,  if  the 
abutment  is  high,  or  if  the  earth  is  deposited  artificially  behind  it, 
ordinarily  it  would  be  safe  to  count  upon  the  pressure  of  the  eartii 
to  assist  in  preventing  the  abutment  from  being  overturned  out- 
wards. Finally,  although  it  may  not  always  be  wise  to  consider  the 
earth  pressure  as  an  active  force,  there  is  always  a  passive  resistance 
which  will  add  greatly  to  the  stability  of .  the  abutment,  and  whose, 
intensity  will  increase  raj^idly  with  any  outward  movement  of  the 
abutment  (see  last  paragraph  of  §  G6(J). 

For  empirical  rules  for  the  dimensions  of  abutments,  see  §§ 
722-23. 

Art.  2.  Rules  Derived  from  Practice. 

713.  In  the  preceding  article  it  was  shown  that  every  theory  of 
the  arch  requires  certain  fundamental  assumptions,  and  that  hence 
the  best  theory  is  only  an  approximation.  Further,  since  it  is  prac- 
tically impossible,  by  any  theory  (§  693),  to  include  the  effect  of 
passing  loads,  theoretical  results  are  inapplicable  when  the  moving 
load  is  heavy  compared  with  the  stationary  load.  It  was  shown 
also  that  the  stability  of  a  masonry  arch  does  not  admit  of  exact 
mathematical  solution,  but  is  to  some  extent  an  indeterminate 
problem.  At  best  the  strains  in  a  masonry  arch  can  never  be  com- 
puted anything  like  as  accurately  as  those  in  metallic  structures. 
However,  this  is  no  serious  matter,  since  the  material  employed  in 
the  former  is  comparatively  cheap. 

Considered  practically,  the  designing  of  a  masonry  arch  is 
greatly  simplified  by  the  many  examples  furnished  by  existing 
structures  which  afford  incontrovertible  evidence  of  their  stability 
by  safely  fulfilling  their  intended  duties,  to  say  nothing  of  the 
history  of  those  structures  which  have  failed  and  thus  supplied 
negative  evidence  of  great  value.  In  designing  arches,  theory 
should  be  interpreted  by  experience  ;  but  experience  should  be 
studied  by  the  light  of  the  best  theory  available. 

This  article  will  be  devoted  to  the  presentation  of  current  prac- 
tice as  shown  by  approved  empirical  formulas  and  practical  rules, 
and  by  examples. 


ART.  2.]  RULES    DERIVED    FROM    PRACTICE.  495 

714.  Empirical  Formulas.  Numerous  formulas  derived  from 
existing  structures  have  been  proposed  for  use  in  designing  masonry 
arches.  Such  formulas  are  useful  as  guides  in  assuming  propor- 
tions to  be  tested  by  theory,  and  also  as  indicating  what  actual 
practice  is  and  thus  affording  data  by  which  to  check  the  results 
obtained  by  theory. 

As  proof  of  the  reliability  of  such  formulas,  they  are  frequently 
accompanied  by  tables  showing  their  agreement  with  actual  struct- 
ures. Concerning  this  method  of  proof,  it  is  necessary  to  notice 
that  (1)  if  the  structures  were  selected  because  their  dimensions 
agreed  with  the  formula,  nothing  is  proven ;  and  (2)  if  the  struct- 
ures were  designed  according  to  the  formula  to  be  tested,  nothing 
is  proven  except  that  the  formula  represents  practice  which  is 
probably  safe. 

At  best,  a  formula  derived  from  existing  structures  can  only 
indicate  safe  construction,  but  gives  no  information  as  to  the  degree 
of  safety.  These  formulas  usually  state  the  relation  between  the 
principal  dimensions  ;  but  the  stability  of  an  arch  can  not  be  de- 
termined from  the  dimensions  alone,  for  it  depends  upon  various 
attendant  circumstances, — as  the  condition  of  the  loading  (if  earth, 
upon  whether  loose  or  compact ;  and  if  masonry,  upon  the  bonding, 
the  mortar,  etc.),  the  quality  of  the  materials  and  of  the  workman- 
ship, the  manner  of  constructing  and  striking  the  centers,  the 
spreading  of  the  abutments,  the  settlement  of  the  foundations,  etc. 
The  failure  of  an  arch  isa  very  instructive  object  lesson,  and  should 
be  most  carefully  studied,  since  it  indicates  the  least  degree  of 
stability  consistent  with  safety.  Many  masonry  arches  are  excessively 
strong ;  and  hence  there  are  empirical  formulas  which  agree  with 
existing  structures,  but  which  differ  from  each  other  300  or  400  per 
cent.  All  factors  of  the  problem  must  be  steadily  borne  in  mind  in 
comparing  empirical  formulas  either  with  each  other  or  with  theo- 
retical results. 

A  number  of  the  more  important  empirical  formulas  will  now  oe 
given,  but  without  any  attempt  at  comparisons,  owing  to  the  lack 
of  space  and  of  the  necessary  data. 

715.  Thickness  of  the  Arch  at  the  Crown.  In  designing  an  arch, 
the  first  step  is  to  determine  the  thickness  at  the  crown,  i.  e.,  the 
depth  of  the  keystone. 


496  ARCHES.  [chap.  XVlil. 

Let  d  =  the  depth  at  the  crown,  in  feet  ; 

p  =  the  radius  of  curvature  of  the  intrados,  in  feet ; 
r  =  the  rise,  in  feet ; 
s  =  the  span,  in  feet. 

716.  American  Practice.  Trautwine's  formula  for  the  depth 
of  the  keystone  for  a  first-class  cut-xtoiie  arch,  whether  circular  or 
elliptical,  is 

^^±ii+ifi  +  0.3 (27) 

''For  second-class  worh,  this  depth  may  be  increased  about  one 
eighth  part ;  and  for  hiHck  worh  or  fair  rubble,  about  one  third." 

717.  English  Practice.  Eankiue's  formula  for  the  depth  of 
keystone  for  a  single  arch  is 

d  =  4/0.13/3  ; (38) 

for  an  arch  of  a  sei'ies, 

d  =  V  0.17  p  ; (39) 

and  for  tunnel  arches,  where  the  ground  is  of  the  firmest  and  safest, 

=  1/0.134", 
and  for  soft  and  slipping  materials, 


d=y  0.12- , (30) 


d=  1/0.48-- 


.     (•") 

The  segmental  arches  of  the  Refinies  and  the  Stephensons,  which 
are  generally  regarded  as  models,  ''have  a  thickness  at  the  crown 
of  from  -jIq  to  -^^  of  the  span,  or  of  from  ^'g-  to  -5^  of  the  radius  of 
the  intrados."" 

718.  French  Practice*  Perronnet,  a  celebrated  French  engi- 
Jieer,  is  frequently  credited  with  the  formula, 

d=li.  +  i^s, (32) 

♦From  "Proportions  of  Arches  from  French  Practice,"  by  E.  Sherman  Gould 
in  Van  Nostrand's  Engin'g  Mag.,  vol.  xxix,  p.  450. 


ART.   2.]  RULES    DERIVED    FROM    PRACTICE.  497 

as  being  applicable  to  arches  of  all  forms — semi-circular,  segmental, 
elliptical,  or  basket-handled, — and  to  railroad  bridges  or  arches 
sustaining  heavy  surcharges  of  earth.  '^Perronnet  does  not  seem, 
however,  to  have  paid  much  attention  to  the  rule  ;  but  has  made 
his  bridges  much  lighter  than  the  rule  would  require."  Other 
formulas  of  the  above  form,  but  having  different  constants,  are  also 
frequently  credited  to  the  same  authority.  Evidently  Perronnet 
varied  the  proportions  of  his  arches  according  to  the  strength  and 
weight  of  the  material,  the  closeness  of  the  joints,  the  quality  of 
mortar,  etc. ;  and  hence  different  examples  of  his  work  give  differ- 
ent formulas. 

Dejardin's  formulas,  which  are  frequently  employed  by  French 
engineers,  are  as  follows  : 

For  circular  arches. 


d  =1  +  0.1  p; (33) 

d  =  l-{-0.05p;     ....     (34) 
d  =  l-\-  0.035  p ;     .     .     .     .     (35) 


if 

r 

s 

= 

^> 

if 

r 

s 

= 

h 

if 

r 

s 

= 

i. 

r 


if  -  =  ^,     d  =  1  +  0.02  p;      ....     (36) 

For  elliptical  and  basket-handled  arches, 

if  -=  I,      d  =1+0.07 p (37) 

Croizette-Desnoyers,  a  French  authority,  recommends  the  fol- 
lowing formulas  : 

if  ->    i,     ^  =  0.50  + 0.28  VTp";      .    .     .     (38) 


r 


if  -=    I,     t?  =  0.50  +  0.26  VTp;      .     .     .     (39) 
s 


a  -=  ^,     d  =  0.50  +  0.20  V2p;    .     .     .     r40) 


498 


ARCHES. 


[chap.  XYIII. 


719.  Notice  that  in  none  of  the  above  formulas  does  the  char- 
acter of  the  material  enter  as  a  factor.  Notice  also  that  none  of 
ihem  has  a  factor  depending  upon  the  amount  of  the  load. 

Table  62  is  given  to  facilitate  the  comparisons  of  the  preceding 
formulas  with  each  other  and  with  actual  structures.  Values  not 
given  in  the  table  can  be  interpolated  with  sufficient  accuracy.  It  is 
remarkable  that  according  to  all  formulas  credited  to  Perronnat  the 
thickness  at  the  crown  is  independent  of  the  rise,  and  varies  only 
with  the  span.  Notice  that  by  Dejardin's  formulas  the  thickness 
decreases  as  the  rise  increases, — as  it  should. 

TABLE  63. 
Comparison  of  Empirical,  Formulas  for  Depth  of  Keystone. 


Proportion  op  Rise  to  Span. 

Semi-circle. 

Rise   _  , 
Span  "  ' 

Rise 
Span  ~  ^ 

Formula. 

Span. 

Span. 

Span. 

10 

50 

100 

10 

50 

100 

10 

50 

100 

Trautwine's,  for  flrst-class  work 
"    second  "       " 
"    third     •' 

.99 
1.11 
1.38 

.77 
1.51 
1.50 
1.38 

1.9S 
2.23 
2.64 
1.73 
•3.26 
3.50 
2.48 

2.70 
3  04 
3.60 
2.45 
5  43 
6.00 
3.30 

1.11 
1.25 
1.48 
1.00 
1  51 
1.42 
1.56 

2.23 
2.51 
2.97 
2.25 
3.26 
3  07 
2.86 

3.09 
3.43 
4.12 
3.16 
5.43 
5.17 
3.85 

1.26 
1.44 
1.68 
1.25 
1.51 
1.26 
1.62 

2.57 
2.89 
3.42 
2  79 
3.26 
2  30 
3.01 

3.55 
4.00 
4.73 
3.95 

5.43 

2.60 

Oroizette-Desnoyers's 

4.05 

720.  Thickness  of  the  Arch  at  the  Springing.  Generally  the 
thickness  of  the  arch  at  the  springing  is  found  by  an  application  of 
theory ;  and  hence  but  few  empirical  formulas  are  given  for  this 
purpose. 

Trautwine  gives  a  formula  for  the  thickness  of  the  abutment, 
which  determines  also  the  thickness  of  the  arch  at  the  springing 
(see  §  722). 

''The  augmentation  of  thickness  at  the  springing  line  is  made, 
by  the  Stephensons,  from  20  to  30  per  cent. ;  and  by  the  Kennies, 
ibout  100  per  cent." 

721.  If  the  loads  are  vertical,  the  horizontal  component  of  the 
compression  on  the  arch  ring  is  constant ;  and  hence,  to  have  the 
mean  pressure  on  the  joints  uniform,  the  vertical  projection  of  the 


AKT.   2.]  RULES    DERIVED    FROM    PRACTICE.  499 

joints  should  be  constant.  This  principle  leads  to  the  following 
formula,  which  is  frequently  employed  :  Tlie  length,  measured  radi- 
ally, of  each  joint  hetiveen  the  joint  of  rupture  and  the  croivn 
should  be  such  that  its  vertical  projection  is  equal  to  the  depth  of 
the  keystone.     In  algebraic  language,  this  rule  is 

/  1=  fZsec  a, (41) 

in  which  /  is  the  length  of  the  joint,  d  the  depth  at  the  crown,  and 
a  the  angle  the  joint  makes  with  the  vertical. 

The  length  of  the  joint  of  rupture,*  i.  e.,  the  thicknesss  of  the 
arch  at  the  practical  springing  line,  can  be  computed  by  the  above 
formula.  The  following  arc  the  values  for  circular  and  segmental 
arches  : 

If  ^  >    l:     1=  2.J00  d; (42) 

''-=.     i,     l^lAOd; (43) 

''-=    h     ^-1.24^: (44) 

r 


j\,     l=l.ud;     .     .    .     .    ,     (45) 


r 


1  =  J^,     l  =  l.lOd (46) 

722.  Thickness  of  the  Abutment,  t     Trautwi7ie's  toTmnla,  is 

^  =0.2p+0.1 /•  +  2.0, (47) 

in  which  t  is  the  thickness  of  the  abutment  at  the  springing,  p  the 
radius,  and  r  the  rise, — all  in  feet.  '''  The  above  formula  applies 
equally  to  the  smallest  culvert  or  the  largest  bridge — whether  cir- 
cular or  elliptical,  and  whatever  the  proportions  of  rise  and  span — 
and  to  any  height  of  abutment.  It  applies  also  to  all  the  usual 
methods  of  filling  above  the  arch,  whether  witli  solid  masonry  to 
the  level  of  the  top  of  the  crown,  or  entirely  with  earth.  It  gives 
a  thickness  of  abutment  which  is  safe  in  itself  without  any  back' 
ing  of  earth  behind  it,  and  also  safe  against  the  pressure  of  the 

*  Concerning  the  method  of  determining  the  joint  of  rupture,  see  §§  68Q-8L 
I-  For  a  theoretical  discussion  of  tliis  subject,  see  §§  711-12. 


500 


AECHES. 


[chap.   XVIII. 


earth  when  the  bridge  is  unloaded.  It  gives  abutments  which 
alone  are  safe  when-the  bridge  is  loaded  ;  but  for  small  arches,  the 
formula  supposes  that  earth  will  be  deposited  behind  the  abut- 
ments to  the  height  of  the  roadway.  In  small  bridges  and  large 
culverts  on  first-class  railroads,  subject  to  the  jarring  of  heavy 
trains  at  high  speeds,  the  comparative  cheapness  with  which  an 
excess  of  strength  can  be  thus  given  to  important  structures  has  led, 
in  many  cases,  to  the  use  of  abutments  from  one  fourth  to  one  half 
thicker  than  those  given  by  the  preceding  rule.  If  the  abutment  is 
of  rough  rubble,  add  6  inches  to  the  thickness  by  the  above  formula, 
to  insure  full  thickness  in  every  part."* 

To  find  the  thickness  of  the  abutment  at  the  bottom,  lay  off,  in 
Fig.  139, 071=  t  as  computed  by  the  above  equation  ;  vertically  above 


Fig.  139. 

n  lay  off  a7i  =  half  the  rise  ;  and  horizontally  from  a  lay  off  ab  =  one 
forty-eighth  of  the  span.  Then  the  line  h)i  prolonged  gives  the 
back  of  the  abutment,  provided  the  width  at  the  bottom,  sp,  is  not 
less  than  two  thirds  of  the  height,  os.  "  In  practice,  os  will  rarely 
exceed  this  limit,  and  only  in  arches  of  considerable  rise.  In  very 
high  abutments,  the  abutment  as  above  will  be  too  slight  to  sustain 
the  earth  pressure  safely."* 

To  find  the  thickness  of  the  arch,  compute  the  thickness  ce  liy 
equation  (27),  page  496,  draw  a  curve  through  e  parallel  to  tl  e 
intrados,  and  from  J  draw  a  tangent  to  the  extrad«s;  and  then  will 
bfe  be  the  top  of  the  masonry  filling  above  the  arch.  Or,  instead  of 
drawing  the  extrados  as  above,  find,  by  trial,  a  circle  which  will 
pass  through  b,  e,  and  b',  the  latter  being  a  point  on  the  left  abut- 
ment corresponding  to  b  on  the  right. 

*  Trautwine's  Engineer's  Pocket-book. 


ART.   2.]  EULES   DERIVED   FROil    PRACTICE.  501 


Trautwine's  rule,  or  a  similar  one,  for  proportioning  tlie  abut- 
ment and  the  backing  is  frequently  emploved.  For  examples,  see 
Plates  IV  and  V. 

723.  Rankine  says  that  in  some  of  the  best  examples  of  bridges 
the  thickness  of  the  abutment  ranges  from  one  third  to  one  fifth  of 
the  radius  of  curvature  of  the  arch  at  its  crown. 

The  following  formula  is  said  to  represent  German  and  Russian 
practice, 

i;  =  1  +  0.04  (5  5  +  4/0, (48) 

in  which  h  is  the  distance  between  the  springing  line  and  the  top  of 
the  foundation. 

724.  Dimensions  of  Actual  Arches.  Table  G3  (pages  502-3) 
gives  the  dimensions  of  a  number  of  actual  structures,  which,  from 
their  wide  distribution  and  the  frequency  with  which  most  of  them 
are  cited  as  examples,  may  be  taken  to  represent  average  practice. 
Unfortunately  the  details  concerning  most  of  them  are  very 
meager,  the  following  and  those  in  the  table  being  all  that  can  be 
obtained. 

No.  1  is  the  longest  span  ever  built. 

No.  2  is  the  longest  span  in  existence.*  The  arch  is  a  circular 
arc  of  110°.  It  carries  a  conduit  (clear  diameter  9  feet)  and  a  car- 
riage-way (Avidth  20  feet).  The  top  of  the  roadway  is  101  feet  above 
the  bottom  of  the  ravine.  The  voussoirs  are  Quincy  (Mass.)  granite, 
and  are  2  feet  thick,  4  feet  deep  at  the  crown,  and  6  feet  at  the 
springing.  The  spandrel  filling  is  composed  of  Seneca  sandstone, 
which,  for  a  distance  above  the  arch  of  4  feet  at  the  crown  and  15 
feet  at  the  springing,  is  laid  in  regular  courses  with  joints  radial  to 
the  intrados  ;  and  hence  the  effective  thickness  of  the  arch  is  about 
8  feet  at  the  crown  and  about  21  feet  at  the  springing  (see  Fig.  159. 
page  525).  The  abutments  are  prevented  from  spreading  by  the 
bed-rock  in  the  side-hills. 

No.  9  is  a  remarkable  bridge.  It  was  built  by  an  "  uneducated '' 
mason  in  1750;  and  although  a  very  rude  construction,  is  still  in 
perfect  condition.  A  former  bridge  of  the  same  general  design  at 
the  same  place  fell,  on  striking  the  centers,  by  the  weight  of  the 
haunches  forcing  up  the  crown  ;  and  hence  in  building  the  preseni 
structure  the  load  on  the  haunches  of  the  arch  was  lightened  bt 

*  Concerning  arched  dams,  see  foot  of  page  330  and  top  of  331. 


502 


ARCHES. 


[chap.  XVIII. 


TABLE 
Data  Concerning 


■Ref. 

No. 


Location  and  Description. 


1 

2 

3 

4 

5 

6 

7 

8 

9 
10 
11 
12 
13 
14 
15 
16 
17 
18 
19 
20 
21 
22 
23 
24 

2.T 

26 
27 
28 
29 
30 
31 
Si 
33 
34 
35 
36 
37 
3S 
39 
40 
41 
42 
43 
44 
45 
46 
47 
48 
49 
50 
51 
53 
53 
54 
55 
56 
.57 
58 
59 
60 
61 


Trezzo,  Italy:  built  in  1380,  destroyed  in  1427;  granite 

Cabin  John,  Washington  (D.  C);  aqueduct;  granite  (see  §  724,  p.  501) 

Grosvenor  bridge,  Chester,  England 

Ballochmyle,  over  the  Ayr,  Scotland 

London  bridge,  England;  street;  granite 

Gloucester,  England • . . 

Turin,  Italy 

Alma  bridge,  Paris;  small  rough  rubble  in  cement;  railroad 

Pont-y-Prydd,  Wales;  rough  rubble  in  lime  mortar  (see  §  724,  p.  501) 

Maidenhead,  England;  brick  in  cement;  railroad 

Neuilly,  France;  five  spans  (see  page  504) 

Bourbonnais  Railway  bridge ;  France  ;  cut  granite  (see  page  504) 

Waterloo  bridge,  London,  England;  granite 

Tongueland.  England ;  turnpike 

Napoleon  bridge,  Paris;  small  rough  rubble  in  cement;  railroad 

Mantes,  over  Seine,  France 

Etherow  river,  England ;  railroad ;  four  spans 

Bishop  Auckland,  England:  turnpike;  built  in  1388 

Wellington  bridge,  Leeds,  England 

Louis  XIX 

Dean  bridge,  near  Edinburgh,  Scotland ;  turnpike 

Licking  Aqueduct,  Chesapeake  &  Ohio  canal 

Dnrlaston 

Over  the  Oise,  France;  railroad 

Trilport,  France :  railroad 

Conemaugh  viaduct,  Pennsylvania  R.  R. ;  sandstone  in  lime  (no  sand) 

Royal  Border  viaduct,  England ;  brick  in  cement 

Posen  viaduct.  Germany:  brick  in  cement 

Orleans,  Fi-ance :  railroad 

Hutcheson  bridge,  Glasgow,  Scotland 

Falls  bridge,  Philadelphia  &  Reading  R.  R 

St.  Maxeiice,  over  the  Oise,  France 

Westminster  bridge,  London 

Allentown,  England;  turnpike 

Staines,  England :  turnpike.  

Black  Rock  Tunnel  bridge,  Philadelphia  &  Reading  R.  R 

Edinburgh , 

Swatara,  Philadelphia  &  Reading  R.  R. ;  brick ■ 

Brent  R.  R.  viaduct.  England ;   brick  in  cement 

Wellesley  bridge  at  Limerick 

Bow  bridge.  England ;  turnpike 

Houghton  river,  England ;  railroad 

Bewdly,  England;  turnpike 

Chestnut  Street  bridge,  Philadelphia;  brick  in  cement  

Carrollton  viaduct,  near  Baltimore;  railroad :  granite 

Llanwast,  in  Denbighshire,  Wales;  built  in  1636;  turnpike . 

Monocacy  viaduct,  Chesapeake  &  Ohio  canal 

Over  the  Forth,  at  Stirling , 

Nemours.  France 

Abattoir  Street,  Paris;  railroad 

Dole,  over  the  Doubs,  France 

Chateau  Thierry,  France 

Avon  viaduct,  England;   brick  in  cement. .... 

Filbert  St.,  Extension  Pennsylvania  R.  R.,  Philadelphia;  brick  in  lime  mortar. 

James  River  aqueduct.  Virginia 

Des  Basses-Granges.  Orleans  a  Tours,  France 

Over  the  Salat.  France 

Pesmes.  over  the  Ougnon.  France 

Philadelphia  &  Reading  R.  R 

Couturette,  Arbois,  France 

Tonoloway  culvert,  under  Chesapeake  &  Ohio  canal;  rubble  in  cement 


*  C  =  semi-circle  ;  E  =  elliptical ;  B  =  basket-bandied. 


ART.  2.] 


RULES  DERIVED  FROM  PRACTICE. 


503 


63 

A-CTUAL  Arches. 


Engineer. 


Meigfs 

Hartley..  . 
Miller.   ... 

Rennie 

Telford.  . 

Mosca 

Darcel 

Edwards. . 

Brunei 

Perronnet 
Yaudray. . 

Rennie 

Telford... 
Couche . . . 

Haskoll. . . 

Rennie 

Perronnet, 
Telford.... 
Fisk 


Stephenson 

Perronnet.. 

Labelye 

Stephenson 

Rennie 

Robinson. .. 

Mylne 

Osborne 

Brunei 

Walker 

Haskoll 

Telford 

Kneass 

Jones , 

Fisk 

Perronnet.. 

Perronnet,. 
Vignoles 

Ellet  

Bertrand... 
Steele 

Fisk 


Curve 

of 

Intrados. 


Span. 


feet. 

251 

220 

200 

181 

152 

150 

148 

141 

140 

128 

128 

124 

120 

118 

116 

115 

100 

100 

100 

94 

90 

90 

87 

83 

81 

80 

80 

80 


74 
72 
72 
70 
70 
70 
66 
65 
60 
60 
58 
58 
54 
53 
53 
53 
52 
51 
50 
50 
50 
49 
46 
45 
44 
43 
40 


Rise. 


feet. 
88 
57 
42 
90.5 
29.5 
35 
18 
28 
35 
24 
32 

6.92 
32 
38 
14.8 
34 
25 
22 
15 

9.75 
30 
15 

13  5 
11.75 
28 
40 
40 
16 

26.3 
13 
25 

6.40 
38 
11.50 

9.25 
16.5 
36 
25 
17.6 
17.5 
13.75 
32.5 
20 
18 
29 
17 

9 
10.25 

3.75 

5.11 
17.50 
17.0 
15 


24.5 
6  27 
3.83 
8 
6.13 

15 


Radius 

at 
Crown. 


feet. 
133 
134 
140 

90 
162 

98 
160 
103 

88 
169 
159 
281 
112 

65 
120 


43 

119 
38 


44 


28.3 


Thickness. 


Crown. 


feet. 

4.00 

4      t 

4.60 

4  50 

4.75 

4.50 

4.92 

4.92 

1.50 

5.25 

5.13 

2.67 

4.50 

3.50 

4.00 

6.40 

4.00 

1.83 

3.00 

3.67 

3.00 

2. as 

3  50 
4.60 
4.45 
3.00 
2.66 

4  66 
3.95 
3.50 
3.00 
4.80 
7.60 
2.50 
3.00 
2.75 
2.75 
3.50 
3.00 
2.00 
2.50 
2.75 
2.20 
2.50 
2.50 
1.50 
2.50 
2.75 
3.16 
2.97 
3,75 
3.75 
2.00 

2  00 
2.66 

3  95 
3.63 

3. as 

2. 50 
2.97 
2.00 


Spring- 
ing. 


feet. 


7.00 
6  00 
9.00 


1.50 
7.16 


3.60 
8.00 


4.00 
1.83 
7.00 


3.50 


4.50 


14.00 
3.00 
6.00 
2.75 

3.50 


2.75 
2.50 


t  See  §  720,  and  also  Fig.  159,  page  525. 


604  ARCHES.  [chap.  XVIII. 

leaving  horizontal  cylindrical  openings  (see  third  paragraph  of 
§  730)  through  the  spandrel  filling.  The  outer,  or  showing,  arch 
stones  are  only  2.5  feet  deep,  and  that  depth  is  made  up  of  two 
stones;  and  the  inner  arch  stones  are  only  1.5  feet  deep,  and  but 
from  6  to  9  inches  thick.  The  stone  quarried  with  tolerably  fair 
natural  beds,  and  received  little  or  no  dressing.  It  is  a  wagon-road 
bridge,  and  has  almost  no  spandrel  filling,  the  roadway  being  dan- 
gerously steep.  A  strain  sheet  of  the  arch  shows  that  the  line  of 
resistance  remains  very  near  the  center  of  the  arch  ring  (see  §  730). 
The  mean  pressure  at  the  crown  is  about  244  pounds  per  square 
inch.     On  the  whole  it  is  an  example  of  creditable  engineering. 

No.  11,  as  designed,  had  a  radius  at  the  crown  of  160  feet ;  but 
the  arch  settled  2  feet  on  removing  the  center,  and  increased  the 
radius  to  about  250  feet. 

No.  12  is  noted  for  its  boldness.  This  design  was  tested  by 
building  an  experimental  arch — at  Soujoes,  France— of  the  propor- 
tions given  in  the  table,  and  12  feet  wide.  The  center  of  the  ex- 
perimental arch  was  struck  after  four  months,  when  the  total  set- 
tlement was  1.25  inches,  due  mostly  to  the  mortar  joints,  which 
were  about  one  quarter  inch  ;  and  it  was  not  injured  by  a  dis- 
tributed load  of  500  pounds  per  square  foot,  nor  by  a  weight  of  5 
tons  falling  1.5  feet  on  the  key. 

No.  46  is  said  to  have  "  approached  a  horizontal  line  in  conse- 
quence of  the  substitution  of  vehicles  for  pack-horses." 

725.  Table  63  affords  some  striking  comparisons.  For  exam- 
ple, Nos.  8  and  9  have  practically  the  same  span  ;  and  as  the  rise 
of  the  former  is  four  fifths  that  of  the  latter,  the  thickness  at  the 
crown  of  the  former  should  be  only  about  one  and  a  .quarter  times 
that  of  the  latter,  while  in  fact  it  is  3.3  times  as  thick.  How- 
ever, the  former  carries  a  railroad,  and  the  latter  a  turnpike  ;  but, 
on  the  other  hand,  the  former  is  laid  in  cement,  and  the  latter  in 
lime. 

Nos.  11  and  12  have  nearly  the  same#span,  but  the  rise  of  the 
former  is  4.7  times  the  latter  ;  and  if  the  thickness  at  the  crown 
were  in  like  proportion — as  it  should  be, — that  of  the  former 
would  be  only  0.6  feet.  Also  compare  No.  32  with  No.  33  ;  and 
No.  33  with  Nos.  9  and  18. 

726.  Dimensions  of  Abutments.  For  examples  of  the  abutments 
of  railway  culverts,  see  Tables  49-52  (pages  425-31).      Table  64, 


AET.  2.] 


RULES    DERIVED   FROM    PRACTICE. 


505 


below,  gives  the  dimensions  of  a  number  of  abutments  represen- 
tative of  French  railroad  practice. 

TABLE  64. 
Dimensions  of  Abutments  from  French  Railroad  Practice.* 


Designation  of  Bridge. 


I       Circular  Arches. 

De  crochet,  chemiti  de  fer  de  Paris  k  Chartres 

De  Lotifir-Sauts,  chemin  de  fer  de  Paris  ft.  Cliartres. 

D'Eiifcliien,  cheiiiiii  de  fer  du  Nord 

De  Pantin,  caual  St.  Martiu 

De  la  Bastille,  canal  St.  Martin 

De  Basses-Grauges,  Orleans  k  Tours 


Segmental  Arches. 

Des  Fruitiers,  chemiu  du  fer  du  Nord 

De  Paisia 

De  M6ry,  chemin  de  fer  du  Nord.   

De  Couturette,  at  Arbois 

Over  the  Salat 

iDe  la  rue  des  Abattoirs,  at  Paris,  chemin  de  fer  de 

Strasbourg    

Over  the  Forth,  at  Stirling 

St.  Maxence,  over  the  Oise 

Over  the  Oise,  chemin  de  fer  du  Nord  

De  Dorlaston 


Elliptical  or  False-Elliptical  Arches. 

I  De  Charolles  

I  Du  Canal  St.  Denis  

j  De  Chateau-Thierry 

De  Dole,  over  the  Doubs  

Wellesley,  at  Limerick   

D'Orleans,  chemin  de  fer  de  Vierzon 

De  Trilport 

I  De  Nantes,  over  the  Seine  

De  Neuilly,  over  the  Seine 


feet. 
13.3 
16.5 
24.4 
27.0 
36.3 
49.4 


13.2 
16.5 
25.2 
42.9 
46.1 

52.9 
53.5 

77.2 
82.7 
87.0 


19.8 
39.5 
51.3 
52.4 
70.0 
79.5 
80.7 
115.2 
128.0 


feet. 


2.31 
2.64 
2.97 
6.13 
6.27 

5.11 
10.25 

6.40 
11.75 
13.50 


7.55 
14.85 
17.10 
17.50 
17.50 
26.30 
27.80 
34.40 
32.00 


5  >> 


feet. 
1.65 
1.81 
1.95 
2.47 
3.95 
3.95 


1.81 
1.72 
2.14 
2.97 
3.63 

2.97 
2.75 
4.80 
4.60 
3.50 


1.95 
2.95 
3.75 
3.75 
2.00 
3.95 
4.45 
6.40 
5.35 


M;3 


feet. 

13.20 
9.90 
6  60 

11.85 

20.75 
6.60 


13.20 
6.60 

14.20 
6.60 

24.49 

12.96 
20.75 
27.85 
17.90 
16.55 


1.30 
10.20 
13.65 
1.35 
12.00 
2.85 
6.40 
3.20 
7.55 


feet. 
4  95 
5.90 
6.93 

o.no 

12.50 


5.94 
5.01 
11.71 
17.16 
19.14 

33.00 
16.00 
38.94 
31.65 
.32.20 


5.25 
12.35 
15.00 
11.85 
16.50 
IS. 40 
19.30 
28.90 
35.50 


727.  Illustrations  of  Actual  Arches.— For  illustrations  of 
stone  arches  for  railroad  culverts,  see  Plates  II-V.  Fig.  1-1:3  (page  509) 
shows  a  50-foot  stone  arch  on  the  Pennsylvania  Railroad.  For 
brick  arches  for  sewers,  see  Figs.  148  and  149  (pages  513  and  514). 
For  an  example  of  a  brick  tunnel-arch,  see  Fig.  147  (page  512). 
Cabin  John  arch,  the  longest  span  in  the  world  (see  No.  2  of  Table 
63,  page  502),  is  shown  incidentally  in  Fig.  159  (page  525). 

728.  Minor  Details.  Backing.  The  backing  is  masonry  of 
inferior  quality  laid  outside  and  above  the  arch  stones  proper,  to 
give  additional  security.  The  backing  is  ordinarily  coursed  or  ran- 
dom rubble,  but  sometimes  concrete.     Sometimes  the  upper  ends 


*  E.  Sherman  Gould,  in  Van  Nostrand's  Engin'g  Mag.,  vol.  xxix,  i>  450. 


50G 


ARCHES. 


[chap.   XVIII. 


of  the  arch  stones  are  cut  with  horizontal  surfaces,  in  which  case 
the  backing  is  built  in  courses  of  the  same  depths  as  these  steps 
and  bonded  with  them.  The  backing  is  occasionally  built  in  ra- 
diating courses,  whose  beds  are  prolongations  of  the  bed-joints  of 
the  arch  stones  ;  but  it  usually  consists  of  rubble,  laid  in  horizontal 
courses  abutting  against  the  arch  ring,  with  occasional  arch  stones 
extending  into  the  former  to  bond  both  together.  The  radial 
joints  possess  some  advantages  in  stability  and  strength,  particu- 
larly above  the  joint  of  rupture  ;  but  below  that  joint  the  horizon- 
tal and  vertical  joints  are  best,  since  this  form  of  construction  the 
better  resists  the  overturning  of  the  arch  outward  about  tlie 
springing  line.  Ordinarily,  the  backing  has  a  zero  thickness  at  or 
near  the  crown,  and  gradually  increases  to  the  springing  line  ;  but 
sometimes  it  has  a  considerable  thickness  at  the  crown,  and  is  pro- 
portionally thicker  at  the  springing. 

It  is  impossible  to  compute  the  degree  of  stability  obtained  by 
the  use  of  backing ;  but  it  is  certain  that  the  amount  ordinarily 
employed  adds  very  greatly  to  the  stability  of  the  arch  ring.  In 
fact,  many  arches  are  little  more  than  abutting  cantilevers  ;  and  it 
is  probable  that  often  the  backing  alone  would  support  the  struct- 
ure, if  the  arch  ring  were  entirely  removed. 

729.  Spandrel  Filling.  Since  the  roadway  must  not  deviate 
greatly  from  a  horizontal  line,  a  considerable  quantity  of  material  is 

required  above  the  backing  to  bring  the 
roadway  level.  Ordinarily  this  space  is 
filled  with  earth,  gravel,  broken  stone, 
cinders,  etc.  Sometimes,  tc  save  filling, 
small  arches  are  built  ovei-  the  haunches 
of  the  main  arch,  as  shown  in  Fig.  140. 
The  interior  longitudinal  walls  may  be 
strengthened  by  transverse  walls  between 
them.  To  distribute  the  pressure  uni- 
formly, the  feet  of  these  walls  should 
be  expanded  by  footings  where  they  rest 
upon  the  back  of  the  arch. 

730.   When  the  load  is  entirely  sta- 
tionary— as    in   an     aqueduct   or   canal 
Fia.  140.  bridge — or  nearly  so — as  in  a  long  span 

arch  under  a  high  railroad  embankment, — the  materials  of  the 


ART.  3.] 


EULES   DERIVED   FROil    PRACTICE. 


507 


spandrel  filling  and  the  size  and  position  of  the  empty  spaces  may 
be  such  as  to  cause  the  line  of  resistance  to  coincide,  at  least  very 
nearly,  with  the  center  of  the  arch  ring. 

For  example,  A  BCD,  Fig.  141,  represents  a  semi-arch  for  which 
it  is  required  to  find  a  disposition  of  the  load  that  will  cause  the 
line  of  resistance  to  coincide  with  the  center  line  of  the  arch  ring. 


Fig.  141. 

Divide  the  arch  and  the  load  into  any  convenient  number  of  divi- 
sions, by  vertical  lines  as  shown.  From  P  draw  radiating  lines  par- 
allel to  the  tangents  of  the  center  line  of  the  arch  ring  at  a,  h,  c, 
etc. ;  and  then  at  such  a  distance  from  P  that  01  shall  represent, 
to  any  convenient  scale,  the  load  on  the  first  section  of  the  arch  ring 
(including  its  own  weight),  draw  a  vertical  line  through  0.  The 
intercepts  0-1,  1-3,  3-3,  etc.,  represent,  to  scale,  the  loads  which  the 
several  divisions  must  have  to  cause  the  line  of  resistance  to  coincide 
with  the  center  of  the  arch  ring.  Lay  off  the  distances  0-1,  1-3, 
etc.,  at  the  centers  of  the  respective  sections  vertically  upwards  from 
the  center  line  of  the  arch  ring,  and  trace  a  curve  through  their 
upper  ends.  The  line  thus  formed — EF,  Fig.  141— shows  the  re- 
quired amount  of  homogeneous  load;  i.  e.,  EF  is  the  contour  of  the 
homogeneous  load  that  will  cause  the  line  of  resistance  to  pass  ap- 
proximately through  the  center  of  each  joint. 

Hence,  by  choosing  the  material  of   the   spandrel  filling  and 


508 


ARCHES. 


[chap.   XVIII. 


arranging  the  empty  spaces  so  that  the  actual  load  shall  be  equiv- 
alent in  intensity  and  distribution  to  the  reduced  load  obtained  as 
above,  the  voussoirs  can  be  made  of  moderate  depth.  The  vacant 
spaces  may  be  obtained  by  tlie  method  shown  in  Fig.  140  (page 
506)  ;  or  by  that  shown  in  Fig,  142,  in  which  .4  is  a  small  empty 
cylindrical  arch  extending  from  the  face  of  one  end  wall  to  that  of 
the  other.     (See  the  description  of  arch  No.  9,  §  724,  p.  501.) 

Notice  that  the  lines  radiating  successively  from  P,  Fig.  141 
(page  507),  will  intercej)t  increasing  lengths  on  the  load-line  ;  and 
that,  therefore,  the  load  which  will  keep  a  circular  arch  in  equilib- 
rium must  increase  in  intensity  per  horizontal  foot,  from  the  crown 
towards  the  springing,  and  must  become  infinite  at  the  springing  of 

a  semi-circular  arch.  Hence 
it  follows  that  it  is  not  practi- 
cable to  load  a  circular  arch, 
beyond  a  certain  distance  from 
the  crown,  so  that  the  line  ol 
resistance  shall  coincide  with 
the  center  line  of  the  arch 
ring. 

731.  Drainage.    The  drain- 
age of  arch    bridges  of  more 
than  one  span  is  generally  ef- 
fected by  giving  the  top  sur- 
FiG- 142.  face  of  the  backing  a  slight 

inclination  from  each  side  toward  the  center  of  the  width  of  the 
bridge  and  also  from  the  center  toward  the  end  of  the  span.  The 
water  is  thus  collected  over  the  piers,  from  whence  it  is  discharged 
through  pipes  laid  in  the  masonry. 

To  prevent  leakage  through  the  backing  and  through  the  arch 
sheeting,  the  top  of  the  former  should  be  covered  with  a  layer  of 
puddle,  or  plastered  with  a  coat  of  best  cement  mortar  (see  §  141), 
or  painted  with  coal  tar  or  asphaltum  (see  §  264). 

732.  For  an  illustration  of  the  method  of  draining  a  series  of 
arches,  and  also  of  several  minor  details  not  mentioned  above,  see 
Fig.  143,  which  represents  "  Little  Juniata  bridge  No.  12  "  on  the 
Pennsylvania  Railroad.* 


*  Published  by  permission  of  Wm.  H.  Brown,  chief  engineer. 


ART. 


RULES    DERIVED   FROM    PRACTICE. 


509 


610  ARCHES.  [chap.   XYIII. 

733.  Beick  Arches.  The  only  matter  requiring  special  mention 
in  connection  with  brick  arches  is  the  bond  to  be  employed.  When 
the  thickness  of  the  arch  exceeds  a  brick  and  a  half,  the  bond  from 
the  soffit  outward  is  a  very  important  matter.  There  are  three 
principal  methods  employed  in  bonding  brick  arches.  (1)  The  arch 
may  be  built  in  concentric  rings  ;  i.  c,  all  the  brick  may  be  laid  as 
stretchers,  with  only  the  tenacity  of  the  mortar  to  unite  the  several 
rings  (see  Fig.  144).  This  form  of  construction  is  frequently  called 
roivlock  bond.  (2)  Part  of  the  brick  may  be  laid  as  stretchers  and 
part  as  headers,  as  in  ordinary  walls,  by  thickening  the  outer  ends 
of  the  joints — either  by  using  more  mortar  or  by  driving  in  thin 
pieces  of  slate, — so  that  there  shall  be  the  same  number  of  bricks  in 


Fm.  144.  Fig.  145.  Fig.  14B. 


each  ring  (see  Fig.  145).  This  form  of  construction  is  known  as  header 
and  stretcher  bond,  or  is  described  as  being  laid  with  contmuous 
radial  joints.  (3)  Block  in  course  bond  is  formed  by  dividing  the 
arch  into  sections  similar  in  shape  to  the  voussoirs  of  stone  arches, 
and  laying  the  brick  in  each  section  with  any  desired  bond,  but 
making  the  radial  joints  between  the  sections  continuous  from 
intrados  toextrados.  With  this  form  of  construction,  it  is  custom* 
ary  to  lay  one  section  in  rowlock  bond  and  the  other  with  radial 
joints  continuous  from  intrados  to  extrados,  the  latter  section  being 
much  narrower  than  the  former  (see  Fig.  146). 

1.  The  objection  to  laying  the  arch  in  concentric  rings  is  that, 
since  the  rings  act  nearly  or  quite  independent  of  each  other,  the 
proportion  of  the  load  carried  %  each  can  not  be  determined.  A 
ring  may  be  called  upon  to  support  considerably  more  than  its  proper 
share  of  the  load.  This  is  by  far  the  most  common  form  of  bonding 
in  brick  arches,  and  that  this  difficulty  does  not  more  often  mani- 


ART.  2.]  RULES    DERIVED    FROM    PRACTICE.  611 

fest  itself  is  doubtless  due  to  the  very  low  unit  working  pressure 
employed.  The  mean  pressure  on  brick  masonry  arches  ordinarily 
varies  from  20  to  40  pounds  per  square  inch,  under  which  condition 
a  single  ring  might  carry  the  entire  pressure  (see  Tables  19  and  20, 
pages  164  and  166).  The  objection  to  this  form  of  bond  can  be 
partially  removed  by  using  the  very  best  cement  mortar  between 
the  rings. 

The  advantages  of  the  ring  bond,  particularly  for  tunnel 
and  sewer  arches,  are  as  follows :  It  gives  4-inch  toothings  for  con- 
necting with  the  succeeding  section,  while  the  others  give  only 
2-inch  toothings  along  much  of  the  outline.  It  requires  less 
cement,  is  more  rapidly  laid,  and  is  less  liable  to  be  poorly  executed. 
It  possesses  certain  advantages  in  facilities  for  drainage,  when  laid 
in  the  presence  of  water. 

2.  The  objection  to  laying  the  arch  with  continuous  radial  joints 
is  that  the  outer  ends  of  the  joints,  being  thicker  than  the  inner, 
will  yield  more  than  the  latter  as  the  centers  are  removed,  and 
hence  concentrate  the  pressure  on  the  intrados.  This  objection 
is  not  serious  when  this  bond  is  employed  in  a  narrow  section 
between  two  larger  sections  laid  in  rowlock  courses  (see  Fig.  146). 

3.  When  the  brickwork  is  to  be  subject  to  a  heavy  pressure, 
some  form  of  the  block  in  course  bond  should  be  employed.  For 
economy  of  labor,  the  ''blocks"  of  headers  should  be  placed  at  such 
a  distance  apart  that  between  each  pair  of  them  there  shall  be  one 
more  course  of  stretchers  in  the  outer  than  in  the  inner  ring  ;  but  a 
moment's  consideration  will  show  that  this  would  make  each  section 
about  half  as  long  as  the  radius  of  the  arch, — which,  of  course, 
is  too  long  to  be  of  any  material  benefit.  Hence,  this  method 
necessitates  the  use  of  thin  bricks  at  the  ends  of  the  rings. 

734.  Examples  of  Brick  Arches.  The  method  of  bonding  shown 
in  Fig.  146  (page  510)  is  frequently  employed — as,  for  example,  in 
the  70-foot  brick  arch  of  the  Swatara  bridge  (Philadelphia  and 
Reading  R.  R.).  The  bonding  employed  in  arching  the  Vosburg 
tunnel  (Lehigh  Valley  R.  R.)  is  shown  in  Fig.  147  (page  512).* 

735.  Fig.  148  (page  513)  shows  the  standard  forms  of  large 
brick  sewers  employed  in  the  city  of  Philadelphia,  f     "  They  are 


*From  Rosenberg's  "  The  Vosburg  Tunnel,"  by  permission. 
tR.  Hering,  in  Trans.  Am.  Soc.  of  C.  E.,  vol.  Tii,  pp.  252-57.      The  illustrations 
are  reproduced  from  those  in  the  original,  the  force  diagrams  being  omitted  here. 


512 


ARCHES. 


[chap.    XVIII. 


designed  for  a  maximum  pressure  on  the  brick-work  of  80  pounds 
per  square  inch,"  which,  considering  the  usual  specifications  for 
such  work  (see  §  2G0,  p.  176),  seems  unnecessarily  small  (see  Tables 
19  and  20,  pages  164  and  166). 

Fig.  149  (page  514)  shows  the  standard  forms  of  sewers  in 
Washington,  D.  C*  "The  invert  as  shown  is  the  theoretical  form, 
although  the  concrete  is  rammed  into  the  trench  and  nearly  always 
extends  beyond  the  limits  shown."  The  largest  sewers  have  a  trap- 
rock  bottom ;  the  intermediate  sizes  have  a  semi-circular  vitrified 


Fig.  1J7.— Bond  and  Center  of  Vosburg  Tunnel. 

pipe  in  the  bottom  ;  and  the  smallest  sizes  consist  of   sewer  pipes 
bedded  in  concrete. 

736.  Owing  to  their  great  number  of  joints,  brick  arches  are 
liable  to  settle  much  more  than  stone  ones,  when  the  centers  are  re- 
moved ;  and  hence  are  less  suitable  than  the  former  for  large  or  fiat 
arches.  Nevertheless  a  number  of  brick  arches  of  large  span  have 
been  built  (see  Table  63,  page  503).  Trautwine  gives  the  following 
description  of  some  bold  examples.  "  On  the  Filbert  Street  exten- 
sion of  the  Pennsylvania  R.  R.,  in  Philadelphia,  are  four  brick  arches 
-of  50  feet  span,  and  with  the  very  low  rise  of  7  feet.  The  arch  rings 
:are  2h  feet  thick,  except  on  their  showing  faces,  where  they  are  but  2 
feet.     The  joints  are  in  common  lime  mortar,  and  are  about  ^  inch 


*  Report  of  the  Commissioners  of  the  District  of  Columbia,  for  the  year  ending 
June  30,  1884,  p.  175.  For  details  of  quantities  of  material  required,  and  for  esti- 
mates of  cost,  see  report  for  preceding  year,  pp.  277-79. 


ART.  2. J 


RULES  DERIVED  FROM  PRACTICE. 


513 


514 


ARCHES. 


[chap.   XVIII. 


Fia.  149.— Standard  Forms  of  Brick  sewers.— Washington,  D.  C. 


ART.  3.]  CEN-TERS.  515 

thick.  These  four  arches,  about  ^00  yards  apart,  with  a  large  num- 
ber of  others  of  26  feet  span,  form  a  viaduct.  The  piers  between 
the  short  spans  are  4^  feet  thick,  and  those  at  the  ends  of  the  oO-foot 
spans  are  18j  feet.  The  road-bed  is  about  100  feet  wide,  giving  room 
for  9  or  10  tracks.  The  springing  lines  of  all  the  arches  are  about 
6  to  8  feet  above  the  ground.  One  of  the  50-foot  arches  settled  3 
inches  upon  permanently  striking  the  center  ;  but  no  further  settle- 
ment has  been  observed,  although  the  viaduct  has,  since  built  (1880), 
had  a  very  heavy  freight  and  passenger  traffic  at  from  10  to  20  miles 
per  hour.-" 

737.  Specifications  for  Stone  Arches.  The  specifications  for 
arch  masonry  employed  on  the  Atchison,  Topeka  and  Santa  Fe 
Railroad  are  as  follows  :  * 

738.  First-Class  Arch  Masonry  shall  be  built  in  accordance  with  the  speci- 
fications for  first-class  masonrj-  [see  §  207],  with  the  exception  of  the  arch  sheet, 
iug  and  ring  stones.  The  ring  stones  shall  be  dressed  to  such  shape  as  the 
engineer  shall  determine.  The  ring  stones  and  the  arch  sheeting  shall  be  not 
less  than  ten  inches  (10")  thick  on  the  intrados,  and  shall  have  a  depth  equal 
tn  the  specified  thickness  of  the  arch.  The  joints  shall  be  at  right  angles  to 
the  intrados,  and  their  thickness  shall  not  exceed  three  .eighths  of  an  inch  (f  ). 
The  face  of  the  sheeting  stones  shall  be  dressed  so  as  to  make  a  close  center- 
ing joint.  The  ring  stones  and  sheeting  shall  break  joints  not  less  than  one 
foot  (1 ). 

The  wings  shall  be  neatlj-  stepped  with  selec>*'d  stones  of  the  full  width  of 
the  wing,  and  of  not  less  than  ten  inches  (10")  in  thickness,  overlapping  by 
not  less  than  one  and  one  half  feet  (U');  or  they  shall  be  finished  with  a  neatly 
capped  newel  at  the  end  of  each  wing,  and  a  coping  course  on  the  wing.  The 
parapets  shall  be  finished  with  a  coping  course  of  not  less  than  ten  inches 
(10  ■)  in  thickness,  having  a  projection  of  six  inches  (6"). 

739.  Second-Class  Arch  Masonry  shall  be  the  same  as  first-class  masonry  (see 
§  207).  The  stones  of  the  arch  sheeting  .shall  be  at  least  four  inches  (4)  in  thick- 
ness  on  the  intrados  ;  shall  have  a  depth  equal  to  the  thickness  of  the  arch  ; 
shall  have  good  bearings  throughout ;  and  shall  be  well  bonded  to  each  other 
and  to  the  ring  stones. 

740.  Specifications  for  Brick  Arches.  See  §§  2G0-61  (pages 
lTG-77). 

Art.  3.  Arch  Cexters. 

741.  A  ce7iter  is  a  temporary  structure  for  supporting  an  arch 
while  in  process  of  construction.  It  usually  consists  of  a  number  of 
frames   (commonly  called   ribs)  placed  a  few  feet  apart  in  planee 

*  For  general  specifications  for  raihroad  masonry,  see  Appendix  I. 


516  ARCHES.  [chap.   XVIII. 

perpendicular  to  the  axis  of  the  arch,  and  covered  with  narrow 
planks  (called  laggings)  running  parallel  to  the  axis  of  the  arch, 
upon  which  the  arch  stones  rest.  The  center  is  usually  wood — 
either  a  solid  rib  or  a  truss, — but  is  sometimes  a  curved  rolled-iron 
beam.  In  a  trussed  center,  the  pieces  upon  which  the  laggings  rest 
are  called  hack-pieces.  The  ends  of  the  ribs  may  be  supported 
by  timber  struts  which  abut  against  large  timbers  laid  upon  the 
ground,  or  they  may  rest  upon  a  shoulder  on  the  abutment. 

The  framing,  setting  up,  and  striking  of  the  centers  (§§  752-55) 
is  a  very  important  part  of  the  construction  of  any  arch,  particularly 
one  of  long  span.  A  change  in  the  shajje  of  the  centei',  due  to 
insufficient  strength  or  improper  bracing,  will  be  followed  by  a 
change  in  the  curve  of  the  intrados  and  consequently  of  the  line  of 
resistance,  which  may  endanger  the  safety  of  the  arch  itself. 

742.  Load  to  be  Supported.  If  there  were  no  friction,  the  load 
to  be  supported  by  the  center  could  be  computed  exactly  ;  but  fric- 
tion between  the  several  arch  stones  and  between  these  and  the 
center  renders  all  formulas  for  that  purpose  very  uncertain. 
Fortunately,  the  exact  load  upon  the  center  is  not  required  ;  for  the 
center  is  only  a  temporary  structure,  and  the  material  employed  in 
its  construction  is  not  entirely  lost.  Hence  it  is  wise  to  assume 
the  loads  to  be  greater  than  they  really  will  be.  Some  allowance 
must  also  be  made  for  the  accumulation  of  the  material  on  the 
center  and  for  the  effect  of  jarring  during  erection.  The  following 
analysis  of  the  problem  will  show  roughly  what  the  forces  are  and 
why  great  accuracy  is  not  possible. 

To  determine  the  pressure  on  the  center,  consider  the  voussoir 
DEFG,  Fig.  150,  and  let 

a  =  the  angle  which  the  joint  DE  makes 

with  the  horizontal ; 
fx  =  the  co-efficient  of  friction  (see  Table  ."'0. 
page  315),  i.  e.,  jj.  is  the  tangent  of 
the  angle  of  repose  ; 
0  —  the  angular  distance  of  any  point  from 

the  crown  ; 
W  =  the  weight  of  the  voussoir  DEFG  ; 
N  =■  the  radial  pressure  on  the  center  due  to 
^'°-  ^^-  the  weight  of  DEFG, 

If  there  were  no  friction,  the  stone  DEFG  would  be  supported 


ART.  3;]  CENTERS.  517 

by  the  normal  resistance  of  the  surface  DE  and  the  radial  reac- 
tion of  the  center.  The  pressure  on  the  surface  DE  would  then 
be  W  cos  a,  and  the  pressure  in  the  direction  of  the  radius  W  sin  a. 
Friction  causes  a  slight  iiidetermination,  since  part  of  the  weight 
of  the  voussoirs  may  pass  to  the  abutment  either  through  the  arch 
ring  or  through  the  back-pieces  (perimeter)  of  the  center.  Owing 
to  friction,  both  of  these  surfaces  will  offer,  in  addition  to  the 
above,  a  resistance  equal  to  the  product  of  the  perpendicular  pres- 
sure and  the  co-efficient  of  friction  (foot-note,  page  276).  If  the 
normal  pressure  on  the  joint  DE  is  IT  cos  a,  then  the  frictional 
resistance  is  yu  IF  cos  a.  Any  frictional  resistance  in  the  joint  DE 
will  decrease  the  pressure  on  the  center  by  that  amount ;  and  conse- 
quently, with  friction  on  the  joint  DE,  the  radial  pressure  on  the 
center  is 

N  =  ir  (sin  a  —  J.I  cos  a) (49) 

On  the  other  hand,  if  there  is  friction  between  the  arch  stone  and 
the  center,  the  frictional  resistance  between  these  surfaces  will 
decrease  the  pressure  upon  the  joints  DE,  as  computed  above  ;  and 
consequently  the  value  of  N  will  not  be  as  in  equation  (49). 

Notice  that  in  passing  from  the  springing  toward  the  crown  the 
pressure  of  one  arch  stone  on  the  other  decreases.  Near  the  crown 
this  decrease  is  rapid,  and  consequently  the  friction  between  the 
voussoirs  may  be  neglected.  Under  this  condition,  the  radial  pres- 
sure on  the  center  is 

N=WcosO (50) 

As  a  rough  approximation,  equation  (50)  may  be  applied  for 
the  first  30°  from  the  crown,  although  it  gives  results  slightly 
greater  than  the  real  pressures ;  and  for  the  second  30°,  equation 
(49)  may  be  employed,  although  it  gives  results  less  than  the  actual 
pressure  ;  and  for  the  third  30°,  the  arch  stones  may  be  considered 
self-supporting. 

743.  The  value  of  the  co-efficient  to  be  employed  in  equation 
(49)  is  somewhat  uncertain.  Disregarding  the  adhesion  of  the 
mortar,  the  co-efficient  varies  from  about  0.4  to  0.8  (see  Table  36, 


518  ARCHES.  [chap.' XVIII. 

page  315)  ;  and,  including  the  adhesion  of  good  cement  mortar,  it 
may  be  nearly,  or  even  more  than,  1.  (It  is  1  if  an  arch  stone 
remains  at  rest,  without  other  support,  when  placed  upon  another 
one  in  such  a  position  that  the  joint  between  them  makes  an  angle 
of  45°  with  the  horizontal.)  If  the  arch  is  small,  and  consequently 
laid  up  before  the  mortar  has  time  to  harden,  probably  the  smaller 
value  of  the  co-efficient  should  be  used  ;  but  if  the  arch  is  laid  up 
so  slowly  that  the  mortar  has  time  to  harden,  a  larger  value  could, 
with  equal  safety,  be  employed.  As  a  general  average,  we  will 
assume  that  the  co-efficient  is  .58,  i.  e.,  that  the  angle  of  repose 
is  30°. 

Notice  that  by  equation  (49)  N  =  0,  if  tan  a  =  /u ;  that  is  to 
say,  N  =  0,  if  a  =  30°.  This  shows  that  as  the  arch  stones  are 
placed  upon  one  another  they  would  not  begin  to  press  upon  the 
center  rib  until  the  plane  of  the  lower  face  of  the  top  one  reaches 
an  angle  of  30°  with  the  horizon. 

Table  65  gives  the  value  of  the  radial  pressure  of  the  several  por- 
rions  of  the  arch  upon  the  center ;  and  also  shows  the  difference 
between  applying  equation  (49)  and  equation  (50).  Undoubtedly 
the  former  should  be  applied  when  the  angle  of  the  lower  face  of 
any  arch  stone  with  the  horizontal  does  not  differ  greatly  from  30°; 
and  when  this  angle  is  nearly  90°,  then  equation  (50)  should  be  ap- 
plied. It  is  impossible  to  determine  the  point  at  which  one  equation 
becomes  inapplicable  and  the  other  applicable  ;  but  it  is  probably 
safe  to  apply  equation  (49)  up  to  60°  from  the  horizontal. 

744.  Example.  To  illustrate  the  method  of  using  Table  65, 
assume  that  it  is  required  to  find  the  pressure  on  a  back-piece  of  a 
20-foot  semi-circular  arch  which  extends  from  30°  to  60°  from  the 
horizontal,  tlie  ribs  being  5  feet  apart,  and  the  arch  stones  being  2 
feet  deep  and  weighing  150  pounds  per  cubic  foot.  Take  the  sum 
of  the  decimals  in  the  middle  column  of  Table  65,  which  is  3.19. 
Tins  must  be  multiplied  by  the  weight  of  the  arch  resting  on  2°  of  the 
center.  (In  this  connection  it  is  convenient  to  remember  tliat  an 
arc  of  1°  is  equal  to  0.0175  times  the  radius.)  The  radius  to  the 
middle  of  the  voussoir  is  11  feet,  and  the  length  of  2°  of  arc  is  0.38 
feet.  The  volume  of  2°  is  0.38x5x2  =  3.8  cubic  feet;  and  the 
weight  of  2°  is  3.8x150  =  570  pounds.  Therefore  the  pressure 
on  the  back-piece  is  570x3.19  =  1,818  pounds. 


ART.  3.] 


CENTERS. 


519 


TABLE  65. 

The  Radial  Pressure  op  the  Arch  Stones 
OP  A  Semi-Arch,   on  the  Center. 


Radial  Pressure  in  Terms  of  the 

Angle  of  the  Lower 

Weight  of  the  Arch  Stone. 

Face  with  the 
Horizontal. 

By  Equation  (49). 

By  Equation  (50). 

30° 

0.00 

33° 

0.04 

34° 

0.08 

36° 

0.12 

38° 

0.16 

40° 

0.20 

42° 

0.24 

0.67 

44° 

0.28 

0.69 

46° 

0.33 

0.72 

48° 

0  36 

0.74 

50° 

0.40 

0.76 

55° 

0.45 

0.83 

60° 

0.54 

0.86 

65° 

.... 

0.91 

70° 



0.94 

80° 



0.98 

90° 

1.00 

745.  Outline  Forms  of  Centers.     Solid  Wooden  Rib.    For 

flat  arches  of  10-foot  span  or  under,  the  rib  may  consist  of  a  plank, 
a,  a,  Fig.  151,  10  or  12  inches  wide  and  1^  or  3  inches  thick,  set 


Fig.  151. 


f^dgewise,  and  another,  b,  of  the  same  thickness,  trimmed  to  the 
curve  of  the  intrados  and  placed  above  the  first.  The  two  should 
r>e  fastened  together  by  nailing  on  two  cleats  of  narrow  boards  as 
snown.     These  centers  may  be  placed  3  or  3  feet  apart. 


520 


ARCHES. 


[chap.  XVI  II» 


746.   Built  Wooden  Rib.     For  flat  arches  of  10  to  30  feet  span, 

the  rib  may  consist  of  two  or 
three  thicknesses  of  short 
boards,  fitted  and  nailed  (or 
bolted)  together  as  shown  m 
Fig.  152.  An  iron  plate  is 
often  bolted  on  over  the  Joints 
(see  Fig.  147,  page  512),  which 
adds  materially  to  the  rigidity 
of  the  rib.  Centers  of  this 
form  have  an  astonishing  strength.  Trantwine  gives  the  two  fol- 
lowing examples  which  strikingly  illustrate  this. 

In  the  first  of  these  examples,  this  form  of  center  was  employed 
for  a  semi-circnlar  arch  of  35  feet  span,  having  arch  stones  2  feet 
deep.  "  Each  rib  consisted  of  two  thicknesses  of  2-inch  plank,  in 
lengths  of  abont  6.5  feet,  treenailed.  together  so  as  to  break  joint. 
Each  piece  of  plank  was  12  inches  deejj  at  the  middle,  and  8  inches, 
at  each  end,  the  top  edge  being  cut  to  suit  the  curve  of  the  arch.  The 
treenails  were  1.25  inches  in  diameter,  and  12  of  them  were  used  to 
each  length.  These  ribs  were  placed  17  inches  apart  from  center  to 
center,  and  were  steadied  together  by  a  bridging  piece  of  1-inch 
board,  13  inches  long,  at  each  joint  of  the  planks,  or  about  3.25  feet 
apart.  Headway  for  traffic  being  necessary  under  the  arch,  there 
were  no  chords  to  unite  the  opposite  feet  of  the  ribs.  The  ribs  were 
covered  with  close  board-lagging,  which  also  assisted  in  steadying 
them  together  transversely.  As  the  arch  approached  about  two 
thirds  of  its  height  on  each  side,  the  ribs  began  to  sink  at  the 
haunches  and  rise  at  the  crown.  This  was  rectified  by  loading  the 
crown  with  stone  to  be  used  in  completing  the  arch,  which  was  then 
finished  without  further  trouble." 

The  other  example  was  an  elliptic  arch  of  60  feet  span  and  15 
feet  rise,  the  arch  stones  being  3  feet  deep  at  the  crown  and  4  feet 
at  the  springing.  '^'^Eacli  frame  of  the  centre  was  a  simple  rib  6 
inches  thick,  composed  of  three  thicknesses  of  2-inch  oak  plank, 
in  lengths  (about  7  to  15  feet)  to  suit  the  curve  and  at  the  same 
time  to  preserve  a  width  of  about  16  inches  at  the  middle  of  each 
length  and  12  inches  at  each  of  its  ends.  The  segments  broke 
joints,  and  were  well  treenailed  together  with  from  ten  to  sixteen 


AET.  3.]  CENTERS.  521 

treenails  to  each  length.  There  were  no  chords.  These  ribs  were 
placed  18  inches  from  center  to  center,  and  were  steadied  against 
one  another  by  a  board  bridging-piece,  1  foot  long,  at  every  5  feet. 
When  the  arch  stones  had  approached  to  within  about  12  feet  of 
each  other,  near  the  middle  of  the  span,  the  sinking  at  the  crown 
and  the  rising  at  the  haunches  had  become  so  alarming  that  pieces 
of  12-  X  12-inch  oak  were  hastily  inserted  at  intervals  and  well 
wedged  against  the  arch  stones  at  their  ends.  The  arch  was  then 
finished  in  sections  between  these  timbers,  which  were  removed  one 
by  one  as  the  arch  was  completed." 

Although  the  above  examples  can  not  be  commended  as  good 
construction — the  flexibility  of  the  ribs  being  so  great  as  to  endanger 
the  stability  of  the  arch  during  erection  and  to  break  the  adhesion 
of  the  mortar,  thus  decreasing  the  strength  of  the  finished  arch, — 
they  are  very  instructive  as  showing  the  strength  attainable  by  this 
method. 

747.  The  above  form  of  center  is  frequently  employed,  partic- 
ularly m  tunnels,  for  spans  of  20  to  30  feet,  precautions  being  taken 
to  have  the  pieces  break  joints,  to  secure  good  bearings  at  the 
joints,  and  to  nail  or  bolt  the  several  segments  firmly  together. 
The  centers  for  the  25-foot  arch  of  the  Musconetcong  (N.  J.)  tun- 
nel (Lehign  Valley  R.  R.)  consisted  of  segments  of  3-inch  plank, 
5  feet  8  inches  long,  14  inches  wide  at  the  center,  and  8  inches  at 
the  ends,  bolted  together  with  four  -l^-inch  and  four  f-inch  bolts 
each,  and  14-  x  8-inch  pieces  of  plate-iron  over  the  joints.  Where 
the  center  was  required  to  support  the  earth  also,  a  three-ply  rib 
was  employed ;  but  in  other  positions  two-ply  ribs,  spaced  4  to  5 
feet  apart,  were  used.  Centers  of  this  form  have  successfully  stood 
very  bad  ground  in  the  Musconetcong  and  other  tunnels;*  and 
hence  we  may  infer  that  they  are  at  least  sufficiently  strong  for  any 
ordinary  arch  of  30  feet  span. 

Although  not  necessary  for  stability,  it  is  wise  to  connect  the 
feet  of  the  rib  by  nailing  a  narrow  board  on  each  side,  to  prevent 
the  end  of  the  rib  from  spreading  outwards  and  pressing  against  the 
masonry — thus  interfering  with  the  striking  of  the  center, — and  also 
to  prevent  deformation  in  handling  it. 

*  Drinker's  Tunneling,  p.  548. 


522 


ARCHES. 


[chap.  XVIII. 


748.  Braced  Wooden  Rib.  For  semi-circular  arches  of  15  to  30 
feet  span,  a  coustruction  similar  to  that  shown  in  Fig.  147  (page  512) 
may  be  employed.  The  segments  should  consist  of  two  thicknesses 
of  1-  or  2-inch  plank,  according  to  span,  from  8  to  12  inches  wide 
at  the  middle,  according  to  the  length  of  the  segments.  The  hori- 
zontal chord  and  the  vertical  tie  may  each  be  made  of  two  thick- 
nesses of  the  plank  from  which  the  segments  are  made. 

For  greater  rigidity,  the  rib  may  be  further  braced  by  any  of 
the  methods  shown  in  outline  in  Figs.  153,  154,  155,  or  by  obvious 


Fig.  153. 


Fig.  154. 


Fig.  155. 


modifications  of  them.  The  form  to  be  adopted  often  depends  upon 
the  passage-way  required  under  the  arch.  Fig.  153  is  supported  by 
a  post  under  each  end;  in  extreme  cases,  Fig.  154  may  be  supported 
at  the  middle  point  also;  and  Fig.  155  may  be  supported  at  both 
middle  points  as  well  as  at  the  ends. 

Since  the  arch  masonry  near  the  springing  does  not  press  upon 
the  center,  it  may  be  laid  with  a  template  before  the  center  is  set 
up;  and  hence  frequently  the  center  of  a  semi-circular  arch  does 
not  extend  down  to  the  springing  line.  For  examples,  see  Figs. 
147  and  158  (page  512  and  524). 

Center  frames  are  put  together  on  a  temporary  platform  or  the 
floor  of  a  large  room,  on  which  a  full-size  drawing  of  the  rib  is  first 
drawn. 

749.  Trussed  Center.  When  the  span  is  too  great  to  employ 
any  of  the  centers  described  above,  it  becomes  necessary  to  construct 
trussed  centers.     It  is  not  necessary  here  to  discuss  the  principles 


Fig.  156 


of  trussing,  or  of  finding  the  strains  in  the  several  pieces,  or  of 
determining  the  sections,  or  of  joining  the  several  pieces, — all  of 


ART.  3.]  CENTERS.  533 

wliich  are  fully  described  in  treatises  on  roof  and  bridge  construc- 
tion. There  is  a  very  great  variety  of  methods  of  constructing  such 
centers.  Figs.  156  and  157  show  two  common,  simple,  and  efldcient 
general  forms. 

750.  Camber.  Strictly,  the  center  should  be  constructed  with 
a  cambi'r  ju<t  equal  to  the  amou.nt  it  will  yield  when  loaded  with 
the  arch ;  but,  since  the  load  is  indeterminate,  it  is  impossible  to 
compute  what  this  will  be.  Of  course,  the  camber  depends  upon 
the  unit  strain  in  the  material  of  the  center.  The  rule  is  frequently 
given  that  the  camber  should  be  one  four -hundredth  of  the  radius; 
but  this  is  too  great  for  the  excessively  heavy  centers  ordinarily 
used.  It  is  probably  better  to  build  the  centers  true,  and  guard 
against  undue  settling  by  giving  the  frames  great  stiffness;  and 
then  if  unexpected  settling  does  take  place,  tighten  the  striking 
wedges  slightly. 

The  two  sides  of  the  arch  should  be  carried  up  equally  fast,  to 
prevent  distortion  of  the  center. 

751.  Examples  of  Actual  Centers.  For  an  examj^le  of  a 
center  employed  in  a  tunnel,  see  Fig.  1-47  (page  512). 

Fig.  158  (page  o2-4)  shows  the  center  designed  for  the  60-foot 
granite  arches  of  the  recently  completed  Washington  bridge  over 
the  Harlem  River,  Xew  York  City.*  The  bridge  is  80  feet  wide, 
and  fifteen  ribs  were  employed.  N"otice  that  the  center  does  not 
extend  to  the  springing  line  of  the  arch  ;  the  first  fifteen  feet  of 
the  arch  were  laid  by  a  template. 

Fig.  159  f  (page  525)  shows  the  center  employed  in  constructing 
the  Cabin  John  arch,  which  carries  the  Washington  (D.  C.)  aque- 
duct over  a  creek,  and  which  is  the  largest  masonry  arch  in  the 
world  (see  No.  2,  Table  63,  page  502).  The  arch  is  20  feet  wide, 
and  five  ribs  were  emploved. 

752.  Striking  the  Center.  The  Method.  The  ends  of  the  ribs 
or  center-frames  usually  rest  upon  a  timber  lying  parallel  to.  and 
near,  the  springing  line  of  the  arch.  This  timber  is  supported  by 
wedges,  preferably  of  hard  wood,  resting  upon  a  second  stick,  which 
is  in  turn  supported  by  wooden  posts — usually  one  under  each  end  of 
each  rib.     The  wedges  between  the  two  timbers,  as  above,  are  used 

*  Published  by  permission  of  Wm.  R.  Hutton,  chief  engineer. 
+  Compiled  from  photographs  taken  during  the  progress  of  the  work  (1856-60),  by 
courtesy  of  Gen.  M.  C.  Meigs,  chief  engineer. 


524 


ARCHES. 


CHAP.   XVITT. 


ART.  3.] 


CEIS^TEKS. 


5:^5 


526  ARCHES.  [chap.  XVIII. 

in  removing  the  center  afier  the  arch  is  completed,  and  are  known 
as  striking  wedges.  They  consist  of  a  pair  of  folding  wedges — 1  to 
2  feet  long,  6  inches  wide,  and  having  a  slope  of  from  1  to  5  to  1  to 
10 — placed  under  each  end  of  each  rib.  It  is  necessary  to  remove 
the  centers  slowly,  particularly  for  large  arches  ;  and  hence  the 
striking  wedges  should  have  a  very  slight  taper, — the  larger  the  span 
the  smaller  the  taper. 

The  center  is  lowered  by  driving  back  the  wedges.  To  lower 
the  center  uniformly,  the  wedges  must  be  driven  back  equally. 
This  is  most  easily  accomplished  by  making  a  mark  on  the  side  of 
each  pair  of  wedges  before  commencing  to  drive,  and  then  moving 
each  the  same  amount. 

753.  Instead  of  separate  pairs  of  folding  wedges,  as  above,  a 
compound  wedge.  Fig.  160,  is  sometimes  employed.     The  pieces 


Fig.  160. 

A  and  B  are  termed  striking  plates.  The  ribs  rest  upon  the  former, 
and  the  latter  is  supported  by  the  wooden  posts  before  referred  to. 
The  wedge  C  is  held  in  place  during  the  construction  of  the  arch 
by  the  keys.  A",  K,  etc.,  each  of  which  is  a  pair  of  folding  wedges. 
To  lower  the  center,  the  keys  are  knocked  out  and  the  wedge  Cis 
driven  back. 

The  piece  C  is  usually  as  long  as  the  arch,  and  supports  one  end 
of  all  the  ribs  ;  but  with  large  arches,  say  80  to  100  feet  span,  it  is 
customary  to  support  each  rib  on  a  compound  wedge  running 
parallel  to  the  chord  of  the  center  (jDcrpendicular  to  the  axis  of  the 
arch).  Instead  of  cutting  the  striking  plates  A  and  B  as  shown  in 
Fig.  160,  the  compound  wedge  may  play  between  tapered  blocks 
gained  into  A  and  B.  The  piece  C  is  usually  made  of  an  oak 
stick  10  or  13  inches  square.  The  individual  wedges  are  from  4  to 
6  feet  long. 

For  the  larger  arches,  the  compound  wedge  is  driven  back 
with  a  heavy  log  battering-ram  suspended  by  ropes  and  swung 
back  and  forth  by  hand.     The  inclined   surfaces  of  the  wedges 


ART.  3.]  CEXTEKS.  527 

should  be  lubricated  when  the  center  is  set  up,  so  as  to  facilitate 
the  striking. 

754.  An  ingenious  device,  first  employed  at  the  Pont  d'Alma 
arcli — 141  feet  span  and  28  feet  rise, — consisted  in  supporting  the 
center-frames  by  wooden  pistons  or  plungers,  the  feet  of  which 
rested  on  sand  confined  in  plate-iron  cylinders  1  foot  in  diameter 
and  about  1  foot  high.  Xear  the  bottom  of  each  cylinder  there  was 
a  plug  which  could  be  withdrawn  and  rejDlaced  at  pleasure,  by  means 
of  which  the  outflow  of  the  sand  was  regulated,  aud  consequently 
also  the  descent  of  the  center.  This  method  is  particularly  use- 
ful for  large  arches,  owing  to  the  greater  facility  with  which  the 
center  can  be  lowered.     See  Fig.  158,  page  5.24:. 

755.  The  Time.  There  is  a  great  difference  of  opinion  as  to  the 
proper  time  for  striking  centers.  Some  hold  that  the  center  should 
be  struck  as  soon  as  the  arch  is  completed  and  the  spandrel  filling 
is  in  jolace ;  while  others  contend  that  the  mortar  should  be 
given  time  to  harden.  It  is  probably  best  to  slacken  the  centers  as 
soon  as  the  keystone  is  in  place,  so  as  to  bring  all  the  Joints  under 
pressure.  The  length  of  time  which  should  elapse  before  the  centers 
are  finally  removed  should  vary  Avith  the  kind  of  mortar  employed 
(see  Fig.  5,  page  89)  and  also  with  its  amount.  In  brick  and  rubble 
arches  a  large  proportion  of  the  arch  ring  consists  of  mortar  ;  and 
if  the  center  is  removed  too  soon,  the  compression  of  this  mortar 
might  cause  a  serious  or  even  dangerous  deformation  of  the  arch. 
Hence  the  centers  of  such  arches  should  remain  until  the  mortar 
has  not  only  set,  but  has  attained  a  considerable  part  of  its  ultimate 
strength  (see  Fig.  5,  page  89), — this  depending  somewhat  upon  the 
maximum  compression  in  the  arch.  It  is  probable  that  a  knowledge 
of  the  elasticity  and  of  the  ''set"  of  mortar  would  give  some  light 
as  to  the  best  time  to  strike  centers  ;  but  unfortunately  our  infor- 
mation on  those  topics  is  very  limited  (see  §  146). 

Frequently  the  centers  of  bridge  arches  are  not  removed  for 
three  or  four  months  after  the  arch  is  completed  ;  but  usually  the 
centers  for  the  arches  of  tunnels,  sewers,  and  culverts  are  removed 
as  soon  as  the  arch  is  turned  and,  say,  half  of  the  spandrel  filling  is 
in  plaee. 


APPENDIX    I. 


SPECIFICATIONS    FOE  MASONRY.* 

Contents. 

General  Railroad  Masonry  page  529 

Masonry  for  Railroad  Buildings "     534 

Architectural  Masonry "     5,39 

Laying  Masonry  in  Freezing  Weather "     543 

Railroad   MASOXRT.f 

General  Provisions.  All  stone  used  for  the  different  classes  of  masonry 
must  be  furnished  from  the  best  quarries  in  the  vicinity,  subject  to  the  ap- 
proval of  the  engineer.  Brick  masonry  shall  at  all  times  be  substituted  for 
stone,  when  so  desired  by  the  engineer. 

Inspection.  All  materials  will  be  subject  to  rigid  inspection,  and  any  that 
have  been  condemned  must  be  immediately  removed  from  the  site  of  the  work. 
The  work  will  be  done  under  the  supervision  of  an  inspector,  whose  duties 
will  be  to  see  that  the  requirements  of  these  specifications  are  carried  out;  but 
his  presence  is  in  no  way  to  be  presumed  to  release  in  any  degree  the  responsi- 
bility or  obligation  uf  the  contractor. 

Laying  Masonry.  All  classes  of  masonry  laid  in  cement  must  be  neatly 
pointed  with  cement  mortar,  finely  tempered.  No  masonry  of  any  kind  must 
be  covered  until  it  has  been  inspected  and  accepted  by  the  engineer.  No  ma- 
sonry will  be  allowed  to  be  laid  in  freezing  weather.  [Many  specifications 
omit  this  condition.  See  "  Specifications  for  Laying  Masonry  in  Freezing 
Weather,"  page  543.] 

Measurement  of  Masonry.  All  masonry  and  brick-work  will  be  built  ac- 
cording to  the  plans  and  instructions  furnished  by  the  engineer,  and  will  be 
estimated  and  paid  for  by  the  cubic  yai-d,  computing  ouly'the  actual  solidity 
thereof.  No  constructive  or  conventional  measurement  will  be  allowed,  any 
rule  or  custom  in  the  section  of  the  country  through  which  the  road  passes  to 
the  contrarj-  notwithstanding.  The  price  per  cubic  yard  paid  for  masonry 
and  brick-work  will  include  the  furnishing  of  all  material,  scaffolding,  cen- 
tering, and  all  other  expenses  necessary  to  the  construction  and  compleHon  of 
the  masonry  or  brick-work.  All  "  dressed  "  or  "cut-stone"  work— such  as 
copings,  bridge-scats,  cornices,  belt-courses,  water-tables,  brackets,  corbels, 
etc  — furnished  under  the  plans  of  the  engineer  will  be  paid  for  b}'  the  cubic 
yard,  imder  the  classification  of  the  masonry  in  which  they  occur,  with  an 
iidditional  price  per  square  foot  of  the  entire  superficial  surface  of  the  stones 
"  dressed."  or  "  cut,"  or  "  bush-hammered." 

Allowance  for  Extras.  No  allowance  will  be  made  for  timber,  or  work  on 
same,  used  in  .scaffolding,  shoring,  or  centering  for  arches, — excepting  only 
timber,  sheet-piling,  or  foundation  plank,  necessarily,  and  by  order  of  the  en- 
gineer, left  in  the  ground.     No  allowance  will  be  made  to  the  contractor  for 


•  See  also  the  specifications  in  the  body  of  the  bo  k.    See  "  Specifications  '  in  Index. 

+  These  specifications  are  the  same,  except  ip  form,  as  those  employed  in  the  construction 
of  the  "  West  Shore"  Railroad,  but  do  not  differ  materially  from  those  used  in  other  roadSf 
and  have  frequently  been  accepted  as  the  standard. 

529 


530  SPECIFICATIONS   FOR   MASONRY.  [APP.  I. 

any  damage  he  may  sustain  by  reason  of  floods  or  other  causes;  but  such 
draining,  bailing,  or  pumping  from  foundations  as  the  engineer  may  decide  to- 
be  necessary  will  be  paid  for  at  a  price  to  be  tixed  by  the  engineer. 

First-class  Masonry  will  consist  of  quarrj^-faced  ashlar  [see  §§  200-07] 
laid  in  horizontal  courses  having  parallel  beds  and  vortical  joints,  of  not  less 
than  ten  inches  (10")  nor  more  than  thirty  inches  (30")  in  thickness, — decreas- 
ing in  thickness  regularly  from  the  bottom  to  the  top  of  the  wall, — laid  Hush 
in  cement  mortar  of  the  quality  hereinafter  speciiled.  Each  course  must  be 
thoroughly  grouted  before  the  succeeding  one  is  laid. 

Size  of  Stones.  Stretcliers  must  be  not  less  than  two  and  one  half  feet(2i'> 
nor  more  than  six  feet  (6')  in  length,  and  not  less  than  one  and  one  half  feet 
(H')  in  width,  nor  less  in  width  than  one  and  one  half  (li)  times  their  depth. 
Headers  must  not  be  less  than  three  and  one  half  feet  (SV)  nor  more  than  tour 
and  one  half  feet  (4+)  in  length — where  the  thickness  of  the  wall  will  admit 
of  the  same, — and  not  less  than  one  and  one  half  feet  (U')  in  width,  nor  less 
in  width  than  they  are  in  depth  of  course. 

Cutting.  Every  stone  must  be  laid  on  its  natural  bed.  All  stones  must 
have  their  beds  well  dressed,  parallel  and  true  to  the  proper  line,  and  made  al- 
ways as  large  as  the  stone  will  admit  of.  The  beds  and  sides  of  the  stone  must 
be  cut,  before  being  placed  on  the  work,  so  as  to  form  joints  not  exceeding  one 
half  inch  (^")  in  width.  No  hammering  on  a  stone  will  be  allowed  after  it  is 
set;  but  if  any  inequalities  occur,  they  must  be  pointed  olf.  The  vertical 
joints  of  the  face  must  be  not  less  than  eight  inches  (8")  in  from  the  face,  and 
as  much  more  as  the  stone  will  admit  of.  All  corners  and  batter  lines  must  be 
run  with  a  neat  chisel  draft  one  and  one  half  inches  (1+")  on  each  face.  The 
projections  of  the  quarry  face  beyond  the  draft  lines  must  not  exceed  four  inches 
(4");  and  in  the  side-walls  of  tunnels  this  projection  m^st  not  exceed  two 
inches  (2"). 

Bond.  The  masonry  shall  consist  of  headers  and  stretchers  alternating.  At 
least  one  fourth  of  it  shall  consist  of  headers  extending  entirely  through  the 
wall,  and  every  header  shall  be  immediately  over  a  stretcher  of  the  underlj'in* 
course.  The  stones  of  each  course  shall  be  so  arranged  as  to  form  a  proper 
bond — in  no  case  less  than  one  foot  (1') — with  the  stones  of  the  underlying  course. 

Backing^  The  backing  shall  be  of  good-sized,  well-shaped  stones,  laid  so 
as  to  break  joints  and  thoroughly  bond  the  work  in  all  directions,  and  leave  no 
spaces  between  them  over  six  inches  (6")  in  width,  which  spaces  shall  be  filled 
with  small  stones  and  spalls  well  grouted. 

Coping.  All  bridge-seats  and  tops  of  walls  will  be  finished  with  a  coping 
course  of  such  dimensions  and  projections  as  may  be  ordered  bj"  the  engineer, 
dressed  and  cut  to  a  true  surface  on  top  and  front  edges,  in  conformity  with 
diagrams  for  same  which  will  be  furnished  by  the  engineer. 

Foundation  Courses.  All  foundation  courses  must  be  laid  with  selected 
large  flat  stones;  not  less  than  twelve  inches  (12")  thick,  nor  of  less  superficial 
surface  than  fifteen  (15)  square  feet. 

Second-class  Masonry  [^g  208-12]  will  consist  of  broken  range  rubble 
of  superior  qualit}^  laid  with  horizontal  beds  and  vertical  joints  on  the  face, 
with  no  stone  less  than  eight  inches  (8  ')  in  thickness — unless  otherwise  directed 
by  the  engineer, — well  bonded,  and  leveled  as  well  as  can  be  without  hammer- 
dressing.  No  mortar  joint  shall  exceed  three  quarters  of  an  inch  (f '  )  in  thick- 
ness. AH  corners  and  quoins  shall  have  hammer-dressed  beds  and  joints;  and 
all  corners  and  batter  lines  shall  be  run  with  an  inch-and-one-half  (U")  chisel 
draft.  At  least  one  fourth  {\)  of  the  stones  in  the  face  must  be  headers  evenly 
distributed  through  the  wall. 

Bridge-seats  and  tops  of  walls  shall  be  coped  in  the  same  manner  as  specified 
for  first-class  masonry.  Stones  in  foundaLion  courses  shall  be  not  less  than 
twelve  inches  (12")  thick,  and  shall  contain  not  less  than  twelve  (12)  square 
feet  of  surface. 


RAILROAD    MASONRY.  531 


Third-class  Masonry  will  consist  of  good  substantial  rubble  [§§213-1 7] 
laid  in  cement  mortar.  All  stones  shall  be  perfectly  sound,  and  sufficiently 
large  to  make  good,  well-bonded,  strong  work;  and  shall  be  laid  on  their 
natural  beds,  in  the  most  substantial  manner,  and  with  as  much  neatness  as 
this  description  of  work  admits  of.  The  stones  in  the  foundations  must  be  not 
less  than  ten  inches  (10  ')  thick,  and  shall  contain  not  less  than  ten  (10)  square 
feet  of  surface;  and  each  shall  be  tirmlj',  solidly,  and  carefully  laid. 

First-class  Arch-culvert  Masonry  shall  be  built  in  accordance 
with  the  specitications  for  first-class  masourj',  with  the  exception  of  the  arch 
sheeting  and  the  ring-stones.  The  rings  shall  be  dressed  to  such  size  and 
shape  as  the  engineer  shall  direct.  The  ring-stones  and  sheeting-stones  shall 
not  be  of  less  thickness  than  ten  inches  (10")  on  the  intrados,  and  shall  be 
dressed  with  three  eighths  inch  (f ")  joints,  and  shall  be  of  the  full  depth  speci- 
fied (by  drawings  or  otherwise)  for  the  thickness  of  the  arch.  The  joints  must 
be  made  on  truly  radial  lines,  and  the  face  of  the  sheeting-stones  must  be 
dressed  to  make  close  joints  with  the  center.  The  ring-stone  and  sheeting- 
stones  shall  break  joints  by  not  less  than  one  foot  (1'). 

The  wing  walls  shall  be  neatly  stepped,  in  accordance  with  the  drawings 
furnished,  with  selected  stones  of  the  full  width  of  the  wing  and  of  not  le^s 
than  ten  inches  (10  ')  in  thickness,  no  stone  being  covered  less  than  eighteen 
inches  (18  )  by  the  one  next  above  it;  or  the  wing  shall  be  finished  with  a 
neatl}^  capped  newel  at  the  end,  and  a  coping  course, — as  may  be  selected  by 
the  engineer.  The  parapet  shall  be  finished  with  a  coping  course  of  full  width 
of  parapet,  with  such  projection  as  may  be  directed  by  the  engineer,  the  stone 
to  be  not  less  than  ten  inches  (10")  thick. 

Second-class  Arch-culvei't  Masonry  shall  be  of  the  same  general 
character  and  description  as  second-class  masonry,  with  the  exception  of  the 
ring-stones  and  the  arch  sheeting.  The  former  shall  be  dressed  as  specified 
for  first-class  arch-culvert  masonry.  The  latter  shall  consist  of  selected  stones 
of  the  full  depth  of  the  arch,  and  shall  have  a  good  bearing  throughout  the 
thickness  of  the  arch,  and  shall  be  well  bonded.  No  stone  shall  be  less  than 
six  inches  (6")  in  thickness  on  the  intrados. 

Box-culv^ert  Masonry  will  be  good  rubble  [see  §g  313-17],  neatly  laid 
up  with  square-shaped  stones  of  a  size  and  quality  satisfactory  to  the  engineer. 
The  end  parapet  walls  and  also  the  side  walls  for  three  feet  (3)  from  the  ends 
shall  be  laid  in  good  cement  mortar.  "When  box  culverts  are  ordered  to  be 
laid  up  entirely  in  cement  mortar  [see  §  214],  they  will  be  classified  as  third- 
class  masonry,  and  must  conform  to  the  specifications  for  the  same. 

The  covering-stone  for  all  box  culverts  shall  be  not  le.ss  than  ten  inches 
(10")  in  thickness,  and  must  have  a  good,  solid,  well-leveled  bearing  on  the 
side  walls  of  not  less  than  fifteen  inches  (15"). 

Vitrified  Pipe.  In  localities  where  but  a  small  quantity  of  water  passes, 
vitriMed  pipe  will  be  .substituted  for  culverts  when  so  ordered  by  the  engineer. 
Sizes  of  twelve  (12"),  fifteen  (15"),  or  eighteen  (18")  inches  in  diameter  may  be 
used,  and  must  be  of  the  best  qualitj'  double  strength,  vitrified  culvert  pipe, 
subject  to  the  approval  of  the  engineer.  Vitrified  pipes  must  be  well  and  care- 
fully bedded  and  laid  [see  Figs.  97-99,  pages  409-10],  in  accordance  with  the 
instructions  of  the  engineer. 

Ketainins'  Walls  will  be  classified  as  second-  or  third-class  masonry 
laitl  dry,  as  nitiy  be  ordered  in  each  particidar  case. 

Slope  Walls  will  be  of  such  thickness  and  slope  as  directed  by  the  en- 
gineer. Tlie  stones  must  reach  entirely  tlirough  the  wall,  and  be  not  less  than 
four  inches  (4")  thick  and  twelve  inches  (12")  long,  laid  with  close  joints,  and 
as  free  as  possible  from  spalls.  The  foundations  must  be  prepared  and  laid  as 
directed  by  the  engineer. 

Stone  Paviiig"  shall  be  made  by  setting  on  edge  stone  from  eight  (8")  to 


532  SPECIFICATIONS    FOR   MASONRY.  [APP.  I. 

fifteen  inches  (15")  in  depth,  laid  either  dry  or  grouted  with  strong  cement 
mortar,  as  may  be  directed  by  the  engineer. 

Riprap.  When  required  by  the  engineer,  the  face  of  embankments  and 
the  foot  of  slopes  shall  be  protected  from  the  action  of  water  bj'  a  facing  of 
riprap  stone,  or  of  brush  and  stones,  or  by  a  retaining  wall,  as  may  be  directed. 
The  riprap,  when  used,  shall  be  laid  by  hand  by  competent  workmen,  and 
shall  be  of  such  thickness  and  slope  and  of  such  undressed  stone  as  the  en- 
gineer may  direct 

Brick  Masoury.  The  brick  must  be  of  the  best  quality  [see  ^  57], 
well  tempered,  hard  burned,  and  8J  X  4  X  2f  inches.*  No  bats,  ciueked, 
crooked,  or  salmon  bricks  will  under  any  circumstances  be  allowed  in  the 
work.  Tlie  brick  shall  be  well  soaked  in  water  before  being  laid,  and  shall 
be  laid  in  hydraulic  cement  mortar  of  the  quality  hereafter  specitied,  with 
such  thickness  of  joint  and  style  of  bond  [§  242  and  ^'  783]  as  shall  be  presciibed 
by  the  engineer.  Grout  will  be  substituted  for  mortar  when  ordered  by  the 
engineer. 

Brick  arching  must  be  covered  on  the  back  with  a  coat  of  strong  cement 
mortar  one  inch  (1")  thick.  In  tunnel  arching  wherever  a  seam  of  water  is 
met,  the  arch  must  be  covered  with  rooting  felt;  or  with  a  course  of  asphaltum 
(applied  hot)  of  such  thickness  as  may  be  directed  by  the  engineer,  and  this 
covered  with  a  plastering  of  cement  mortar  so  as  to  make  the  arch  impervious 
to  water.  A  properly  formed  drainage  channel  shall  be  left  in  the  backing  of 
the  arch  and  side  walls,  with  suitable  openings  for  the  escape  of  the  water,  at 
such  points  and  of  such  size  as  may  be  directed  by  the  engineer.  The  keying 
of  all  arches  shall  be  most  carefully  done,  and  in  such  manner  as  may  from 
time  to  time  be  directed  by  the  engineer.  The  packing  between  the  arch  and 
tunnel  roof  shall  never  be  put  in  until  at  least  forty-eight  (48)  hours  after  the 
section  has  been  keyed. 

Cement.     The  cement  must  be  of  the  best  quality  of  freshly  ground  hy- 
draulic cement  [of  tlie  Rosendale  type — see  §  72],  and  be  equal  in  quality  to  the 
best  brands  of  .  .  .  ...  cement.     It  will  be  subject  to  test  by  the  engineer  or  his 

appointed  inspector,  and  must  stand  a  tensile  stress  of  tifty  (50)  pounds  per 
square  inch  of  sectional  area  on  specimens  allowed  a  set  of  thirty  (30)  minutes 
in  air  and  twenty-four  (24)  hours  under  water  [see  §  90,  and  art.  5  of  Chapter 
III). 

Mortar.  The  mortar  shall  in  all  cases  be  composed  of  one  (1)  part  in  bulk 
of  the  above  specitied  hydraulic  cement  to  two  (2)  parts  in  bulk  of  clean, 
sharp  sand,  well  and  thoroughly  mixed  together  in  a  clean  box  of  boards,  be- 
fore the  addition  of  the  water.  It  must  be  used  immediately  after  being 
mixed;  and  no  mortar  left  over  night  will,  under  any  pretext,  be  allowed  to 
be  used.  The  sand  and  cement  used  will  at  all  times  be  subject  to  inspection, 
test,  and  acceptance  or  rejection  by  the  engineer. 

Concrete.  The  concrete  shall  be  composed  of  two  (2)  parts  in  bulk  of 
hard,  sound,  and  acceptable  stone — broken  to  a  size  that  will  pass  in  any  direc- 
tion through  a  two-inch  (2")  ring,  thoroughly  clean  and  free  from  mud,  dust, 
dirt,  or  any  earthy  admixture  whatever, — and  one  (1)  part  of  freshly-made 
cement  mortar  of  the  quality  above  described.  The  concrete  shall  be  quickly 
laid  in  sections,  in  laj'ers  not  exceeding  nine  (9)  inches  in  thickness,  and  shall 
be  thoroughly  rammed  until  the  water  Hushes  to  the  surface.  It  shall  be  al- 
lowed at  least  twelve  (12)  hours  to  set  before  any  work  is  laid  on  it. 

Foundations.  Excavations.  Foundations  for  masonry  shall  be  excavated 
to  such  depths  as  may  be  necessary  to  secure  a  solid  bearing  for  the  masonry, 
— of  which  the  engineer  shall  be  the  judge.     The  materials  excavated  will  be 

*  Instead  of  the  dimensions  as  above,  the  speciflcations  of  which  these  are  a  revision  and 
also  many  others  contain  the  term  "standard  size,"  but  until  recently  that  term  could  have 
had  no  special  significance  (see  §  62,  page  46). 


RAILROAD   MASONRY.  533 


classified  and  paid  for,  as  provided  for  in  the  Specifications  for  Grading.  All 
materials  taken  from  the  excavations  for  foundations,  if  of  proper  quality, 
shall  be  deposited  in  the  contiguous  embankment;  and  any  material  uutit  for 
such  purpose  shall  be  deposited  outside  the  roadwa}-,  or  in  such  place  as  the 
engineer  shall  direct,  and  so  that  it  shall  not  interfere  with  any  drain  or  water 
course.  In  case  of  foundations  in  rock,  the  rock  must  be  excavated  to  such 
depth  and  in  such  form  as  may  be  required  bj-  the  engineer,  and  must  be 
dressed  level  to  receive  the  foundation  course. 

Artificial  Foundations.  When  a  safe  and  solid  foundation  for  the  masonry 
can  not  be  found  ut  a  reasonable  depth  (of  which  the  engineer  is  to  be  the 
judge),  the  contractor  shall  prepare  such  artificial  foundations  as  the  engineer 
may  direct. 

Paving.  Box  culverts  and  small  bridge  abutments  may  have  a  paved  foun- 
dation, if  so  directed  by  the  engineer,  by  setting  stones  on  edge,  breaking 
joints,  and  extending  across  tlie  entire  width  of  the  foundation. 

Timber.  Timber  foundations  shall  be  such  as  the  engineer  may  by  drawing? 
or  otherwise  prescribe,  and  will  be  paid  for  bj-  the  thousand  feet,  board  meas 
ure. — the  price  to  include  the  cost  of  material,  framing,  and  putting  in  place. 
All  timber  must  be  sound,  straight-grained,  and  free  from  sap,  loose  or  rotten 
knots,  wind  shakes,  or  any  other  defect  that  would  impair  its  streugth  or 
durability.  It  must  be  sawed  (or  hewed)  perfectly  straight  and  to  e.xact 
dimensions,  with  full  corners  and  square  edges.  All  framing  must  be  done 
in  a  thoi'ough,  workmanlike  manner.  Both  material  and  workmanship  will 
be  subject  to  the  inspection  and  acceptance  of  the  engineer. 

Piling.  All  piles  shall  be  of  youug,  sound,  and  thrifty  white  oak,  yellow 
pine  or  other  timber  equally  good  for  the  purpose,  acceptable  to  the  engineer. 
They  must  be  at  least  eight  inches  (8  ')  in  diameter  at  the  small  end  and  twelve 
inches  (12")  in  diameter  at  the  butt  when  sawn  off;  and  must  be  perfectly 
straight  and  be  trimmed  close,  and  have  the  bark  stripped  off  before  they  are 
driven.  The  piles  must  be  driven  into  hard  bottom  uutil  they  do  not  move 
more  than  one  half  inch  (I")  under  the  blow  of  a  hammer  weighing  two  thou- 
sand (3,000)  pounds,  falling  twenty-five  feet  (25)  at  the  last  blow.  They  must 
be  driven  verticall}^  and  at  the  di.stances  apart,  transversely  and  longitudinally, 
required  by  the  plans  or  directions  of  the  engineer.  They  must  be  cut  off 
square  at  the  butt  and  be  well  sharpened  to  a  point;  and  when  necessary,  in 
the  opinion  of  the  engineer,  shall  be  shod  with  iron  and  the  heads  bound  with 
iron  hoops  of  such  dimensions  as  he  may  direct, — which  will  be  paid  for  the 
same  as  other  iron-work  used  in  foundations. 

The  necessary  length  of  piles  shall  be  ascertained  by  driving  test  piles  in 
different  parts  of  the  localities  in  which  they  are  to  be  used.  In  case  a  single 
pile  shall  not  prove  long  enough  to  reach  hard  bottom,  two  shall  be  spliced 
together  as  follows:  The  head  shall  be  sawed  off  square,  and  a  hole  two  inches 
(2)  in  diameter  and  twelve  inches  (12")  deep  shall  be  bored  into  it;  and  into 
this  hole  a  circular  white  oak  treenail  twenty-three  inches  (23 ')in  length  shall 
be  well  driven.  Then  another  pile  similarly  squared  and  bored,  and  of  as 
large  a  diameter  at  the  small  end  as  can  be  procured,  shall  be  placed  upon  the 
lower  pile,  brought  to  its  proper  position,  and  driven  as  before  directed.  All 
piles,  when  driven  to  the  required  depth,  are  to  be  cut  off  truly  .square  and 
horizontal  at  the  height  given  by  the  engineer;  and  only  the  actual  number  of 
lineal  feet  of  the  piles  left  for  use  in  the  foundations  after  being  sawed  off, 
will  be  paid  for. 

Iron.  All  wrought  and  cast-iron  work  ordered  by  the  engineer  will  be 
paid  for  by  the  pound, — the  price  to  include  the  cost  of  material,  manufac- 
ture, and  placing  in  the  work. 

Coffer-dams.  Where  coffer-dams  are,  in  the  opinion  of  the  engineer,  re- 
quired for  foundations,  the  prices  provided  in  the  contract  for  timber,  piles, 
and  iron  in  foundations,  will  be  allowed  for  the  material  and  work  on  same, 


534:  SPECIFICATIOIS^S    FOR    MASOXRY.  [aPP.    I. 

which  is  understood  as  covering  all  risks  from  high  water  or  otherwise,  drain- 
ii.g,  bailing,  pumping,  and  all  materials  connected  with  the  colfer-dams. 
Sheet-piling  will  be  classed  as  plank  in  foundations;  and  if  left  in  the  ground 
will  be  paid  for  by  the  thousand  feet  (1,000),  board  measure. 

Kailroad  Buildings.^ 

Tools.  All  tools  necessary  for  the  execution  of  the  contract,  including 
mortar  boxes,  will  be  furnished  by  the  contractor  at  his  own  expense. 

Stag'iiig.  All  staging  required  for  the  execution  of  the  work  done  under 
contract  shall  be  furnished  by  the  contractor  at  his  own  expense.  The  rail- 
way company  will,  however,  upon  the  completion  of  any  structure,  purchase 
of  the  contractor  such  staging  material  as  it  can  advantageously  use,  and  pay 
the  contractor  for  such  material  an  amoimt  which,  in  the  opinion  of  the  rail- 
way company's  engineer,  shall  seem  reasonable  and  just. 

Excavations.  Dry  excavations,  or  excavations  above  water,  will  be 
made  by  the  contractor  when  so  ordered  by  the  railway  company.  Wet  exca- 
vations, or  excavations  below  water,  will  be  made  by  the  railwaj'^  company, 
excepting  when  a  special  arrangement  is  made  with  the  contractor.  All  exca- 
vations will  be  classified  as  either  earth,  loose  rock,  or  solid  rock. 

When  the  excavation  for  any  structure  is  made  entirely  by  the  contractor, 
it  shall  be  measured  in  cubic  yards,  and  paid  for  at  the  price  per  cubic  yard 
specified  in  the  contract.  When  an  excavation  is  made  in  part  by  the  railway 
company's  force  and  is  finished  by  the  contractor's  force,  or  when  contractor's 
force  assists  railway  company's  force  in  making  any  excavation,  contractor  will 
be  paid  for  the  actual  time  that  his  force  is  employed,  at  laborer's  current  rate 
per  da}'  plus  ten  per  cent.  In  case  contractor  furnishes  a  foreman  for  such 
work,  time  charged  for  foreman  must  not  exceed  one  day  for  foreman  for 
each  ten  days  of  laTjor,  and  contractor  will  be  paid  for  the  services  of  .such 
foreman  at  a  rate  per  day  not  to  exceed  the  current  wages  paid  foremen  of 
laborers  plus  ten  per  cent.  lu  case  contractor  uses  masons,  foremen  of  masons, 
or  other  skilled  labor  for  the  execution  of  the  above  "extra"  or  "time"  work, 
the  wages  and  time  allowed  will  be  the  same  as  it  would  be  if  the  work  had 
been  performed  and  supervised  by  laborers  and  foremen  of  laborers.  When 
"extra"  or  "  time"  work  is  performed  by  contractor's  force,  and  is  supervised 
by  contractor's  foreman,  who  at  the  same  time  and  place  has  charge  of  and 
is  supervising  "contract"  work,  no  pay  will  be  allowed  contractor  for  such 
supervi.sion,  except  when,  in  the  opinion  of  the  railway  company's  engineer, 
it  may  seem  reasonable  and  just. 

All  excavations  shall  be  made  strictly  in  accordance  with  the  plans  fur- 
nished by  the  railway  companj'  and  the  stakes  set  by  the  railway  comjiany's 
engineer,  and  .shall  be  executed  in  a  neat  and  workmanlike  manner.  Where 
excavations  are  made  under  the  supervision  of  the  contractor,  his  agent  or 
foreman,  any  erroneous  or  unnecessary  excavation,  and  any  masonry  conse- 
quent to  such  erroneous  or  unnecessary  excavation,  shall  be  entirely  at  the 
contractor's  expense,  unless  the  contractor  can  show  that  such  unnecessary 
work  was  caused  by  errors  in  the  plans  furnished  by  the  railway  compan}',  or 
by  errors  in  the  railway  company's  engineer's  stakes  or  instruction!?. 

When  excavation  is  made  for  concrete,  great  care  must  be  taken  to  make 
the  pits  or  trenches,  as  the  case  may  be,  of  the  exact  width  and  depth  required 
for  tiie  concrete,  and  any  unnecessary  excavation  made  or  concrete  used  on 
accouiit  of  lack  of  such  care  on  the  part  of  the  contractor  will  be  at  his  ex- 
pense.    Excav^tiaQs  for  stone  footing  courses  will  be  made,  when  not  other- 

*  A)!tcbi80n,  Topeka  and  Santa  Fe  Railroad. 


EAILROAD   BUILDINGS. 


■wise  ordered,  eight  inches  (8")  (four  inches  (4")  on  each  side)  wider  than  the 
footing  course.  Excavations  for  walls  not  having  footing  courses  will  be 
iiuule,  when  not  otherwise  ordered,  twelve  inches  {V^")  (six  inches  (6')  on 
each  side)  wider  than  the  wall  is  thick. 

Before  masonr}^  is  built,  excavations  must  be  cleared  of  all  loose  earth, 
mud,  or  other  objectionable  material. 

Stone.  Stone  will  be  furnished  by  the  contractor  at  his  own  expense,  and 
he  of  a  qualitj'  suitable  for  the  dill'erent  cla.sses  of  masonry  beieinafter  speci- 
tied,  and  be  subject  to  the  inspection  and  acceptance  of  the  railway  company's 
engineer.  Stone  will  be  loaded  on  cars  and  unloaded  by  the  contractor  at  his 
own  expen.se.  Stone  will  be  delivered  by  the  railway  company  on  the  nearest 
available  side  track  to  the  work,  and  no  charges  whatsoever  will  be  allowed 
contractor  for  hauling  .stone  from  cars  to  the  work,  except  in  extreme  cases, 
where,  in  the  opinion  of  the  railway  company's  engineer,  such  charges  may 
appear  rea.sonable  and  just. 

Sand.  All  sand  for  mortar  or  concrete  will  be  furnished  by  the  contractor 
at  his  own  expense.  When,  in  the  opinion  of  the  railway  company's  engineer, 
sand  can  not  be  secured  by  contractor  within  reasonable  distance  by  wagon 
haul  and  at  a  reasonable  price,  transportation  by  rail  will  be  furnished  by  the 
railway  compan^^  it  being  optional  with  the  railway  company  at  what  point 
saiul  sliall  be  procured.  When  railway  company  furnishes  transportation  for 
sand,  cars  shall  be  loaded  and  unloaded  by  contractor  at  his  own  expense. 

All  sand  furnished  by  contractor  shall  be  clean  and  sharp,  and  subject  to 
the  inspection  of,  and  rejection  by,  the  railway  company's  engineer.  When, 
in  the  opinion  of  the  railw^ay  company's  engineer,  sand  requires  screening,  it 
shall  be  .screened  by  the  contractor  at  his  own  expense. 

Ceineiit  and  Lime.  All  cement  and  lime  will  be  furnished  by  the 
railway  company  at  its  own  expense;  and  will  be  delivered  on  cars  on  the 
nearest  available  side  track  to  the  work.  It  shall  be  unloaded  by  the  con- 
tractor at  his  own  expense,  and  shall  be  piled  up  in  such  manner  by  him  as  the 
railwaj'  company's  engineer  may  direct.  Cement  and  lime  shall  be  covered 
and  protected  from  the  weather  by  the  contractor  at  his  own  expense,  in  such 
manner  as  seems  suitable  to  the  railway  company's  engineer;  and  the  con- 
tractor will  be  held  responsible  for  the  value  of  any  cement  damaged  on  ac- 
count of  unsuitable  protection. 

Water.  Water  required  for  all  work  done  under  contract  shall  be  fur- 
nished l;y  the  contractor  at  his  own  expense.  No  charges  made  b}'  contractor 
lor  hauling  water  will  be  allowed.  When,  in  the  opinion  of  the  railway  com- 
pany's engineer,  water  can  not  be  procured  by  the  contractor  within  reason- 
able wagon  haul,  or  at  a  reasonable  expense,  it  will  be  furnished  by  the  rail- 
way comjiany. 

31<)rtar.  Except  when  otherwise  ordered,  all  mortar  shall  be  thoroughly 
mixed  in  a  box,  in  the  foF-owing  proportions:  One  (1)  part  cement,  two  (3) 
parts  sand,  with  sufficient  w^ater  to  render  the  mixture  of  the  proper  consist- 
ency. Care  must  be  taken  to  thoroughly  mix  the  .sand  and  cement  drj',  in  the 
proportions  specitied,  before  the  introduction  of  water  into  the  mixture.  Mor- 
tar shall  not  be  mixed  except  as  it  is  u.sed,  and  no  mortar  must  be  allowed  to 
stand  over  night  in  mortar  boxes  or  elsew^here. 

CoiM'iH'te.  All  concrete  shall  consist  of  one  (1)  part  cement,  two  (2)  parts 
sand,  and  six  (i>)  parts  broken  .stone,  together  with  sufficient  w'ater  to  mix  the 
sand  and  cement  to  the  consistency  of  good  moi'tar  for  masonry.  The  pro- 
portion of  .sand,  cement,  broken  stone,  and  the  quantity  of  water  used  in  the 
mixture,  may  be  varied  at  the  option  of  th'e  railway  company's  engineer. 

Stone  sliall  be  of  a  quality  acceptable  to  the  railway  company's  engineer, 
and  be  broken  .so  that  seventy-live  (75)  \^ev  cent,  will  pass  through  a  two-inch 
(3')  ring  antl  so  that  all  wiil  jia.ss  through  a  two  and  one  half  inch  {2^") 
ring.     Broken  stone  shall  be  free  from  mud,  dirt,  and  other  objectionable 


536  SPECIFICATIONS    FOE    MASONRY.  [aPP.   I. 

material,  and  shall  be  subject  to  the  inspection  of,  and  rejection  by,  the  rail- 
way company's  engineer. 

The  sand  and  cement  must  be  thoroughly  mixed  dry,  in  a  clean,  tight 
mortar  box,  before  the  introduction  of  water;  and  after  water  is  apitlied  to  the 
mixture,  the  whole  must  be  worked  over  with  hoes  until  a  good  mortar  of 
proper  consistency  is  secured.  After  the  mortar  is  made,  the  broken  stone 
must  be  thoroughly  drenched  whh  clean  water,  and  then  shall  be  added  to 
the  mixture  in  the  proportion  stated  above — or  in  any  other  proportion  which 
the  railway  company's  engineer  may  specify.  The  concrete  must  then  be 
worked  over  and  mixed  until  each  stone  is  completely  covered  with  mortar 
and  all  spaces  between  the  stones  entirely -filled  with  same. 

The  concrete  shall  be  deposited  in  horizontal  layers  not  exceeding  twelve 
inches  (12  ')  in  depth,  and  shall  be  thoroughly  tamped  when  so  required  by  the 
railway  company's  engineer. 

Kubble  Masoui'y.  Rubble  masonry  will  be  classified  as  either  heavy 
rubble,  foundation  rubble,  pier  rubble,  or  uncoursed  hammer-squared  rubble. 
The  latter  will  be  called  for  convenience  .squared  rubble  [see  §§2U8-12]. 

J'-^avy  Rubble.  When  not  otherwise  specihed  or  shown  on  the  plans,  foot- 
ing courses  will  be  built  of  rubble  masonry.  When  footing  courses  exceed 
thfrty  inches  cjO  )  in  width,  the  masonry  will  be  classitied  as  heavy  rubble; 
and  when  thirt}'  inches  (SO")  or  less  in  width,  the  masonry  will  be  classitied  as 
foundation  rubble. 

Heavy  rubt)le  footing  courses  shall  be  built  of  well-selected  stone,  which 
shall  have  a  thickness  not  less  than  the  height  of  the  footing  course.  Each 
stone  shall  have  a  bottom  bed  of  good  surface  over  its  entire  area,  which  shall 
be  horizontal  when  the  stone  is  in  po.sition.  As  much  of  the  upper  .surface  of 
each  stone  as  will  be  directly  vmder  the  ma.sonry  to  be  put  above  the  footing 
course  shall  be  uniform  and  parallel  to  the  bottom  bed.  At  least  one  third  (i). 
of  the  length  of  the  footing  course  shall  be  built  of  through-stone,  and  a 
larger  proportion  shall  be  furni.shed  by  the  contractor  when,  in  the  opinion 
of  the  railway  company's  engineer,  more  through-stone  are  required  to  secure 
stability.  No  stone  shall  be  used  which  will  not  bond  or  extend  under  the- 
masonry  to  be  built  above  the  footing  course  a  distance  equal  to  at  least  one 
third  {^)  the  thickness  or  width  of  the  masonry;  and  not  more  than  two  .stones 
shall  be  used  at  any  section  to  make  up  the  total  width  of  the  footing  cour.se, 
and  the  exposed  face  of  each  stone  shall  he  at  least  twelve  inches  (12  )  in  length. 

All  stones  must  be  roughly  jointed  with  a  hammer  for  a  distance  back 
from  their  faces  equal  to  the  projection  or  offset  of  the  footing  cour.se.  No 
spaces  to  exceed  forty  (40)  square  inches  in  area  shall  he  filled  with  spalls  or 
chips,  and  the  total  area  of  all  spaces  must  not  exceed  five  (5)  per  cent,  of  the 
area  of  the  footing  course. 

All  stone  when  placed  in  position  must  be  thoroughly  rammed  until  firmly 
embedded  in  a  bed  of  mortar,  which  shall  first  be  placed  in  bottom  of  excava- 
tion or  trench,  and  after  stone  are  placed  in  position,  all  joints  must  be  well 
grouted  with  mortar.  When  so  required  by  the  railway  company's  engineer, 
fooling  courses  shall  be  built  exactly  to  the  dimensions  shown  on  drawings  or 
specifications,  or  with  their  edges  built  to  a  line. 

Foundation  Bubble.  In  general,  and  when  not  otherwise  specified,  all  masonry 
below  the  bottom  of  water  table  or  below  the  top  of  rail  for  stone  buildings, 
and  all  masonry  below  the  sill  of  wooden  buildings,  will  be  classified  as  foiui- 
dation  rubble,  except  footing  courses  more  than  thirty  inches  (30")  in  width, 
which  will  be  classified  as  heavy  rubble.  Foundation  rubble  may  be  required, 
however,  for  any  portion  or  for  all  the  masonry  in  any  structure,  in  which 
case  no  additional  price  shall  be  allowed,  except  when,  in  the  opinion  of  the 
railway  company's  engineer,  it  shall  seem  reasonable  and  just. 

In  this  class  of  masonry  no  stone  having  an  exposed  face  shall  be  less, 
than  one  twentj'-fourth  (^y  of  a  foot  in  cubical  contents  nor  less  than  two 
inches  -(2")  thick.     Any  stone  smaller  than  this  will  be  considered  a  spall;. 


RAILKOAD   BUILDINGS.  53? 

and  spalls  are  not  to  be  used  to  exceed  seven  (7)  per  cent,  of  the  entire  mass. 
The  contractor  will  not  be  required  to  furnish  stone  (except  for  through- 
stone)  larger  than  one  and  one  half  feet  (IV)  in  cubical  contents,  but  the  stone 
used  shall  not  average  less  than  one  half  (|)  of  a  cubical  foot  in  contents.  No 
stone  shall  be  used  which  does  not  bond,  or  extend  into  the  wall,  at  least  six 
inches  (6").  One  through-stone,  whose  face  area  shall  not  be  less  than  one 
half  (i)  of  a  superHcial  foot,  will  be  required  for  each  sixteen  (16)  superticial 
feet  of  face  measurement  of  wall,  and  more  than  this  may  be  required  by  the 
railway  company  when,  in  the  opinion  of  its  engineer,  a  larger  proj)oriiou  of 
tiiroiigh-stoue  is  required  to  secure  stability;  provided,  however,  that  the  con- 
tractor shall  in  no  case  be  required  to  furnish  through-stone  to  exceed  ten  (10> 
per  cent,  of  the  entire  mass.  At  least  twenty  (20)  per  cent,  of  the  entire  ma- 
sonry shall  consist  of  headers,  or  bond  stones.  In  walls  twenty-four  inches 
(24  ')  thick  or  less,  these  headers  shall  be  at  least  two  thirds  (f )  the  thickness  of 
the  wall  in  length;  and  in  walls  more  than  twenty-four  inches  (24)  thick,  they 
shall  he  of  sufficient  length  and  be  so  placed  as,  in  the  opinion  of  the  railway 
compau3'^'s  engineer,  seems  necessary  to  secure  well-bonded  and  stable  work. 

Each  stone  shall  be  laid  in  its  quarry  bed,  and  any  stone  set  on  edge,  or 
with  the  planes  of  its  stratiricatiou  vertical,  will  be  rejected  and  ordered  re- 
moved at  the  expense  of  the  contractor.  Stones  shall  be  tirmly  bedded  in 
mortar,  and  all  spaces  and  joints  thoroughly  grouted  with  same. 

Pier  Rubble.  Piers  or  pedestals  whose  horizontal  sectional  area  is  nine  (9) 
s(iuare  feet  or  less  will  be  classitied  as  pier  rubble.  When  this  area  exceeds 
nine  (9)  square  feet,  the  masonry  will  be  classed  as  foundation  rubble.  Foot- 
ing courses  for  such  piers,  when  not  exceeding  sixteen  (16)  square  feet  in  area, 
will  be  classed  as  pier  rubble;  and  when  exceeding  this  area,  they  will  be 
classitied  as  heavy  rubble. 

Footing  courses  must  be  built,  so  far  as  practicable,  in  accordance  with  the 
preceding  specifications  for  heavy  rubble  masonry.  iVIasonry  in  piers  above 
footing  courses  must  be  carefully  built  of  well-selected  stone,  having  horizon- 
tal beds  and  vertical  joints,  and  be  thoroughly  bonded;  corners  and  facea 
must  be  built  true  and  plumb.  The  specifications  for  foundation  rubble,  sa 
far  as  practicable,  shall  apply  to  this  class  of  masonry. 

Each  pier  or  pedestal  shall  be  furnished  with  a  hammer-dressed  cap-stone 
not  less  than  six  inches  (6")  thick,  of  same  area  as  pier,  which  must  be  accu- 
rately set  at  the  required  level.  The  price  of  this  cap-stone  must  be  included 
in  the  contract  price  per  cubic  yard  for  this  class  of  masonry. 

Squared  Rubble.  When  not  otherwise  specified,  the  walls  of  all  stone  build- 
ings above  the  bottom  of  the  water-table  wiU  be  built  of  uncoursed  squared 
rubble. 

In  general  the  specifications  for  foundation  rubble  will  apply  to  this  class 
of  masonry,  the  difference  between  the  two  classes  being  in  the  construction 
and  finish  of  the  outside  face.  The  outside  face  of  the  wall  will  be  built  of 
well -selected  stones,  as  nearly  uniform  in  color  as  possible,  which  shall  be 
neatly  squared  to  rectangvilar  faces,  and  which  in  all  cases  shall  be  laid  on 
their  natural  or  quarry  beds.  The  beds  of  the  stones  shall  be  horizontal  and 
the  side  joints  vertical,  and  no  joints  to  exceed  three  fourths  (f)  of  an  inch  will 
be  allowed.  No  stone  having  a  face  area  of  less  than  eighteen  (l^<)  square 
inches  or  a  thickness  less  than  three  inches  (3")  shall  be  used;  and  the  average 
face  of  all  the  stones  shall  not  be  less  than  seventy-two  (72)  square  inches. 

The  inside  face  shall  be  built  and  finished  in  accordance  with  the  specifica- 
tions for  foundation  rubble. 

Corners  of  all  buildings  shall  be  built  up  with  quoin  stones,  uniform  in  size 
and  arrangement,  for  which  no  extra  pay  will  be  allowed  contractor.  Drafts 
will  be  cut  on  the  corners  when  so  specified  or  shown  on  plans.  All  joints 
shall  be  cleaned  or  raked  out  for  a  distance  of  three  quarters  of  an  inch  (f"), 
and  neatly  pointed  with  a  raised  joint.  The  mortar  used  for  pointing  shall  be 
composed  of  such  material  as  the  railway  company's  engineer  may  select. 


538  SPECIFICATIO]Sl"S    FOR   MASOXRY.  [aPP.  I. 

Openings  for  windows,  doors,  or  for  other  purposes,  will  be  made  in  walls 
when  specified  or  shown  on  plans.  The  jambs  of  such  openings  shall  be 
neatly  cut  to  a  true  and  smooth  surface,  and  be  drove  tooled,  craudalled,  or 
tooth-axed  [see  pages  12.5-34,  particularly  12ti  and  133],  as  may  be  re(iuired 
by  the  railway  company's  engineer.  Bed-joints  of  jamb-stones  must  be  care- 
fully cut,  so  that  no  joint  to  exceed  one  half  an  inch  (+  )  will  appear  on  the 
exposed  face  of  the  jambs.  Jamb-stones  shall  be  uniform  in  height,  and  one 
half  shall  be  through-stones.  In  general  the  arrangement  of  jamb-stones  will 
be  shown  on  drawings. 

The  contract  price  for  any  opening  shall  include  the  cost  of  cut-stone  sills, 
lintels,  arches,  jamb-stones,  or  any  other  cut-stone  work  required  for  that 
opening.  In  case  no  contract  price  is  made  for  any  opening,  the  contractor 
will  be  paid  such  price  as,  in  the  opinion  of  the  railway  company's  engineer, 
seems  reasonable  and  just. 

Cut  stone  shall  be  furnished  and  put  in  place  by  the  contractor  when  so  re- 
quired by  the  railway  company.  The  stone  furnished  shall  be  of  the  quality 
required  for  the  work,  and  acceptable  to  the  railway  company's  engineer;  and 
must  be  cut  strictly  in  accordance  with  the  plans  and  specitications  in  each 
case,  and  must  be  so  cut  as  to  lie,  when  in  position,  on  natural  or  quarry  beds. 
Cut  stone  will  be  paid  for  at  the  price  specified  in  contract,  and  in  case  cut 
stone  is  furnished  by  the  contractor  for  which  theie  is  no  contract  price,  a 
price  will  be  paid  which,  in  the  opinion  of  the  railway  company's  engineer, 
seems  reasonable  and  just. 

Cut  stone,  or  dimension  stone  for  cut-stone  work,  may  be  furnished  by  the 
railway  company  at  its  own  expense,  and  the  contractor  required  to  set  the  cut 
stone  in  position,  or  to  cut  and  .set  the  rough  dimension  stone,  in  which  case  the 
contractor  will  be  paid  for  the  work  either  as  "extra"  or  "time"  work,  or  at 
a  price  which,  in  the  opinion  of  the  railway  company's  engineer,  may  seem 
reasonable  and  just. 

AVall  Masonry.  All  walls  shall  be  built  to  a  line  both  inside  and  out- 
side, and  both  faces  shall  be  finished  with  a  smooth  and  uniform  surface, 
which  shall  be  flat-pointed  with  a  trowel,  in  a  neat  and  workmanlike  manner. 

The  upper  courses  of  all  walls,  when  leveled  or  finished  for  the  reception  of 
superstructure,  .shall  be  provided  with  a  through-stone  at  each  end,  and  also 
one  through-stone  for  at  least  each  five  (5)  linearfeet  of  wall.  These  through- 
stone  shall  be  dressed  on  tlieir  top  beds  and  accurately  set  to  a  level  one  half 
inch  a")  below  the  level  of  the  bottom  of  the  supenstructure.  Between  these 
through-stone  the  walls  must  be  carefully  laid,  with  the  upper  beds  of  the 
stones  brought  up  flush  with  the  top  of  the  above-described  through-stones  so 
as  to  secure  a  perfectly  level  surface  for  the  top  of  the  wall.  In  no  case  shall 
spalls  or  chips  be  used,  except  in  vertical  joints. 

The  contractor  will  make  such  openings  in  walls  as  are  required  for 
windows,  doors,  or  other  purposes.  Ko  additional  pay  will  be  allowed  for 
such  openings,  except  where  jambs  are  to  be  cut,  and  cut-stone  sills  or  lintels 
are  required,  in  which  case  such  price  per  opening  will  be  allowed  as,  in  the 
opinion  of  the  railway  couipanj^'s  engineer,  may  seem  reasonable  and  just.  Cut 
or  dressed  dimen.sion-.stone  will  be  furnished  and  set  in  position  when  so  re- 
quired by  plans  or  specifications,  and  will  be  paid  for  by  the  railway  company 
at  such  price  as  may,  in  the  opinion  of  its  engineer,  .seem  reasonable  and  just. 
Wood,  iron,  or  other  material  which  may  be  required  to  be  built  into  the  ma- 
sonry shall  be  properly  put  into  position  by  the  contractor,  and  no  extra  pay 
shall  be  allowed  for  such  work.  The  cubical  contents  of  such  material,  how- 
ever, will  not  be  deducted  from  the  measurement  of  the  masonry. 

When  so  required,  the  contractor  shall  plaster  the  outside  surface  of  base- 
ment or  other  walls  with  hydraulic  mortar,  composed  of  such  materials  as  the 
railway  company  may  select,  and  tor  such  work  the  railway  company  will  pay 
the  contractor  a  price  per  square  yard  in  addition  to  the  contract  price  ot  the 
masonry. 


ARCHITECTURAL   MASOXRY.  539 

Fouiidatioiis  for  Trestles.  Foundations  for  trestle  bents,  such  as  are 
built  for  coal  chutes,  will  he  classihed  as  foundation  rubble,  and  must  be  built 
wiih  i,n-eat  care.  The  lower  footing  course,  when  exceeding  thirty  inches  (80") 
in  width,  will  be  classed  as  heavy  mbble.  The  upper  course  shall  have  one 
hammer-dressed  through-stone  at  each  end  of  wall,  and  at  least  three  such 
through-stones  betweerTthe  end  through-stones;  otherwise  the  top  course  will 
be  tinl^shed  in  accordance  with  the  second  paragraph  under  "wall  masonry" 
al)ove.  This  does  not  apply  to  bent  foundations  inside  of  coal-chute  build- 
ing, which  will  be  built  in  the  same  manner  as  foundation  walls  in  general. 

''Well-wall  3Ias<)ury.  Well-walls  will  be  classified  as  foundation  rubble. 
"Well  masonry  will  be  built  under  the  supervision  of  the  well  foreman  who  has. 
charg-e  of  the"  well  excavation,  and  contractor's  foreman  shall  execute  the  work 
stricFly  in  accordance  with  instructions  given  by  him.  When  well-walls  are 
sunk,  or  settled,  as  the  excavation  is  made  great  care  must  be  taken  to  make 
the  outside  surface  perfectly  smooth  and  uniform;  and  as  many  headers,  not 
to  exceed  the  maximum  heretofore  specified,  may  be  required  as,  in  the  opin- 
ion of  the  railway  company's  engineer  or  well  foreman,  are  necessary  to 
secure  stability. 

3Ieasiireiiient  of  Masonry.  In  measuring  masonry  paid  for  by  the 
cubic  yard,  all  openings  will  be  deducted,  and  the  number  of  cubic  yards 
will  be  the  actual  cubical  contents  of  the  masonry  built.  The  cubical  contents 
of  cut  stone,  iron  work,  timber  or  other  material,  built  into  the  masonry  by  the 
contractor,  will  not  be  deducted  from  the  cubical  contents  of  the  whole  mass. 

Architectural  Masoxet.* 

Permit.  The  contractor  for  the  masonry  shall  take  out  a  building  per- 
mit, including  water  for  himself  and  plasterer  and  all  other  contractors  that 
may  require  water  about  the  building  during  the  progress  of  the  work.  This 
contractor  shall  also  take  out  street  and  obstruction  permit,  and  permit  for 
building  curb  and  retaining  walls.  The  cost  of  the  above  permits  is  to  be  in- 
cluded in  the  estimate. 

Cxrade.  The  inside  grade  at  the  building  shall  be  such  as  the  superintend- 
ent shall  direct.  At  the  time  of  starting  an}-  pier,  this  contractor  shall  ascer- 
tain from  the  superintendent  the  height  the  inside  grade  shall  be  set  above  the 
established  outside  grade,  taking  into  consideration  the  settlement  that  may 
occur  during  the  progress  of  the  work. 

Kxcavatioii.  It  is  the  intention  that  this  contractor  shall  call  at  the 
building  and  examine  for  himself  the  exact  situation  of  the  building  site.  He 
shall  remove  from  the  premises  all  earth  or  debris,  except  that  which  the  super- 
intendent maj'  consider  good  for  use  in  the  grading  required  about  the  build- 
ing. This  contractor  shall  complete  such  grading  about  the  building  as  may 
be  found  necessary.  All  sidewalk  stone  that  may  be  found  in  connection  with 
the  excavation  shall  be  removed  by  the  mason,  the  .said  stone  becoming  his 
property.  The  same  shall  apply  to  any  foundation  stone  or  other  material 
that  may  be  found  in  excavating'  although  none  of  said  material  shall  be  used 
in  connection  with  the  new  work  about  the  building. 

This  contractor  shall  excavate,  according  to  drawings,  for  all  walls,  piers, 
areas,  etc.,  the  intention  being  that  the  general  level  shall  be  excavated  simply 
to  the  level  of  the  finished  basement  floor.  All  trenches  shall  be  excavated  to 
the  neat  size  as  near  as  practicable;  and  each  shall  be  leveled  to  a  line  on  the  bot- 
tom, ready  to  receive  the  foundation.     At  such  time  as  the  superintendent  shall 

*  Except  in  form,  these  specifications  are  those  employed  by  Burnham  &  Root,  archi- 
tects. Chicago,  for  the  Society  of  Savings  Building,  Cleveland,  Ohio,  and  conform  closely  to 
the  general  form  employed  by  these  architects. 


540  SPECIFICATIOXS    FOR   MASONRY.  [APR  I. 

direct,  this  contractor  shall  level  off  the  basement  surfaces  and  floors  of  areas 
to  a  line  finishing  three  inches  (8)  below  the  top  of  the  level  of  the  finished 
basement  floors,  and  leave  the  surface  ready  to  receive  the  work  of  other  con- 
tractors. When  considered  necessary  in  the  judgment  of  the  superintendent, 
all  earth  shall  be  tamped  solidly  and  then  be  wet. 

If  any  pockets  of  quicksand  are  found,  this  contractor  shall  excavate  the 
same,  and  fill  in  solidly  with  concrete  composed  of  clean  broken  stone  of  a  size 
that  will  pass  through  a  two-inch  (2")  ring  and  English  Portland  cement,  pro- 
portioned 1  to  3,  rammed  solidly  into  place  in  the  pockets,  in  layers,  as  the 
superintendent  may  direct.  None  of  the  sand  that  may  be  found  while  ex- 
cavating shall  be  used  in  connection  with  any  of  the  work  about  the  building. 

After  all  foundations  or  retaining  walls  are  in  and  fixed,  this  contractor 
shall  tamp  the  earth  solidly  around  them,  leaving  it  level  to  a  line  within 
eighteen  inches  (18")  of  the  finished  grade,  and  ready  to  receive  the  work  of 
other  contracU  rs. 

Bailing'.  Tliis  contractor  shall  do  all  bailing  and  draining  of  trenches  or 
basement  surfaces  that  may  be  found  necessary  during  the  progress  of  the  work. 

Shoring'.  This  contractor  shall  protect  all  walls  of  the  adjoining  buildings, 
underpin  all  walls  that  may  be  considered  necessary — in  the  judgment  of  the 
superintendent — to  place  the  new  work  or  to  prevent  injury  of  the  old  work, 
make  good  all  repairs,  provide  such  cutting  as  may  be  found  necessary  to 
place  the  work,  and  leave  the  adjoining  buildings  as  good  as  at  the  start.  The 
cost  of  this  work  is  to  be  included  in  his  estimate.  This  contractor  shall 
furnish  and  put  in  place  any  sheet  piling  that  may  be  required  to  retain  the 
earth  while  the  footings  are  being  put  in,  and  include  all  costs  of  the  same  in 
his  estimates. 

Protection.  This  contractor  shall  use  proper  care  and  diligence  in  brac- 
ing and  securing  all  parts  of  the  work  against  storm,  wind,  and  the  action  of 
frost.  Every  night  during  freezing  weather,  each  pier  or  wall  shall  be  covered 
on  top  with  sail-cloth,  and  the  covering  shall  extend  down  over  the  face  of  all 
green  work. 

Concrete  Footing's.  This  contractor  shall  provide  a  frame  of  the  area 
of  the  pier,  composed  of  two-inch  (2")  plank,  so  arranged  that  the  parts  can  be 
withdrawn  and  the  pier  left  isolated  after  the  concrete  is  set  [see  §  800].  All 
footings  not  otherwise  indicated  shall  be  constructed  of  concrete  furnished  by 
this  contractor.  The  cement  shall  be  first-quality,  fresh  Utica,  or  any  other 
equally  good  quality  approved  by  the  architects.  The  contractor  at  the  time 
of  submitting  his  proposal  shall  state  the  kind  of  cement  he  intends  using. 
The  sand  shall  be  clean  and  sharp.  The  stone  shall  be  clean  limestone,  cru.shed 
to  a  size  that  will  pass  through  a  two-inch  (2")  ring,  and  screened.  The  con- 
crete shall  be  composed  of  these  ingredients  in  the  following  proportions:  one 
(l)part  of  hydraulic  cement,  one  (1)  part  of  sand,  and  two  (2)  parts  of  crushed 
limestone.  The  cement  and  sand  shall  be  mixed  dry,  and  the  mixture  wet 
with  a  quantity  of  water  sufficient  to  reduce  it  to  the  consistency  of  mortar. 
The  stone  and  mortar  shall  be  thoroughly  mixed  and  laid  in  trenches  as  soon 
as  possible,  in  layers  of  not  more  than  six  inches  (6')  in  thickness,  and  be 
rammed  until  the  water  rises  freely  to  the  top. 

All  concrete  footings  shall  be  carefully  leveled  or  pitched  with  concrete, 
and  be  left  ready  to  receive  the  piers,  walls,  or  columns,  in  each  case  as  par- 
ticularly indicated  on  the  drawings. 

Railroad-Rail  Footing's.  All  railroad  rails  that  may  be  required  in 
connection  with  the  foundations  shall  be  of  Bessemer  .steel,  weighing  not  less 
than  sixt}'-five  (65)  pounds  per  yard,  straight  and  sound,  cut  to  the  neat  lengths 
indicated  on  the  drawings.  All  railroad  rails  shall  be  furnished  by  this  con- 
tractor, and  by  him  set  in  place  to  centers  and  levels  as  indicated  on  the  dia- 
grams.    None  of  these  railroad  rails  are  to  be  painted. 

The  concrete  used  in  connection  with  steel-rail  footings  shall  be  composed 


AECHITECTURAL   MASONRY.  541 

of  one  (1)  part  of  first-quality  English  Portland  cement— or  any  other  equally 
good  quality  approved  by  the  architects, — one  (1)  part  of  clean  sharp  sand,  and 
two  (2)  parts  of  clean  limestone  crushed  to  chestnut  size.  This  concrete  shall 
be  mixed  as  for  concrete  footings,  and  shall  be  rammed  in  solidly  between  the 
rails;  and  each  tier  shall  be  neaily  squared  at  the  outer  edge. 

liubble  Masonry.  All  piers  colored  blue  on  the  drawings  shall  be 
classed  as  cut  stone,  and  shall  be  furnished  and  set  in  place  by  another  con 
tractor;  but  all  walls  colored  blue  on  the  drawings — referring  particularly  to 
foundation  walls  for  boiler-house,  foundation  wall  for  staircase  way  in  alley, 
area  walls,  curb  walls,  and  curtain  walls  between  piers— shall  be  classed  as  rub- 
ble masonry,  and  shall  be  furnished  and  set  in  place  by  the  mason. 

All  stone  used  in  connection  with  rubble  masonry  shall  be  of  selected,  large 
size,  first-quality  stone,  laid  to  the  lines  on  both  sides,  well  fitted  together  and 
thoroughly  pointed,  frequent  headers  that  extend  through  the  wall  being  pro- 
vided. All  stone  shall  be  not  less  than  two  feet  six  Inches  (2'  6")  long,  one  foot 
six  inches  (1'  6")  wide,  and  eight  inches  (8")  thick,  except  sucli  as  may  be  found 
necessary  to  level  up  a  course  to  the  required  height.  The  intention  is  that  all 
walls  shall  be  laid  in  courses  about  one  foot  six  inches  (1  6")  in  height, 
leveled  off  at  each  course.  Each  stone  shall  have  hammer  dressed  beds  and 
joints,  and  shall  be  firmly  bedded  and  be  well  cushioned  into  place.  All 
joints  shall  be  filled  with  mortar.  The  facing  of  all  walls  .shall  be  laid  ran- 
dom range,  and  the  face  of  the  stone  shall  becoar.se  bush-hammered. 

At  the  time  of  completing  the  retaining  walls,  this  contractor  shall  excavate 
at  least  one  foot  (1')  on  the  outside  of  the  wall,  and  point  up  all  joints  on  the 
outside;  and  then  provide  and  apply  a  coat  of  firsl-quality  Engli.sh  Portland 
cement,  notlessthan  a  half  inch  (i")  thick,  to  the  outside  of  the  wall  from  top 
to  bottom.  No  cement  covering  will  be  required  on  the  curb  walls.  All  joints 
showing  inside  the  building  shall  be  raked  out  and  neatly  pointed  up  with 
cement;  and,  in  addition,  the  face  of  walls  coming  in  connection  with  the  area 
shall  be  squared  up,  the  joints  finishing  not  to  exceed  one  half  inch  (^")  thick. 

All  curb  walls  that  may  be  required  to  receive  the  side-walks  shall  be 
brought  to  such  levels  as  the  superintendent  shall  direct,  and  shall  be  cemented 
on  top  and  left  ready  to  receive  the  sidewalks— which  shall  be  furni.shed  and 
set  by  another  contractor.  None  of  the  screen  walls  shall  be  set  in  place  until 
such  time  as  the  superintendent  shall  direct.  The  foundation  for  the  staircase 
bay  in  the  alley  shall  be  set  in  place,  after  the  building  is  partly  completed,  at 
such  time  as  the  .superintendent  may  direct.  This  contractor,  at  the  time  of 
starting  this  work,  shall  furnish  such  anchors  as  may  be  considered  neces- 
sary, in  the  judgment  of  the  superintendent,  to  attach  his  work  to  that 
already  in  place,  and  shall  do  all  cutting  and  fitting  that  may  be  found  neces- 
sar\'  to  properly  place  his  work. 

Mortar  for  Eubble  Masonry.  All  rubljle  masonry  above  referred  to  shall 
be  laid  in  mortar  composed  of  perfectly'  fresh  Utica  cement— or  other  equally 
as  good  approved  by  the  architects, — mixed  in  the  proportion  of  one  (1)  part 
of  cement  to  two  (2)  parts  of  clean  sharji  coarse  .sand.  The  sand  and  cement 
shall  be  mixed  in  a  box  dry;  then  wet,  tempered,  and  immediately  used. 

Coiniuoii  Brick-work.  All  walls  or  sections  colored  red  on  the  draw- 
ings or  otherwise  indicated  to  be  of  brick,  shall  be  of  selected,  first-quality, 
hard-burned  Chicago  sewer  brick — or  other  equally  good  quality  approved  by 
the  architects.  The  above  quality  of  brick  shall  be  used  throughout  the  entire 
work,  except  that  hollow  fire-clay  brick  shall  be  used  in  connection  with  all 
curtains  between  windows  on  elevations  above  the  first  story,  and  for  the  back- 
ing of  all  stone-work  above  the  top  of  the  eighth-story  floor  beams.  No  bats 
shall  be  used.  No  pressed  or  face  brick  will  be  required  in  connection  with 
this  work. 

All  brick  shall  be  well  wet,  excejn  in  freezing  weather,  before  being  laid. 
Each  brick  shall  be  laid  with  a  shove  joint,  in  a  full  bed  of  mortar,  all  inter- 


542  SPECIFICATION'S    FOR   MASONEY.  [APR  I. 

Slices  being  thoroughly  filled;  and  where  the  brick  conies  in  connection  with 
anchors,  each  one  shall  be  "brought  home"  to  do  all  the  work  possible.  Up 
to  and  including  the  tifth  story,  every  fourth  course  shall  consist  of  a  heading 
course  of  whole  brick  extending  through  the  entire  thickness  of  the  walls; 
above  the  fifth  story,  every  sixth  course  shall  be  a  heading  course.  All  mor- 
tar joints  shall  be  neatly  struck,  as  is  customary  for  "  tirst-^lass  trowel  work." 
All  coursesof  brick- work  shall  be  kept  level,  and  the  bonds  shall  be  accurately 
preserved.  When  necessary  to  bring  any  course  to  the  required  height,  clip- 
ped courses  shall  be  formed,  as  in  no  case  shall  any  mortar  joints  finish  more 
than  one  lialf  inch  (.V)  thick.  All  brick-work  shall  be  laid  to  the  lines,  and 
each  tier  kept  plumb,  the  intention  being  that  none  of  the  window-frames  shall 
be  set  in  place  until  the  roof  is  on. 

All  lintels  over  openings  indicated  in  connection  with  brick  partition  walls 
in  basement  shall  be  of  steel  railroad  rails,  and  shall  be  furnished  and  set  in 
place  by  the  mason.  These  rails  shall  be  painted  one  coat  of  mineral  paint  be- 
fore being  brought  to  the  building. 

All  cut  stone  shall  be  backed  as  fast  as  the  superintendent  m:iy  consider 
proper,  and  the  mason  shall  build  in  all  anchors  that  may  be  furnished  by  the 
contractor  for  the  cut  stone.  When  openings  or  slots  are  indicated  in  connec- 
tion with  walls,  the  size  and  position  of  the  same  shall  be  such  as  the  superin- 
tendent shall  direct,  unless  otherwise  shown.  This  contractor  shall  leave 
openings  to  receive  all  registers  that  may  be  required  in  connection  with  the 
heating  or  ventilating  system,  and  shall  also  leave  openings  in  connection  with 
the  corner  vaults  at  such  places  in  the  floor  and  ceiling  as  the  superintendent 
shall  direct. 

All  masonry  that  may  be  required  at  the  time  of  setting  the  boilers  shall  be 
furnished  and  set  in  place  by  the  contractor  for  steam-heating  apparatus. 

Mortar  for  Brick-work.  AH  mortar  used  in  connection  with  sewer  brick, 
together  with  the  mortar  in  the  brick  parapet  walls  and  the  chimney  above 
the  roof  line,  shall  be  composed  of  two  (2)  parts  of  lime  mortar— made  up  very 
poor, — and  one  (1)  part  of  first-quality  Utica  cement — or  other  equally  good 
approved  by  the  architects.  Said  mortar  shall  be  used  immediately  after  being 
mixed,  and  in  no  case  shall  any  be  used  that  has  stood  over  night. 

The  remaining  brick-work,  including  the  fire-brick  hereinafter  referred  to, 
shall  be  laid  in  mortar  composed  of  best  slaked  lime  and  coarse  sharp  cleaa 
sand  of  approved  quality. 

Brick  Arches.  Where  arches  are  indicated  in  connection  with  the  first- 
story  banking  vault  or  in  connection  with  roadway  in  the  court  on  the  north 
front  of  building,  said  arches  shall  be  formed  with  common  brick  laid  in  row- 
lock courses,  regularly  bonded  [see  §  733].  The  mortar  for  this  work  shall  con- 
sist of  one  (1)  part  Portland  cement  and  three  (3)  parts  clean  sharp  sand.  Each 
brick  shall  be  laid  with  a  shove  joint;  and  each  rowlock  course  shall  be 
cemented  on  top  at  the  time  of  laying  the  next  course.  The  last  course  shall 
be  cemented  on  top,  and  be  left  ready  to  receive  the  concrete  floor  or  roadway 
— which  shall  be  provided  by  another  contractor. 

All  centers  that  may  be  required  in  connection  with  this  work  shall  be 
furnished  and  set  in  place  by  the  carpenter;  and  none  of  said  centers  shall  be 
removed  until  such  time  as  the  superintendent  shall  direct.  After  the  same 
have  been  removed,  this  contractorshall  thoroughly  clean  down  all  face-work. 

All  iron  indicated  in  connection  with  this  work  shall  be  furnished  and  set 
in  place  by  the  contractor  for  constructional  iron  work, — except  the  bearing 
plates,  which  shall  be  bedded  by  the  mason. 

Smoke  Britehing'.  The  smoke  britching  indicated  in  connection  with 
the  main  boiler-stack  will  be  furnished  and  set  in  place  by  the  contractor  for 
constructional  iron,  although  the  mason  shall  back  up  the  same  at  such  time 
as  the  superintendent  shall  direct. 

Fire-brick.     The  lining  shown  to  stand  alone  in  connection  with  the 


ARCHITECTURAL   MASONRY.  543 

boiler  cnimney  in  the  lower  stories  sball  be  laid  with  first-quality  fire-clay 
brick,  laid  iu  stretcher  courses,  regularly  bonded,  with  headers  of  whole  brick 
sixteen  inches  (16  )  apart  in  every  sixth  course  to  stay  the  linings,  care  being 
taken  to  preserve  the  air-space  indicated. 

All  tire-clay  brick  shall  be  laid  in  first-class  fire-clay  mortar,  each  brick 
being  laid  with  a  solid  joint  neatly  struck  on  each  side  with  a  trowel. 

Hollow  Fire-clay  Brick.  All  brick  used  in  connection  with  the 
spandrels  above  the  first  story  ou  all  elevations,  together  with  all  backing  re- 
quired in  connection  with  the  stone  work  above  the  top  of  the  eighth-story  floor- 
beams,  shall  consist  of  first-quality,  hard-burned,  tire-clay,  hollow  brick,  equal 
in  quality  to  sample  to  be  seen  at  tlae  office  of  the  architects.  Each  brick  shall 
be  laid  with  a  shove  joint.  This  contractor  shall  point  up  this  work,  and 
leave  the  surfaces  of  the  walls  smooth  and  ready  to  receive  plastering. 

Cutting'  and  Fitting-.  This  contractor  .shall  do,  promptly  and  at  the 
time  the  superintendent  so  directs,  all  cutting  and  fitting  that  may  be  required 
In  connection  with  the  masou-work  by  other  contractors  to  make  their  work 
come  right,  and  shall  make  good  after  them. 

Setting  Iron-work.  It  is  the  intention  that  all  constructional  iron- 
work shall  be  furnished  and  set  in  place  by  another  contractor,  and  that  all  iron 
shall  be  hoisted  from  the  outside  of  the  building  by  means  of  a  derrick.  In 
setting  the  beams  and  columns  in  place,  the  mason  shall  keep  pace  with  the 
contractor  for  constructional  iron  work,  and  at  no  time  shall  the  mason  be  left 
one  story  behind  the  constructional  iron-work.  Each  beam,  girder,  or  column 
shown  to  rest  on  the  masonrj'  shall  be  provided  with  iron  plates  by  the  con- 
tractor for  constructional  iron,  said  plates  being  furnished  to  the  mason  at  the 
sidewalk;  and  the  mason  shall  set  the  same  in  place,  firmly  bedded  in  mortar, 
at  such  position  or  height  as  the  superintendent  shall  direct. 

All  iron  wall-plates  that  maj'  be  required  to  receive  the  fire-clay  arches 
will  be  furnished  at  the  sidewalk  by  the  constructional-iron  contractor:  and 
this  contractor  shall  set  each  in  such  position  and  at  such  height  as  the  super- 
intendent shall  direct. 

Cut  Stone.  All  parts  colored  blue  on  the  drawings,  or  otherwise  indi- 
cated to  be  of  stone,  or  usually  classed  as  cut  stone,  shall  be  furnished  and  set 
in  place  by  the  contractor  for  cut  stone.  The  same  shall  apply  for  the  terra- 
cotta roof-copings  indicated.  All  mortar,  staging,  or  hoisting  apparatus  that 
may  be  required  in  connection  with  this  work  shall  be  furnished  by  the  con- 
tractor for  cut  stone.  All  cut  stone  will  be  set  from  the  outside;  but  the 
mason  shall  back  up  all  cut-stone  work  in  a  manner  approved  by  the 
superintendent. 

Lattng  Masonry  in  Freezing  Weather. 

Masonry  shall  be  laid  in  freezing  weather  only  in  case  of  absolute  neces- 
sity, aud  then  only  by  permission  of  the  engineer.  When  necessary,  masonry 
maybe  laid  in  freezing  weather,  provided  (1)  that  the  stone  or  brick  while 
being  laid  are  dry  and  perfectly  free  from  snow  or  ice;  (2)  that  there  is  added 
to  the  water  used  in  mixing  the  mortar  1  per  cent,  of  salt  for  each  Fahrenheit 
degree  below  freezing  ;  and  (3)  that  the  mortar  is  mixed  rather  dry.  Any 
masonry  laid  in  freezing  weather  shall  not  be  pointed  until  warm  weather 
in  the  spring.* 

*  For  additional  precautions  that  may  be  prescribed,  see  §§  141-143,  pages  102-4. 


APPENDIX  II. 


SUPPLEMENTAEY   NOTES. 

Note  1.  Labor  Required  in  Quarrying.*  The  following  table  shows  the 
labor  required  in  quarrying  the  stone  [gneiss]  for  the  Boyd's  Corner  dam  on 
the  Croton  River  near  New  York  City.  The  stone  to  be  cut  was  split  out  with 
plugs  and  feathers. 


Labor  Required 

IN  Quarrying  Gneiss. 

Kind 

OF 

Labor. 

Days  per  Cubic  Yard. 

Rough  stone. 

Stone  to  be  cut. 

0.041 
0..339 
0.140 
0.036 
0.035 
0.141 
0.077 

0.114 

Drillers 

0.917 

0  429 

0.102 

0.108 

0  620 

Labor  loading  teams 

0.284 

Note  '2.  Cost  of  Cutting  Granite. f  "  Below  is  given  the  cost  of  cutting 
several  kinds  of  masonry  for  the  New  York  Department  of  Docks,  in  1874-5. 
Between  December  1873  and  May  1875  with  an  average  force  of  40  stone- 
cutters, 2,065  yards  of  granite  of  the  following  kinds  were  cut  in  the  Depart- 
ment yard: 

"  1,524  yards  of  dimension  stone  were  cut  into  headers  and  stretchers. 
This  stone  was  cut  to  lay  ^-iuch  beds  and  joints,  the  faces  being  pointed  work, 
with  a  chisel  draft  l^-inches  wide.  The  headers  averaged  2  feet  on  the  face  by 
3  feet  in  depth;  and  the  stretchers  averaged  6  feet  long  bj'  2  feet  deep,  the  rise 
being  20,  22,  and  26  inches  for  the  different  courses.  The  average  time  of 
stone-cutter  cutting  one  cubic  yard  was  4.53  days  of  8  hours;  and  the  average 
cost  of  cutting  was  $27.54  per  cubic  yard  ($1.02  per  cubic  foot). 

"  310  yards  of  coping  were  cut  to  lay  ^-inch  beds  and  joints,  pointed  on 
the  face  with  chisel  draft  same  as  headers  and  stretchers,  and  8-cut  patent- 
hammered  on  top,  with  a  round  of  3^  inches  radius,  the  dimensions  being  8 
feet  long,  4  feet  wide,  and  2^  feet  rise.  The  average  time  of  stone-cutter 
cutting  one  cubic  yard  was  6.26  days,  and  the  average  cost  of  cutting  |38. 07 
per  cubic  yard  ($1.41  per  cubic  fool). 

"231  yards  of  springers,  ke}'stones,  etc.,  for  arched  pier  at  the  Battery, 

were  cut.     These  stones  were  of  various  dimensions,  part  being  pointed  work 

.-and  jmrt  6- cut  patent-hammered.      The  average  time  of  stone-cutter  cutting 

.  one  cu-i)ic  j'ard  was  6.88  days,  and  the  average  cost  of  cutting  was  $41.85  per 

.  cubic  yard  ($1.55  per  cubic  foot). 

"  The  above  cost  of  cutting  includes,  besides  stone-cutter's  wages,  labor  of 
-moving  stone,  all  material  used — such  as  timber  for  rolling  stone,  new  tools, 
•etc.  — sharpening  tools,  superintendence,  and  interest  on  stone-cutter's  sheds, 
blacksmith  shop,  derrick,  and  railroad.  The.se  expenses,  in  per  cents,  of  the 
total  cost  of  cutting,  are  as  follows:  superintendence  5;  sharpening  tools  15; 
labor  rolling    stones    30;     interest  on  sheds,    derrick,  and  railroad    1;    new 

*  J.  James  S.  Croes,  in  Trans.  Am.  Soc.  of  C.  E.,  Vol.  III.,  page  363. 

t  From  an  article  by  Wm.  VV.  Maolay,  in  Trans.  Am.  Soc.  of  C.  E.,  Vol.  IV.,  pp.  310-11. 

544 


APP.   II.  J 


SUPPLEMENTARY    NOTES. 


545 


tools  and  timber  for  rolling  stone  1;  total  52  per  cent.,  which,  added  to  the 
wages  paid  stone-cutters,  gives  the  total  cost.  During  the  last  year  stone- 
cutters were  required  to  do  at  least  13  superficial  feet  per  day  of  beds  and 
joints,  or  its  equivalent  in  pointed  or  fine  cut  work.  The  average  day's  work 
of  each  stone-cutter,  during  one  year  and  a  half  in  which  118,383  superficial 
feet  of  beds  and  joints  were  cut,  was  13.6  square  feet  per  day,  for  which  he 
received  $4.00. 

"  The  following  table  shows  the  amount  of  granite  that  a  stone-cutter  can 
cut  in  a  day  of  8  hours. 

Labor  Required  in  Cutting  Granite. 


Kind  op  Work. 


Number  op  Superpicial  Feet 


Constituting       a 

Required    as  a 

day's  worli  of  8 

minimum 

hours  in  stone- 

day's      work 

yards  and  con- 

by     the    De- 

tract-workdone 

partment    of 

in    vicinity    of 

Docks,     New 

New  York  City. 

York. 

16 

12 

10 

7.5 

7.27 

5.45 

6.15 

4.61 

5 

3.75 

Averaged  per 
dayof  Shours 
by  stone-cut- 
ters in  the 
Departm  e  n  t 
of  D  o  ck  s  , 
New  York. 


Beds  and  joints 

Pointed  work  with  chiseled  margin,  lines 

all  round 

Pean-hammered 

6cut  patent-hammered 

8-cut       "  '•         


13.6 

8.5 
6.15 
5.22 
4.34 


Note  3.  Cost  of  Cutting  Granite.*  The  average  day's  work  of  a  man 
in  cutting  the  face  of  granite  pitch-faced,  range,  squared-stone  masonry 
(§  197,  page  137)  of  the  Boyd's  Corner  dam,  as  deduced  from  three  and  a  half 
years'  work  in  which  5,200  cubic  yards  were  cut,  was  6,373  square  feet,  the 
dimensions  of  the  stones  being  1.8  feet  rise,  3.6  feet  long,  and  2.7  feet  deep; 
and  the  average  day's  work  in  cutting  the  beds  to  lay  f-inch  joints  was  18.7 
square  feet.  The  granite  coping,  composed  of  two  courses — one  of  12-inch 
rise,  30-inch  bed,  and  3|-feet  average  length,  and  one  of  24-inch  rise,  48-inch 
bed,  and  2|-feet  average  length, — the  top  being  pean-hammered,  the  face 
being  rough  with  chisel  draft  around  it,  and  the  beds  and  joints  cut  to  lay 
^-inch  joints,  required  6.1  days'  work  of  the  cutter  per  cubic  yard. 

' '  In  cutting  the  granite  for  the  gate-houses  of  the  Croton  Reservoir  at  Eighty- 
sixth  Street,  New  York  City,  in  1861-2,  the  minimum  daj-'s  work  for  a  cutter 
was  fi.xed  at  15  superficial  feet  of  joint.  This  included  also  the  cutting  of  a 
chi-sel  draft  around  the  face  of  the  stone,  which  costs  per  linear  foot  about  one 
fourth  as  much  as  a  square  foot  of  joint,  making  the  actual  limit  equivalent 
to  about  17.7  square  feet  of  joint.  On  this  work,  the  proportion  to  be  added 
to  the  cost  of  the  cutters  to  give  the  total  cost  was  as  follows,  the  average  for 
19  months'  work:  for  superintendence  8  per  cent.;  sheds  and  tools  7;  sharpen- 
ing tools  11;  labor  moving  stone  in  j^ard  10;  drillers  plugging  oS  rough  faces 
4:  making  a  total  of  40  per  cent,  to  be  added." 

Note  4.  Cost  of  Laying  Cut  Stone,  f  Most  of  the  cut  stone  was  laid  by 
one  mason,  more  than  two  not  being  employed  at  any  time.  The  mason's 
gang  also  shifted  derricks.  The  cost  of  hauling  stone  to  the  work  varied 
with  the  position  of  the  blocks  in  the  yard  and  whether  they  were  assorted 
there  into  courses  or  lay  promiscuously.  The  amount  of  labor  required  in 
laying  the  masonry  was  as  follows: 

*  From  an  account  of  the  construction  of  the  Boyd's  Corner  dam  on  the  Croton  River 
Dear  New  York  City,  by  J.  James  R.  Croes,  in  Trans.  Am.  Soc.  of  C.  E.,  "V^ol.  III.,  pp.  3C3-^. 
+  Ibid.,  p.  365. 


546 


SUPPLEMENTARY    NOTES. 


[APP.  II. 


Labor  Required  in  Laying  Cut-stone  Masonry. 


Kind  of  Labor. 


Mason,  days 

Laborers,  days 

Mortar  mixer,  days 

Derrick  and  car  men,  days. 

Engine,  hours  . 

Teams  from  yard,  days 

Labor  loading  teams,  days. 

Number  of  cubic  yards  laid. 


Amount  per  Cubic  Yard. 


Hoisted  by  Hand. 

Hoisted  by  Steam. 

5  ft. 

10  to  20  ft. 

20  to  30  ft. 

30  to  50  ft. 

0.120 

0.119 

0.082 

0.108 

0.184 

0.188 

0  145 

0.155 

0.100 

0.82 

0.076 

0.101 

0.327 

0.341 

0.235 

0.261 

0.462 

0.490 

0.100 

0.056 

0.0.56 

0.110 

0.184 

0.223 

0.223 

0.086 

1,070 

2,270 

2,530 

Note  5.  Cost  of  Breaking  Stone  for  Concrete.*  "  The  stone  [gneiss]  for 
the  concrete  was  broken  to  be  not  more  than  2  inches  in  its  largest  dimension. 
A  Blalte  stone-breaker  of  15-inch  jaw,  driven  by  a  IShorse-power  engine,  was 
used.  The  stone,  which  was  obtained  from  the  surface  and  from  old  fence 
walls  in  the  vicinity  of  the  work,  was  tough,  and  used  up  the  jaws  very  fast. 
A  movable  jaw  ordinarily  lasted  30  days.  The  stone  was  delivered  1o  the 
breaker  by  carts,  having  been  tirst  sledged  to  the  proper  size — about  13  inches 
square  by  6  inches  thick.  The  machine,  when  running  at  f'liU  speed,  with 
one  man  feeding,  two  men  supplying  him  with  stone,  one  keeping  the  screen 
clear  and  helping  to  till  barrows,  two  wheeling  away  the  stone,  and  one  on 
the  dump,  could  break  144  cubic  feet  in  an  hour,  or  at  the  rate  of  54.4  cubic 
yards  per  day  of  10  hours.  This  excessive  speed  was  kept  up,  however,  only 
as  long  as  it  was  known  that  an  inspector  was  timing  it.  The  average  rate 
of  breaking  for  the  last  year  was  3.8  cubic  yards  per  hour,  which  may  be 
assumed  as  the  economical  rate  for  the  15-inch  machine.  The  largest  uuichine 
(20-inch)  will  break  8  cubic  yards  per  hour,  if  fed  to  that  capacity;  but  6  cu])ic 
yards  per  hour  is  more  economical.  The  following  table  gives  the  cost  in 
time  of  breaking  the  stone: 

Labor  Required  in  Breaking  Stone  for  Concrete. 


« 

Days  per  Cubic  Yard. 

Kind  op  Labor. 

1867 

1868 

1869            1870 

0.269 

0.049 
0.045 
0.360 

2,410 
22.1 

0  322 
0.051 
0.092 
0.037 
0.238 

4.170 
27.3 

0.224 
0.042 
0.066 
0.027 
0.158 

5,720 
36.8 

0.410 

Laborers  loading  cartst 

Carts  hauling  

Breaking:       Engine  and  machine  t 

0.087 
0.118 
0.026 
0.174 

Total  number  of  cubic  yards  broken 

Average  number  of  cubic  yards  broken  per  day. . . 

3.6.50 
38.0 

*  From  an  account  of  the  construction  of  the  Boyd's  Corner  dam  on  the  Croton  River 
near  New  York  City,  by  J.  James  R.  Croes.  in  Trans.  Am.  Soc.  of  C.  E..  Vol.  HI.,  pp.  .356-58. 

t  "  The  difference  in  sledging  is  accounted  for  thus:  In  1867  many  fence-wall  and  cobble 
stones  were  used,  which  neeiled  no  sledging,  but  were  hard  to  crush.  In  180H  refuse  from 
the  quarry  which  required  sledging,  was  almost  exclusively  used.  In  1869  stone-yard  and 
quarry  spalls  were  used.  In  1870  the  stone  was  quarried  for  the  breaker;  and  consequently 
nearly  liU  'if  it  was  sledged.  The  carting  and  tending  varied  in  the  same  way  as  above,  for 
the  same  reasons."  .  .  .in/Mr» 

$  Includes  cost  of  engine  driver  and  helpers,  fuel  and  repairs  of  e.igine— about  0.05  or 
the  wages  of  a  day  laborer  per  cubic  yard. 


APP.  II.] 


SUPPLEMEXTARY    NOTES. 


547 


iNote  6,  Cost  of  Imbedding  Large  Stones  in  Concrete.*  "The  lars-e  un- 
wrouirlit  stone  laid  in  the  concrete,  from  the  foundations  to  within  45  feel  of 
the  top  of  the  dam,  were  set  in  full  mortar  beds  and  the  surfaces  plastered 
just  before  concrete  was  laid  around  them.  The  setting  was  done  mosll}'  by 
laborers,  one  mason  superintending.  The  cost  in  day's  work  per  cubic  yard 
Tvas  as  follows  : 


Labor  Required  to  Imbed  Large  Stones  in  Concrete. 


Kind  of  Labor. 


Foreman  (mason) 

Laborers  setting  

"         plastering 

mixing  mortar 

at  derricK  

loading  teams 

Teams  transporting  stone 

Total  quantity  laid,  cubic  yards. 
Per  cent,  of  whole  mass 


Days  pkk  Cubic  Yard. 


1867 1 

1868 1 

0.046 

0.057 

0.208 

0.148 

0.085 

0.056 

0.078 

0.083 

0.238 

0.254 

0.305 

0.160 

0.073 

1,234 

2..353 

32.0 

36.6 

"  The  cost  of  the  mass  of  concrete  and  large  stone,  as  laid  in  1867,  was 
89^  per  cent,  of  the  cost  of  the  concrete  alone;  and  in  1868  it  was  84^  per 
cent,  of  such  cost.  If  the  large  stones  do  not  exceed  25  percent,  of  the  mass, 
the  cost  of  the  mass  is  reduced  about  10  per  cent,  below  concrete  cost,  while 
its  specific  gravity  is  increased  about  8  per  cent." 

Note  7.  Crushing  Strength  of  Sewer  Pipe.  Experiments  made  at 
Chicago  in  1879  by  W.  D.  llotchkiss,  and  reported  to  the  author  by  Black- 
mer  and  Post,  of  St.  Louis,  gave  the  strength  of  ordinary  sewer-pipe  as  fol- 
lows, when  tested  as  described  on  page  408:  one  12-inch  and  Uve  15-inch 
pipes  failed  at  an  average  pressure  of  8,504  lbs.  per  sq.  ft.  of  horizontal  sec- 
tion; and  two  12-inch  and  two  15-inch  were  not  crushed  by  an  average  pres- 
sure of  9,068  lbs.  per  sq.  ft. 

Note  8.  Holding  Power  of  Drift  Bolts.  According  to  experiments 
made  under  the  author's  direction, §  the  average  holding  power  of  a  1-inch 
round  rod  driven  into  a  -i^-inch  hole  in  pine,  perpendicular  to  the  grain,  is 
501  pounds  per  linear  iucli  (3  tons  per  linear  foot);  and  under  the  .same  con- 
ditions the  holding  power  of  oak  is  1,300  pounds  per  linear  inch  (7.8  tons  per 
linear  foot).  The  holding  power  of  a  bolt  driven  imrallel  to  the  grain  is 
almost  exactly  half  as  much  as  when  driven  perpendicular  to  the  grain.  If 
the  holding  power  of  a  1-inch  rod  in  a  ||-inch  hole  be  designated  as  1,  the 
holding  power  in  a  j;|-inchhole  is  1.69,  in  a  }f  inch  hole  2.13,  and  in  a  jf-iuch 
hole  1 .09.     The  holding  power  decreases  very  rapidly  as  the  bolt  is  withdrawn. 

Another  series  of  experiments  II  using  round  and  square  drift-bolts  in  the 
same  size  holes  shows  that  round  drift-bolts  have  the  ailvantage  ovei'  square 
ones,  bulb  in  uliiuuite  holding  power  and  in  holding  power  per  pound  of 
metal. 

*  J.  James  R.  Croes  in  Trans.  Am   Soc.  of  C.  E.,  Vol.  IIL,  p.  363. 
t  Stone  lowered  an  average  of  SO  feet. 

t  One  half  lowered  5  feet:  one  quarter  swung  in  level;  one  quarter  hoisted  6  feet. 
§  Selected  papers  of  the  Civil  Engineers'  Club  of  the  University  of  Illinois,  No.  4,  prede» 
cesBor  of  The  Technograph,  pp.  53-5**. 

D  The  Technograph,  University  of  Illinois,  No.  5,  pp.  39-41. 


PLATE    I. 

CAISSON,  CRIB  AND  COFFER-DAM. 

Havre  de  Grace  Bridge. 

FOR  TEXT,    SEE   PAGE  286. 


Plate  I.     PNEUMATIC  CAISSON,  CRIB,  and  COFFER-DAM. 

FOR  TEXT,  SEE  PAGE  286  SCALE  OF  FEET  '  »  »■  —  — 


PLATE     II. 

6-FOOT   ARCH   CULVERT. 

Illlnois  Central  Standard 

rOR  TEXT,   SEE   PAGE  424. 


FLA-TB  II. 

6-FOOT    ARCH    CULVERT. 

ILLINOIS  CENTRAL  STANDARD. 


PLAN     OF    TinBER      UNDER    FOUNDATION 
Flatted  Surface  +<=  cover  kalf  the  -fci-nrfnj-.on 


PLATE    m. 

8-FOOT  AKCH  CULVERT. 

C.   K.   AND  N.  Standard. 

FOR  TEXT,   SEE   PAGE  427. 


CROS5      SECTION 


jpx.a.t:ei  III. 
8-FOOT    ARCH    CULVERT. 

C,  K.  &  N.  STANDARD. 

FOR  TEXT  SEE  PAGE  427. 

SCALE  OF  FEET 


PLATE     IV. 

lO-FOOT  ARCH  CULVERT. 

SEMI-CIRCULAR. 

A.  T.  AND  S.  F.  Standard. 

FOR  T£ZT,  SEE  PAGE  429. 


PLATE  V. 

10-FOOT  ARCH  Cl/LVERT. 

SEGMENTAL. 

A.  T.  AND  S.  F.  Standard 

FOR  TEXT,   SEE  PAGE  429. 


LONOrrUDINAX.  SECTION- 


lO-FooT  Segmental  Arch  Culvert., 


PLATE  VI. 

12-FOOT  STANDARD  ARCH  CULVERT. 

FOB  text:    see   pass  430. 


INDEX. 


ABIT— ARC 

Abutments  of  arches,  dimensions  of  exist- 
ine;,  505 
stability,  empirical  formulas   for,  499 
theory  of,  49^ 
Abutments  of  bridges,  contents,  357,  361,  363 
detailed  plans,  356,  360,  362 
foundation,  364 
general  form,  353 
quality  of  masonry,  365,  385 
T-abutment,  362 
contents,  3G3 
detailed  plan,  362 
U -abutment,  359 
contents,  361 
detailed  plan,  360 
wins'  abutment,  355 
contents,  357 
detailed  plan,  356 
Air-cbamber,  filling,  297 
Air-lock,  for  pneumatic  pile,  281 
for  pneumatic  caisson,  2&4,  291,  299 
position,  290 
Arch,  abutment  of,  stability,  492,  499 
backing,  505 
brick,  510 

center,  camber.  523 
definitions,  515 

examples.  Cabin  John  arch,  525 
stone  bridges, 
tunnel  arch.  512 
AVasliin.Lrton  bridge,  524 
load  sujiiHirted,  516 
outline  forms.  519,  .5-JO,  523 
striking,  method,  .523 
time,  527 
culvert,  419 
cost,  434 
examples.  424 
Atchison,  T.  &  S.  F.,  segmental.  429 
cost,  438 
semi-circular,  429 
cost,  437 
Chicago,   K.  &  N.,  semicircular,  427 

cost,  436 
Illinois  Central,  semi-circular,  424 

cost,  435 
standard  segmental,  429 
cost.  438 
junction  of  wings  to  body,  420 
masonry,  cost  of,  157,  1.59,  160 

quality  of,  4.32 
specifications,  foundations,  432,  533 
masonry.  432,  531 
paving,  148 
segmental  vx.  semi-circular,  421 
splay  of  wings,  419 
definition,  of  kinds  of  arches,  441 
of  parts  of  an  arch,  440 


ARC 

Arches,  dimensions  of  abutments,  505 
of  arches,  502 
rules  derived  from  practice,  494 
thickness  of  abutment,  499 
thickness  at  crown,  American  prac 
tice,  495 
English  practice,  496 
French  practice,  496 
thickness  at  springing,  iSa 
drainage  508 
elastic,  theory  of.  491 
engravings,  505 
inverted,  for  foundations.  212 
joint  of  rupture,  457 
line  of  resistance,  definition,  443 
location.  453 
hypothesis  of  least  preesure.  5.54 
hypothesis  of  least  cro\TO  thrust,  465 

joint  of  ruiJture,  4rj7 
Navier's  principle.  4C'i 
Winkler's  hypothesis,  463 
masonry,  432 
backing,  605 
cost.  157,  159,  ICO 
specifications,  brick,  176,  177 
stone,  432,  51.5.  531 
relieving  arches  for  retaining  walls,  35? 

in  spandrel  filling,  506 
spandrel,  filling,  q.  v..  506 
stability,  criteria  of  safety,  447 
conclusion.  4.52 
crushing,  448 
open  joints,  451 
maximum  pressure,  461 
unit  pressure,  449 
rotation,  448 
sliding,  4.52 
theories,  405 
elastic  arch.  491 
external  forces.  444 
method  of  employing,  466 
method  of  failure.  446 
rational  theory,  466 
criterion,  47-3 
symmetrical  load.  466 
general  solution.  466 
special  solution,  469 
unsymmetrical  load,  471 
Schefflf  r's  theory,  474 
algebraic  solution,  475 
erroneous  solution,  480 
gripiii'Ml  solution,  479 
rclialiility,  4S1 
Rankincs 'theory,  482 
curvjitiirc  of  huear  arch,  482 
met  hod  ofapplyiug,  487 
relialiility.  490 
various  theories  referred  to,  491 

549 


550 


INDEX. 


ART— BEI 

Artiflcial  stone,  112 

B6ton-Coignet,  113 

Frear,  114 

McMurtrie,  113 

Portland,  113 

Raiisome,  114 

Sorel,  115 
Ashlar,  138 

backing,  140 

bond,  139 

definitions,  136 

dressing,  138 

mortar  required  per  yard,  141 

pointing,  141 

specifications,  143 

where  employed,  142 
Atchafalaya  bridge,  foundations,  273 
Atchison,  Topelja  &  Santa  F6,  bridge  abut- 
ment, 359 

culvert,  iron  pipe,  414 

segmental  arch.  429,  431, 438 

semi-circular  arch,  420,  430,  437 
Ax,  and  Tooth-ax,  126 

Batter,  definition,  135 

Bearing  piles,  219 

Bearing  power,  piles,  q.  v.,  233 

soils,  q.  v.,  188 
B6ton,  see  Concrete. 
B6tonCoignet,  113 
Bismarck  bridge,  pressure  on  foundation, 

377 
Blair  bridge,  pier.  383 
pneumatic  foundation,  caisson,  284 
cost.  303 

f lictional  resistance,  297 
rate  of  sinking,  295 
Blasting  in  compressed  air,  295 
Brick,  absorptive  power,  21,  39,  45 
arches,  bond.  510 

examples.  511,  513,  514 
burning,  34 
classification,  35 
cost.  4^ 

flre-brick,  how  made,  35 
elasticity,  co-efflcient  of ,  14 
masonry.  161 
bond,  163 
cost,  1.57,  160 

data  for  estimates,  brick  required,  173 
labor  required,  174 
mortar  required,  174 
impervious  to  water,  178 
joir)ts,  finishing,  162 

thickness,  161 
Streusith,  164 
compressive,  164 

pressure  allowed,  167 
transverse.  167 
specifications,  arches,  177  532,  542 
buildings.  175,  541 
sewers,  176 
vs.  stone  masonry,  177 
moulding.  34 
requisites  for  good,  37 
size,  46 
strength,  crushing,  41 

condition  of  surface,  42 
data,  43-46 
form  of  specimen,  42 
size  of  specimen,  41 
transverse   -40 
data.  13.  45 
weight,  46 
Bridge  abutment,  see  Abutment. 


BRI— CEM 

Bridge  masonry,  cost.  157,  160 
Bridge  piers,  see  Piers. 
Bond,  brick  arches,  510 

brick  masonry,  163 

stone  masonry,  139 
Box-culvert  masoin-y,  cost.  1.57,  160 
Brooklyn-bridge  foundations,  cost,  303 

description,  298 

pressure,  377 
Bush-hammer,  126 
Building-stones,  classification,   24 

requisites  for  good,  3 

tests,  5  ;  see  also  Stone. 
Buildings,  data  for  computing  weight  of, 
200 

specifications  for  brick-work  for,  541 

Cain's  profile  for  masonry  dams,  329 
Cairo  bridge,  frictional  resistance  of  caisson, 
297 
pier,  outline  of,  372 
pressure  on  foundation,  377 
stability  of.  371 
stones  in  a  course  of,  385 
Caisson,  definitions,  266 
diseas*'.  300 
pneumatic.  284.  286 
Blair  bridge.  284 
first  use  of,  280 
guiding.  295 

Havre  de  Grace  bridge,  286 
Canadian  box  culvert,  406 
Cavil,  126 
Cement,  51 
amount  required  per  yard  of  mortar,  88 
burning,  thoroughness  of,  56 
classification,  51 
cost,  54 

data  for  estimates,  86,  88 
lime-cement  mortar,  100 
mortar,  see  Mortar, 
natiu'al.  52 
definition.  52 
specifications,  783 
tests,  see  tests  below, 
weight,  per  barrel,  54 
Portland,  constancy  of  volume,  7Sd 
cost,  .54 
description,  52 

specifications,  78e.  78/,  78g.  787i 
strength,  67,  78a.  78rf,  78^^  78/,  78h 
tests,  see  tests  below, 
weigiit  per  barrel,  54 
Rosendale,  definition,  53 
slag,  54 

specifications,  quality,  American,  78^ 
English,  78e 
French,  78e 
German,  78d 
delivery  and  storage,  7Sh 
tests,  55 
activfty,  57,  60 

burning,  thoroughness  of,  50 
chemical  analysis,  68,  78e 
color,  .55 

constancy  of  volume,  78fi,  78e,  7Sfir.  78h 
fineness,  65,  66.  78d,  78e,  78/,  78gf,'78;i 
set,  time  of,  60 
soundness.  60,  7Sd,  78e,  78(7 

accelerated,  tests  of,  63 
specific  gravity,  50 
strength,  67 
age  when  tested.  76 
data,  78a,  78d,  7He,  7S/,  78a,  78« 
form  of  briquette,  72 


INDEX. 


551 


CEM— CUL 

Cement  tests, stieDgtli, mixing  the  mortar,  71 
rapidity  of  applying  the  stress,  78 
water  required,  68 
weight,  54,  56 

Centrifugal  pump,  364 

Center  of  fuuiidation,  proper  position  of, 

Center  of  gravity  of  trapezoid,  to  find,  318 
Center  of  pressure  on  foundation,  203 
Channeling  and  wedging,  quarrying  by,  123 
Chisel,  pitching,  127 
splitting,  128 
tooth,  128 
Chicago,  K.  &  N.  arch  culvert,  427,  436 
Co  efficient  of  friction,  foundations,  276 

masonry,  315 
Coffer-dam,  definition,  258 
construction,  258,  289 
dovible,  261 

Havre  de  Grace  bridge,  289 
iron,  261 
leakage,  262 
movable,  261 

process,  for  foundations,  214,  258 
Compressed  air,  physiological  effect,  299 
Compressed-air    process   for    foundations, 

see  Foundations,  pneumatic. 
Concrete,  106 
aggregate,  107 
cost,  11  2d,  157,  160,  265 
depositing  imder  water,  112o 
estimates,  data  for,  112/ 
economics  of,  113rt 
ingredients  for  a  yard,  112fir,  112& 
laying,  112n 
mixing.  112/u 

proportions,  theory  of,  109 
strength,  112j) 
compressive,  llSp 
transverse,  112m 
water  required,  112j 
weight,  112v 
Concrete  and  piles  for  foundations,  254 
Connecticut  brown-stone,  30 
1-,—^-,  13ti 

Cost,  see  the  article  in  question. 
Coulomb's  theory  of  retaining  wall,  341 
Cover  stones  for  box  culverts,  398 
theory  for  thickness.  398 
formulas,  399 
practical  data,  401 
Cramps,  136 
Crandall.  127 
Crib  for  coffer-dam,  260 
Culvert,  arch,  see  Arch, 
iron  pipe,  412 
construction,  413 
cost.  410 

dimensions  of  the  pipe,  412 
end  walls,  contents  of,  414 
e.xamples,  414,  415 
Icirge.  416 

weight  of  the  pipe,  412 
stone  box.  396 
Canadian,  406 
contents,  403,  404,  405 
cost,  405 

CDver  stones,  q.  v.,  398 
dimensions,  403,  404,  405 
double,  405 
end  walls,  39S 
examples,  403.  404,  406 
foundation,  397 
masonry,  quality  of,  401 
specifications,  401,  531 


CUL— EFF 

Culvert,  stone  box.  Standard  form,  402,  403 
West  Shore  R.  R.,  402,  404 
timber  box,  417 
timber  barrel,  418 
vitrified  pipe,  407 
construction,  408 
cost  of  the  pipe,  410 
end  walls.  409 
examples.  411 
material  required,  411 
strength  of  the  pipe,  408 
water-way  required,  391 
formulas,  393 
for  quantity  of  flow,  394 
Meyer's  for  the  area,  394 
Talbot's  for  the  area,  394 
practical  method  of  finding,  395 
Cushing  pile  foundation,  255 
Cylindrical  surface,  method  of  forming  in 
stone,  129 

Dam,  arched  vs.  gravity,  330 
bibliography,  334 
curved  gravity,  331 
earth, 335 
gravity,  311 
masonry,  311 
arched,  311 
Cain's  profile,  329 
classification.  311 

gravity,  condition  for  stability  of,  312 
crushing,  320 
maximum  pressure,  392 
tension  in  masonry,  324 
limiting  pressure,  325 
nomenclature,  312 
overturning.  317 
by  moments,  317 
by  resolution  of  forces,  320 
plan,  329 
arched  vs.  gravity,  330 
curved  gravity,  331 
straight  crest  vs.  straight  toe,  399 
pressure  allowable,  325 
profile.  326 
Cain's,  329 
Krantz's.  328 
method  of  finding,  327 
Quaker  Bridge,  328 
sliding,  313 
quality  of  masonry.  333 
when  employed,  335 
width  on  top,  326 
rock-fill.  334 
cost,  337 

when  employed,  -336 
stone-lilled  timber  crib,  335 
Dimensiun  stones.  136 
Disk  piles,  described,  218 

bearing  power,  249 
Dorchester  sandstone,  30 
Dowel,  136 
Dredges,  271 
Milroy.  272 

Morris  &  Cumming's,  272 
mud  inmip.  292 
Dredging  thro'  tul)es.  271 
Drift  bolts,  described,  253 

holding  power,  253 
Drills  used  in  quarrying,  118 
Dj-namite,  121 
driving  piles  with,  227 

Eads'  mud-pump,  292 
Efflorescence  on  brickwork,  181 


UUli 


INDEX. 


ELA— FOU 

Elastic  arch,  theory  of.  491 
Engravings,  for  list  of,  see  Table  of  Con- 
tents. 
Estimates,  data  for,  brick,  46,  47,  1T3,  174 

ceiiient,  8G,  88 

lime,  86 

mortar,  88,  89 

sand,  r9^•,  88 
Excavator,   compressed-air,  272 ;   see   also 

Dredges  and  Pumps. 
Explosives,  119 

dynamite,  r^l 

gunpowder.  119 

nitro-glycerine.  100,  124 

quarrying  by,  117 
Extrados  defined,  440 

Face-hamm°r,  125 
Facing,  defined,  135 
Feathers  and  Plug,  described,  128 
Figures,  for  list  of,  see  Table  of  Contents. 
Footings,  off-set  for  masonry,  208 
steel  rail.  212,  540 
timber,  211 
Forth  bridge,  pneumatic  caisson,  298 
Foundation,  Atchafalaya  bridge,  273 
bearing  power  of  clay,  190 
bearing  power  of  rock,  188 
bearing  power  of  sand,  192 
bearing  power  of  semi-liquid  soil,  193 

summary,  194 
bed  of,  defined,  183 
bridge  piers,  255,  257;  see  also  below. 
buildings,  ISO 
area  reqiiired,  201 

bearing  pnwer  of  soils,  q.  v.  above,  188 
consolidating  the  soil,  197 
depth  required,  195 
drainage,  195 
effect  of  wind.  204 
examination  of  site,  186 
footings,  see  Footings, 
grillage,  q.  v..  215.  254 
load  to  be  supported,  199 
piles,  see  Piles, 
piles  and  grillage,  253 
piles  and  concrete,  254 
preparing  the  bed,  213 
sand  piles,  197 
sand  in  layers,  198 
springs,  19B 
coffer-dam  process,  214,  258 
construction  of  the  dam,  258 
thickness,  259 
puddle  wall,  260 
leakage,  262 

pumps,  q  v..  263 
preparing  the  bed,  264 
cost.  264 
compressed-air    process,    see   pneumatic 

process,  below, 
concrete,  103.  215,  265 
cost  of  various  processes  compared,  310 
crib  and  erect  caisson  process,  266 
construction  of  the  caisson,  267 
construction  of  the  crib,  269 
excavating  the  site,  270 
principle  of  the  method,  267 
definitions.  1S3 
drainage,  195 

dredging  through  wells,  271 
dredges,  q.  v.,  271 
cost,  277 
iron  tubes,  278 
timber  cribs,  278 


FOTJ 

Foundation,  examination  of  site,  186 
examples,  272 
Atchafalaya  bridge,  273 
brick  cvlinders.  275 
Hawkesbury  bridge,  275 
Pouglikeepsie  bridge.  272 
friciional  resistance  in  sinking,  275 
iron,  cast,  276 
wrought,  277 
masonry,  277 
freezing  process,  307 
advantages,  309 
cost,  308 
details,  307 
history,  307 
jirinciple,  307 
footings,  see  Footings  above, 
frictional  resistance,  275 
iron  cylinders.  276 
masonry  cylinders.  277 
pneumatic  caissons,  297 
wood  piles,  247,  248 
grillage.  215 

Hawkesbury  bridge.  275 
independent.  204.  540 
inverted  arch.  212 
lateral  yielding,  255 
pile,  see  Piles, 
piles  and  grillage.  253 
piles  and  concrete,  254 
preparing  the  bed.  213,  264 
Point  Pleasant  bridge,  cost,  265 
Pouglikeepsie  bridge,  described,  273 
pneumatic  piles,  281 
bearing  power,  275,  283,  297 
cost,  304,  305 
pneumatic  process,  278 
advantages,  306 
air-chamber,  284,  297,  298 
air-lock  construction,  281,  284,  290,  299' 

position,  290 
caisson.  284 
Blair  bridge.  284 

Havre  de  Grace  bridge,  q.  v.,  286 
coiiiiiressed-;iir  process.  279 
cost,  Blair,  303 
Brooklyn,  303 

European  examples,  304,  310 
Havre  de  Grace.  302 
Plattsmouth,  304 
Philadelphia.  302,  304 
definitions.  278 
examples.  Brooklyn,  298 
Forth.  298 

Havre  de  Grace,  286 
St.  Louis,  297 
excavators.  291 
blasting.  295 
nuid-pump.  292 
sand-lift.  :iSl 
water-column,  294 
filling  Xhf  air-chamber,' 297 
frictional  resistance,  q.  v.,  275,  283,  297 
guiding  the  caisson.  295 
history.  2';  9 
phy.siological  effect  of  compressed-air^ 

299 
plenum  process,  279 
rate  of  sinking,  295 
vacuum  process,  278 
sand  in  layers,  198 
sand-piles",  197 
steel-iail  footings,  212 
timber  in,  209 
timber  footings,  211,  215 


INDEX. 


552 


FOir— LIM 

Foundation,  under  water,  257 
vacuum  process.  278 
wind,  effect  of,  204 
Freezing  of  mortar,  100 
Freezing  weather,  specificatioa  for  laying 

masonry  in,  543 
Freezing  process  for  foundations,  q.  v.,  307 
Friction-clutch  pile  driver.  223 
Friction,   coefficient    of,  for   foundations, 
27S 
for  masonry,  315 
Fi'ictional  resistance  in  sinking  foundations, 

q.  v.,  247,  248,  275,  297 
Frost  batter,  3t>4 

Grand  Forks  pivot  pier,  380 
Grillage.  215 
Groui,  89 
Gunpowder,  119 

cost.  120 

efficiency  in  blasting,  120 
Gunpowder  pile-driver,  226 

Hammer,  bush,  126 

face.  125 

hand.  127 

patent,  127 
Haunch  of  an  arch,  defined.  440 
Havre  de  Grace  bridge,  pneumatic  founda- 
tions of,  286 

air-lock,  291 

caisson,  286 

coffer-dam,  289 

cost,  302 

dimensions,  290 

frictional  resistance,  297 

guiding  the  caisson,  295 

machinery,  290 

materials,  quantity  of,  290 

mud-pump,  292 

rate  of  sinking,  295 
Henderson  bridge,  top  of  pier,  384 
Hydraulic  cement,  see  Cement. 
Hydraulic  lime,  51,  82 

Ice,  effect  on  stability  of  pier,  368 
Illinois  Central  arch  culverts,  424,  435 
Impervious  brick-work,  178 
Impervious  mortar,  101 
Independent  piers  for  foundations,  204 
Intrados,  defined,  440 
Inverted  arch  for  foundation,  212 
Iron  coffer-dam,  261 

Iron  cylinders  for  foundations,  bearing  pow- 
er of,  283 

cost.  302.  304 

frictional  resistance  in  sinking,  276 

method  of  sinking,  274,  281 
Iron  piles,  216 

Jet  vs.  hammer  pile-driver.  229 
Joint  of  rupture,  defined,  457 
method  of  finding,  457 
Peiifs  theory,  462 

Krantz's  profile  for  masonry  dams,  328 

Laitance.  112p 
Lake  Superior  sandstone,  31 
Laieral  yielding  of  foundations,  255 
Leakage  of  coffer-dams,  262 
Lime,  cost,  .50 

data  fi>r  estimates,  86 

described,  49 

hydraulic,  51 


LIM— MOR 

Lime,  preserving,  50 

testing,  50 

weight  per  barrel,  50 
Lime  mortar,  81 

strength.  91 
Lime-cement  mortar,  100 

Machines,  pile-driving,  221 
Masonry,  ashlar,  see  Ashlar, 
brick,  see  Brick, 
co-efficient  of  friction,  315 
cost,  actual,  arch  culvert,  157,  160 
bridge  pier.  157.  160 
railroad  masonry,  157,  160 
stone.  155 

cutting.  156 
summary.  100 
tunnel  masonry.  157 
U.  S.  public  buildings, 
cutting  the  stone.  156 
mason r J"  complete,  156 
cost,  estimated,  153 
aslilar.  154 
dressing,  153 
quarrying,  153 
rubble,  155 
dressing.  153 
quarrj'ing,  153 
definitions  of  kinds,  136 
footings,  off -sets  for,  208 
general  rules  for.  138 
measurement,  brick.  172,  529 

stone.  151,  529.  539 
mortar  required  perjyard,  87 
off-sets  for  footings.  208 
pedestal,  specifications  for.  385 
specificati  'ns,  see  Specifications, 
squared-stone.  see  Squared-stoue. 
stone,  see  Stone, 
strength  of.  148 
brick,  compressive,  164 

transverse,  167 
stone,  allowed  pressure,  149 
safe  pressure,  150 
rubble,  see  Rubble, 
weight  of.  2U0 
Measurement  of  masonry,  brick,  172,  529 

stone,  151,  529.  539 
Medina  sandstone.  31 
Mortar,  absor-ptive  power,  21 
amount  ivquired  per  yard  of  masonry,  89 
cement,  change  of  volume  in  setting,  62, 
cement-lime.  100  [T8d,  78e,  78^ 

co-efficient  of  elasticity  of,  14 
compression  of,  104 
cost,  95 

elasticity,  14,  104, 
estimates,  data  for,  86 
freezing,  effect  of,  102 
grout,  89 

hydraulic  cement,  83 
hydraulic  lime,  82 
uigredieuits  for  a  yard,  83 
impervions  to  water,  101 
lime,  81 

lime-cement,  100 
natural  vs.  Portland.  92,  95 
Portland  vs.  natural,  92,  95 
proportioning,  method  of,  83 
re-tempering,  99 
strength.  87 
adhesive.  93 
compressive.  92 
increases  with  agt,  91 
tensile,  90 


554 


IXDEX. 


MOB,— PIL 

Mortar,  streiigtli,  transverse,  13 

water  lequii-ed,  68 
Mud-pump,  292 

Nipper  pile-driver.  223 
Nit-ro-glyceriue,  120,  124 


Open  joints  in  an  arch,  451 


Patent  hammer,  127 
Paving,  148,  532 
cost.  15T.  160 

for  foundations.  397,  432,  .'iSS 
Philaitelpliia,  pneumatic  piles,  cost  at,  303 

standaiil  hriolf  sewei's.  513 
Physiological  effect  of  compressed  air,  299 
Pick,  126 

Piers,  contents,  387,  388 
cross  section.  378 

examples.  372.  383,  384,  385 
Cushing's  pile.  255 
dimensions,  bottom.  378 
examples.  372,  380.  383-86 
top,  377,  384 
foundations,  257:  see  also  Foundations, 
iron  tubular,  274.  387 
location.  366 

masonry,  cost  of,  157,  160 
qualtij'  of,  379 
specifications,  381,  537 
pivot.  379 
stability  of,  367 
crushing,  theory  of.  371 

numerical  example,  375 
current,  effect  of.  367 
foundation,  pressure  on,  376 
ice,  effect  of,  368 
overtui-ning.  theory  of.  369,  370 

numerical  example,  374 
resisting  forces.  369 
"     sliding,  theory  of,  367 

numerical  example,  371 
wind,  effect  of.  367 
timber-barrel,  388 
Piles,  bearing  power  of,  disk,  249 
screw,  249 
wood,  actual.  247 
experiments  on,  246 
factor  of  safety,  249 
formulas,  empirical,  241 
author's.  245 
Beaufoy's,  243 
Engineering  News',  245 
HaswelTs,  242 
Mason  s  243 
Ny Strom  s,  243 
Sander's,  244 
Trau twine's,  244 
rational.  234 
author's,  239 
Rankine's,  241 
Weisbach's,  241 
frictioiiAl  resistance  of,  247,  248 
load,  safe,  248 
ultimate,  247 
butt  rs  top  down,  251 
*aps.  2-30 
capping.  2.50 

concrete  and  piles,  in  foundations,  254 
cost,  230 

definitions  of  kinds.  216 
disk,  described.  218 
bearing  power.  249 


PIL— PUM 

Pile  foundations,  250 
concrete.  254 
cost.  310 
grillage.  2.53 
position  of  piles.  250 
sawing  off  the  piles,  252 
iron,  216 
cylinders.  274,281 
cost.  304 

sinking,  frictional  resistance,  273 
method  of,  274,  281 
disk,  q.  v..  218 
screw,  q.  v..  217 
pneumatic.    281  ;    see  also  Foundations, 

pneumatic, 
sand,  197 
sawing  off,  252 
screw.  217 

bearing  power,  249 
sheet,  219 
shoes.  220 

specifications,  220,  533 
splicing,  221 
top  vs.  butt  down,  251 
used  to  consolidate  soil,  197 
wood.  219 
bearing  power.see  bearing  power,  above 
specifications,  220,  533 
Pile-il liver.  221 
drop  hammer,  222 
friction  clutch,  223 
nipper,  223 

steam  vs.  drop  hammer,  225 
dynamite,  227 
friction  clutch.  223 
gunpowder,  22t; 
hammer  vs.  jet,  229 
jet  of  water,  227 
nipper,  223 
steam,  224 

drop  hammer  vs.  steam,  225 
water-jet,  227 
hammer  vs.  jet,  229 
Pile-driving,  cost  of,  230 
bridge  construction,  231 
foundations,  232 
harbor  work,  233 
railroad  construction,  230 
railroad  repairs,  231 
river  protection,  233 
Pitching  chisel,  127 
Pivot  pier,  379 
Plane  surfaces,  method  of  forming  in  stone, 

129 
Plates,  for  list  of,  see  Table  of  Contents. 
Plattsmouth  bridge,  cost  of  concrete  founda- 
tions at,  265 
cost  of  pneumatic  foundations,  304 
pressure  on  foundations.  377 
rate  of  sinking  by  pneumatic  process,  295 
Plug  and  feathers.  128 
Pneumatic  foundations,  see  Foundations. 

pneumaiic. 
Point.  127 

Pointing,  141  [265 

Point  Pleasant  bridge,  cost  of  foimdalion, 
Pouglikeepsie  bridge,  foundation  described, 

272 
Pozzuolana.  53 
Pressure  allowed  on  masonry,  bric'it,  166,  167 

stone.  149,  151 
Puddle.  260 
Pulsometer,  264 
Pumps,  263 
hand,  263 


INDEX. 


555 


PTJM— SEW 

Pump«,  centrifugal,  2u4 
for  water- jet  pile-driver,  228 
mud-pump,  292 
piilsoineter,  264 
steam  siphon,  263 

Quarrying,  116 

by  clianueling  and  wedging,  lig3 

by  explosives,  117 

by  hand  tools,  116 
Quoin,  defined,  136 

Railroad  masonry,  classification,  ^o2 
cost,  157,  160 
specifications,  529,  534 
Rankine's  theory  of  the  arch,  482 
Relieving  arches  for  retaining  walls,  352 
Resistance,   frictional,   in  sinking   founda- 
tions. 247,  248,  275,  297 
Retaining  walls.  Coulomb's  theory,  341 
definitions.  338 
difficulties  in  theories.  339 
dimensions,  eniiiiricai  rules  for, 
Benj.  Baker's,  349 
English,  349 
Trautwine's,  349 
drainage,  350 
failure,  method  of,  338 
land-lies,  351 
Rankine's  theory,  348 
stability,  theory  of,  339,  340 
applicability  of,  348 
assumptions  necessary,  340 
Coulomb's  theory.  341 
surcharged  wall,  343 
reliability.  343 
Rankine's  theorj',  348 
Weyrauch's  theory,  343 
general  formula,  344 
horizontal  earth-surface,  346 
surcharge,  345 
reliability,  "^46 
Weyrauch's  theory,  q.  v.,  343 
Riprap.  148.  .532 

cost,  157,  160 
Rubble  masonry,  145 
cost,  157,  160 
coursed,  137 

mortar  required  per  yard.  89,  146 
specifications.  147,  531,  o'^O,  541 
uncoursed,  137 

Sand,  amount  per  yanl  of  mortar,  88 
cost,  79/c 

data  for  estimates.  '^'^ 
foundations,  used  I        ;':•;■,  i!i8 
requisites  for  gooG.  .:«/ 
cleanness,  79c 
durability,  796 
fineness,  79rf,  79/ 
sharpness,  79b 
voids,  797,  79t 
weight,  79/,  79fc 
Sand-lift,  291 
Sand-pump,  292 

Sandstones,  those  most  frequently  used,  31 
Scheffler's  theory  of  arch,  474 
Schuylkill  bridge.cost  of  pneumatic  pile8,302 
Screw-i>iles.  described,  217 

bearing  power.  249 
Seasoning  of  stone,  18 
Sewers,  brick  arches  for, 
Philadelphia  standard,  513 
Washington  standard,  514 


SEW— STE 

Sewer-pipe,  cost.  410 
strength.  408,  547 
weight,  410 
Sibley  bridge,  guiding  the  caisson,  296 

piers,  specifications  for,  381 
Skew  arch,  defined,  442 
Slope-wall  masonry,  147 
cost,  157.  160 
specifications,  147,  531 
Soap  and  alum  wash  for  brick-work   178 
Soffit,  defined,  440 
Soil,  bearing  power  of,  188 
clay,  190 
rock,  188 
sand,  192 

semi-liquid  soil,  193 
summary,  194 
testing,  method  of,  187 
examining,  method  of,  186 
improving,  method  of,  195 
Spandrel,  defined,  440 
filling,  arches  in,  508 
drainage  of,  508 
Specifications, 
arch  culvert  masonry,  432.  531 
architectui-al  masonry,  534,  539 
ashlai-.  142,  530 
bo.x  culverts,  401,  531 
brickwork,  arches,  177,  532 
buildings,  175,  541 
sewers,  176 
bridge  piers,  381 
cement,  7Sd,  78e,  78/,  78g,  78h 
concrete,  532.  535,  540 
foundations,  432,  533 
masonry, 
arch  culvert, '432 
ashlar.  142 
brick-work,  175,  177 
buildings,  architectural,  539 

railroad,  534 
paving,  148 
pedestal,  385 
pier,  381, 

rubtde,  147,  531,  536,  541 
slope-wall,  147 
squared-stone,  144 
paving,  148 
piers,  .381,  539 
piles.  220.  533 

rubble  masonry,  147,  531,  536,  541 
slope-wall  masonry,  147,  531 
squared-stone  masonry,  144,  530,  538 
Splicing  piles.  221    ' 
Squared-stoue  masonry,  143 
definitions.  137 
pitched-face,  137 
quarry-face,  137 
range  work,  137 
mortar  required  per  yard,  144 
specifications.  144,  .530,  5:38 
Staiidard  arch  culvert,  249 
contents,  433 
cost,  438 
dimensions,  433 
Standard  stone-box  culvert,  402 
contents,  403 
cost,  405 
dimensions.  403 
St.  Genevieve  sandstone.  31 
St.  Louis  bridge  foundations,  297 

maxinnim  jiressure  on,  377 
Steam  pile-driver,  224 

drop-hammer  vs.  steam,  225 
Steam  siphon,  263 


556 


IXDEX. 


STE— STO 

Steel-rail  footings,  aiti,  540 
Stone,  absorbing  power,  21 
Stone,  argillaceous,  a6 
artificial,  1136 
calcareous,  2(5 
cost,  155 

crushing  strength,  8 
cushions,  9 
data.  1 1 

fracture,  form  of,  9 
specimen,  dressing,  13 
form.  8 
size,  8 
slabs.  11 
cut-stone,  132 
axed,  133 

brush-hammered,  134 
crandalled,  133 
diamond  panel,  134 
fine-pointed,  133 
rough- pointed,  132 
rubbed,  134 
tooth-axed,  133 
description,  artificial,  112 
granite,  27 
limestone,  28 
marble,  28 
sandstone,  29 
trap,  27 
durability,  4,  15 
destructive  agents,  16 
preserving,  methods  of,  23 
resisting  agents,  17 
seasoning,  effect  of,  18 
testing,  metliod  of,  20 
artificial,  20 
absorptive  power,  21 
acid,  effect  of,  23 
atmosphere,  effect  of,  23 
Brard's  method,  23 
crushing  strength,  8 
frost,  effect  of,  21 
microscopical  examination,  33 
natural,  20 
elasticity,  14 
granite,  27 
hardness,  7 
limestone,  28 
local  names,  32 
marble,  28 
market  price,  155 
masonry,  q.  v.,  definitions,  135 
measurement  of,  151,  529,  539 
requisites  for  good,  3 
sandstones,  description  of  principal,  29 
siliceous,  2(J 
specific  gravity,  6 
squared,  drafted,  132 


STO— YAZ 

Stone,  squared,  pjtcli-faced,  138 
quarrj-faeed,  132 

strength,  cru!-hing,  q.  v.,  6 
transverse,  13 

tests,  bibliography  of,  15 

toughness,  7 

vs.  brick  masonry,  177 

weight,  7 
Stone-cutting  tools,  described,  125 
St(jne  grinder,  129 
Stone  planer,  129 
Stoue  polisher,  129 
Stone  saws,  128 
Surfaces,  method  of  forming,  129 

method  of  finishing,  131 

Timber,  barrel  culvert,  419 

box  culvert,  417 

footing,  211 

foundations,  269 
Tremie,  107 

Vitrified  pipe,  cost,  410 
strength,  408,  547 
weight,  410 

Wall,  definitions  of  parts  of  a,  135 
Warped  surface,  method  of  forming,  131 
Washington  brick  sewers,  514 
Water  required  for  cement  mortar,  68 

concrete,  H2j 
Water-jet  pile-driver,  227 

■ys.  hammer  pile-driver,  229 
Water-way  for  culverts,  391 

factors,  391 

formulas,  393 

Meyer's,  394 

Talbot's,  394 

Waverly  sandstone,  31 

AVeep  holes,  351 

West  Shore  stone-box  culvert,  402,  404 
Weyrauch's  theory  of  retaining  walls,  343 
"White-wash"  on  brick-w'ork,  181 
Wind,  effect  on  foundation,  204 

pressure,  amount  of,  201 
Wood  bearing-piles,  see  Piles. 
Weight,  bricK,  46 

buildings,  200 

cast-iron  pipe,  418 

cement,  barrel,  54 
cubic  foot,  56 

lime,  50 

masonry,  200 

sand,  79/i; 

stone,  7 

vitrified-pipe,  410 

Yazoo  River  bridge,  guiding  the  caisson,  296 


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3  00 

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2  00 

2  00 

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3  00 

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13 


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16 


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